linux Author Tejun Heo tj kernel org Date October 2015 Control Group v2 cgroup v2 This is the authoritative documentation on the design interface and

Answer:
.. _cgroup-v2:

================
Control Group v2
================

:Date: October, 2015
:Author: Tejun Heo <tj@kernel.org>

This is the authoritative documentation on the design, interface and
conventions of cgroup v2.  It describes all userland-visible aspects
of cgroup including core and specific controller behaviors.  All
future changes must be reflected in this document.  Documentation for
v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`.

.. CONTENTS

   1. Introduction
     1-1. Terminology
     1-2. What is cgroup?
   2. Basic Operations
     2-1. Mounting
     2-2. Organizing Processes and Threads
       2-2-1. Processes
       2-2-2. Threads
     2-3. [Un]populated Notification
     2-4. Controlling Controllers
       2-4-1. Enabling and Disabling
       2-4-2. Top-down Constraint
       2-4-3. No Internal Process Constraint
     2-5. Delegation
       2-5-1. Model of Delegation
       2-5-2. Delegation Containment
     2-6. Guidelines
       2-6-1. Organize Once and Control
       2-6-2. Avoid Name Collisions
   3. Resource Distribution Models
     3-1. Weights
     3-2. Limits
     3-3. Protections
     3-4. Allocations
   4. Interface Files
     4-1. Format
     4-2. Conventions
     4-3. Core Interface Files
   5. Controllers
     5-1. CPU
       5-1-1. CPU Interface Files
     5-2. Memory
       5-2-1. Memory Interface Files
       5-2-2. Usage Guidelines
       5-2-3. Memory Ownership
     5-3. IO
       5-3-1. IO Interface Files
       5-3-2. Writeback
       5-3-3. IO Latency
         5-3-3-1. How IO Latency Throttling Works
         5-3-3-2. IO Latency Interface Files
       5-3-4. IO Priority
     5-4. PID
       5-4-1. PID Interface Files
     5-5. Cpuset
       5.5-1. Cpuset Interface Files
     5-6. Device
     5-7. RDMA
       5-7-1. RDMA Interface Files
     5-8. HugeTLB
       5.8-1. HugeTLB Interface Files
     5-9. Misc
       5.9-1 Miscellaneous cgroup Interface Files
       5.9-2 Migration and Ownership
     5-10. Others
       5-10-1. perf_event
     5-N. Non-normative information
       5-N-1. CPU controller root cgroup process behaviour
       5-N-2. IO controller root cgroup process behaviour
   6. Namespace
     6-1. Basics
     6-2. The Root and Views
     6-3. Migration and setns(2)
     6-4. Interaction with Other Namespaces
   P. Information on Kernel Programming
     P-1. Filesystem Support for Writeback
   D. Deprecated v1 Core Features
   R. Issues with v1 and Rationales for v2
     R-1. Multiple Hierarchies
     R-2. Thread Granularity
     R-3. Competition Between Inner Nodes and Threads
     R-4. Other Interface Issues
     R-5. Controller Issues and Remedies
       R-5-1. Memory

Introduction
============

Terminology
-----------

"cgroup" stands for "control group" and is never capitalized.  The
singular form is used to designate the whole feature and also as a
qualifier as in "cgroup controllers".  When explicitly referring to
multiple individual control groups, the plural form "cgroups" is used.

What is cgroup?
---------------

cgroup is a mechanism to organize processes hierarchically and
distribute system resources along the hierarchy in a controlled and
configurable manner.

cgroup is largely composed of two parts - the core and controllers.
cgroup core is primarily responsible for hierarchically organizing
processes.  A cgroup controller is usually responsible for
distributing a specific type of system resource along the hierarchy
although there are utility controllers which serve purposes other than
resource distribution.

cgroups form a tree structure and every process in the system belongs
to one and only one cgroup.  All threads of a process belong to the
same cgroup.  On creation, all processes are put in the cgroup that
the parent process belongs to at the time.  A process can be migrated
to another cgroup.  Migration of a process doesn't affect already
existing descendant processes.

Following certain structural constraints, controllers may be enabled or
disabled selectively on a cgroup.  All controller behaviors are
hierarchical - if a controller is enabled on a cgroup, it affects all
processes which belong to the cgroups consisting the inclusive
sub-hierarchy of the cgroup.  When a controller is enabled on a nested
cgroup, it always restricts the resource distribution further.  The
restrictions set closer to the root in the hierarchy can not be
overridden from further away.

Basic Operations
================

Mounting
--------

Unlike v1, cgroup v2 has only single hierarchy.  The cgroup v2
hierarchy can be mounted with the following mount command::

  # mount -t cgroup2 none $MOUNT_POINT

cgroup2 filesystem has the magic number 0x63677270 ("cgrp").  All
controllers which support v2 and are not bound to a v1 hierarchy are
automatically bound to the v2 hierarchy and show up at the root.
Controllers which are not in active use in the v2 hierarchy can be
bound to other hierarchies.  This allows mixing v2 hierarchy with the
legacy v1 multiple hierarchies in a fully backward compatible way.

A controller can be moved across hierarchies only after the controller
is no longer referenced in its current hierarchy.  Because per-cgroup
controller states are destroyed asynchronously and controllers may
have lingering references, a controller may not show up immediately on
the v2 hierarchy after the final umount of the previous hierarchy.
Similarly, a controller should be fully disabled to be moved out of
the unified hierarchy and it may take some time for the disabled
controller to become available for other hierarchies; furthermore, due
to inter-controller dependencies, other controllers may need to be
disabled too.

While useful for development and manual configurations, moving
controllers dynamically between the v2 and other hierarchies is
strongly discouraged for production use.  It is recommended to decide
the hierarchies and controller associations before starting using the
controllers after system boot.

During transition to v2, system management software might still
automount the v1 cgroup filesystem and so hijack all controllers
during boot, before manual intervention is possible. To make testing
and experimenting easier, the kernel parameter cgroup_no_v1= allows
disabling controllers in v1 and make them always available in v2.

cgroup v2 currently supports the following mount options.

  nsdelegate
	Consider cgroup namespaces as delegation boundaries.  This
	option is system wide and can only be set on mount or modified
	through remount from the init namespace.  The mount option is
	ignored on non-init namespace mounts.  Please refer to the
	Delegation section for details.

  favordynmods
        Reduce the latencies of dynamic cgroup modifications such as
        task migrations and controller on/offs at the cost of making
        hot path operations such as forks and exits more expensive.
        The static usage pattern of creating a cgroup, enabling
        controllers, and then seeding it with CLONE_INTO_CGROUP is
        not affected by this option.

  memory_localevents
        Only populate memory.events with data for the current cgroup,
        and not any subtrees. This is legacy behaviour, the default
        behaviour without this option is to include subtree counts.
        This option is system wide and can only be set on mount or
        modified through remount from the init namespace. The mount
        option is ignored on non-init namespace mounts.

  memory_recursiveprot
        Recursively apply memory.min and memory.low protection to
        entire subtrees, without requiring explicit downward
        propagation into leaf cgroups.  This allows protecting entire
        subtrees from one another, while retaining free competition
        within those subtrees.  This should have been the default
        behavior but is a mount-option to avoid regressing setups
        relying on the original semantics (e.g. specifying bogusly
        high 'bypass' protection values at higher tree levels).

  memory_hugetlb_accounting
        Count HugeTLB memory usage towards the cgroup's overall
        memory usage for the memory controller (for the purpose of
        statistics reporting and memory protetion). This is a new
        behavior that could regress existing setups, so it must be
        explicitly opted in with this mount option.

        A few caveats to keep in mind:

        * There is no HugeTLB pool management involved in the memory
          controller. The pre-allocated pool does not belong to anyone.
          Specifically, when a new HugeTLB folio is allocated to
          the pool, it is not accounted for from the perspective of the
          memory controller. It is only charged to a cgroup when it is
          actually used (for e.g at page fault time). Host memory
          overcommit management has to consider this when configuring
          hard limits. In general, HugeTLB pool management should be
          done via other mechanisms (such as the HugeTLB controller).
        * Failure to charge a HugeTLB folio to the memory controller
          results in SIGBUS. This could happen even if the HugeTLB pool
          still has pages available (but the cgroup limit is hit and
          reclaim attempt fails).
        * Charging HugeTLB memory towards the memory controller affects
          memory protection and reclaim dynamics. Any userspace tuning
          (of low, min limits for e.g) needs to take this into account.
        * HugeTLB pages utilized while this option is not selected
          will not be tracked by the memory controller (even if cgroup
          v2 is remounted later on).

  pids_localevents
        The option restores v1-like behavior of pids.events:max, that is only
        local (inside cgroup proper) fork failures are counted. Without this
        option pids.events.max represents any pids.max enforcemnt across
        cgroup's subtree.

Organizing Processes and Threads
--------------------------------

Processes
~~~~~~~~~

Initially, only the root cgroup exists to which all processes belong.
A child cgroup can be created by creating a sub-directory::

  # mkdir $CGROUP_NAME

A given cgroup may have multiple child cgroups forming a tree
structure.  Each cgroup has a read-writable interface file
"cgroup.procs".  When read, it lists the PIDs of all processes which
belong to the cgroup one-per-line.  The PIDs are not ordered and the
same PID may show up more than once if the process got moved to
another cgroup and then back or the PID got recycled while reading.

A process can be migrated into a cgroup by writing its PID to the
target cgroup's "cgroup.procs" file.  Only one process can be migrated
on a single write(2) call.  If a process is composed of multiple
threads, writing the PID of any thread migrates all threads of the
process.

When a process forks a child process, the new process is born into the
cgroup that the forking process belongs to at the time of the
operation.  After exit, a process stays associated with the cgroup
that it belonged to at the time of exit until it's reaped; however, a
zombie process does not appear in "cgroup.procs" and thus can't be
moved to another cgroup.

A cgroup which doesn't have any children or live processes can be
destroyed by removing the directory.  Note that a cgroup which doesn't
have any children and is associated only with zombie processes is
considered empty and can be removed::

  # rmdir $CGROUP_NAME

"/proc/$PID/cgroup" lists a process's cgroup membership.  If legacy
cgroup is in use in the system, this file may contain multiple lines,
one for each hierarchy.  The entry for cgroup v2 is always in the
format "0::$PATH"::

  # cat /proc/842/cgroup
  ...
  0::/test-cgroup/test-cgroup-nested

If the process becomes a zombie and the cgroup it was associated with
is removed subsequently, " (deleted)" is appended to the path::

  # cat /proc/842/cgroup
  ...
  0::/test-cgroup/test-cgroup-nested (deleted)

Threads
~~~~~~~

cgroup v2 supports thread granularity for a subset of controllers to
support use cases requiring hierarchical resource distribution across
the threads of a group of processes.  By default, all threads of a
process belong to the same cgroup, which also serves as the resource
domain to host resource consumptions which are not specific to a
process or thread.  The thread mode allows threads to be spread across
a subtree while still maintaining the common resource domain for them.

Controllers which support thread mode are called threaded controllers.
The ones which don't are called domain controllers.

Marking a cgroup threaded makes it join the resource domain of its
parent as a threaded cgroup.  The parent may be another threaded
cgroup whose resource domain is further up in the hierarchy.  The root
of a threaded subtree, that is, the nearest ancestor which is not
threaded, is called threaded domain or thread root interchangeably and
serves as the resource domain for the entire subtree.

Inside a threaded subtree, threads of a process can be put in
different cgroups and are not subject to the no internal process
constraint - threaded controllers can be enabled on non-leaf cgroups
whether they have threads in them or not.

As the threaded domain cgroup hosts all the domain resource
consumptions of the subtree, it is considered to have internal
resource consumptions whether there are processes in it or not and
can't have populated child cgroups which aren't threaded.  Because the
root cgroup is not subject to no internal process constraint, it can
serve both as a threaded domain and a parent to domain cgroups.

The current operation mode or type of the cgroup is shown in the
"cgroup.type" file which indicates whether the cgroup is a normal
domain, a domain which is serving as the domain of a threaded subtree,
or a threaded cgroup.

On creation, a cgroup is always a domain cgroup and can be made
threaded by writing "threaded" to the "cgroup.type" file.  The
operation is single direction::

  # echo threaded > cgroup.type

Once threaded, the cgroup can't be made a domain again.  To enable the
thread mode, the following conditions must be met.

- As the cgroup will join the parent's resource domain.  The parent
  must either be a valid (threaded) domain or a threaded cgroup.

- When the parent is an unthreaded domain, it must not have any domain
  controllers enabled or populated domain children.  The root is
  exempt from this requirement.

Topology-wise, a cgroup can be in an invalid state.  Please consider
the following topology::

  A (threaded domain) - B (threaded) - C (domain, just created)

C is created as a domain but isn't connected to a parent which can
host child domains.  C can't be used until it is turned into a
threaded cgroup.  "cgroup.type" file will report "domain (invalid)" in
these cases.  Operations which fail due to invalid topology use
EOPNOTSUPP as the errno.

A domain cgroup is turned into a threaded domain when one of its child
cgroup becomes threaded or threaded controllers are enabled in the
"cgroup.subtree_control" file while there are processes in the cgroup.
A threaded domain reverts to a normal domain when the conditions
clear.

When read, "cgroup.threads" contains the list of the thread IDs of all
threads in the cgroup.  Except that the operations are per-thread
instead of per-process, "cgroup.threads" has the same format and
behaves the same way as "cgroup.procs".  While "cgroup.threads" can be
written to in any cgroup, as it can only move threads inside the same
threaded domain, its operations are confined inside each threaded
subtree.

The threaded domain cgroup serves as the resource domain for the whole
subtree, and, while the threads can be scattered across the subtree,
all the processes are considered to be in the threaded domain cgroup.
"cgroup.procs" in a threaded domain cgroup contains the PIDs of all
processes in the subtree and is not readable in the subtree proper.
However, "cgroup.procs" can be written to from anywhere in the subtree
to migrate all threads of the matching process to the cgroup.

Only threaded controllers can be enabled in a threaded subtree.  When
a threaded controller is enabled inside a threaded subtree, it only
accounts for and controls resource consumptions associated with the
threads in the cgroup and its descendants.  All consumptions which
aren't tied to a specific thread belong to the threaded domain cgroup.

Because a threaded subtree is exempt from no internal process
constraint, a threaded controller must be able to handle competition
between threads in a non-leaf cgroup and its child cgroups.  Each
threaded controller defines how such competitions are handled.

Currently, the following controllers are threaded and can be enabled
in a threaded cgroup::

- cpu
- cpuset
- perf_event
- pids

[Un]populated Notification
--------------------------

Each non-root cgroup has a "cgroup.events" file which contains
"populated" field indicating whether the cgroup's sub-hierarchy has
live processes in it.  Its value is 0 if there is no live process in
the cgroup and its descendants; otherwise, 1.  poll and [id]notify
events are triggered when the value changes.  This can be used, for
example, to start a clean-up operation after all processes of a given
sub-hierarchy have exited.  The populated state updates and
notifications are recursive.  Consider the following sub-hierarchy
where the numbers in the parentheses represent the numbers of processes
in each cgroup::

  A(4) - B(0) - C(1)
              \ D(0)

A, B and C's "populated" fields would be 1 while D's 0.  After the one
process in C exits, B and C's "populated" fields would flip to "0" and
file modified events will be generated on the "cgroup.events" files of
both cgroups.

Controlling Controllers
-----------------------

Enabling and Disabling
~~~~~~~~~~~~~~~~~~~~~~

Each cgroup has a "cgroup.controllers" file which lists all
controllers available for the cgroup to enable::

  # cat cgroup.controllers
  cpu io memory

No controller is enabled by default.  Controllers can be enabled and
disabled by writing to the "cgroup.subtree_control" file::

  # echo "+cpu +memory -io" > cgroup.subtree_control

Only controllers which are listed in "cgroup.controllers" can be
enabled.  When multiple operations are specified as above, either they
all succeed or fail.  If multiple operations on the same controller
are specified, the last one is effective.

Enabling a controller in a cgroup indicates that the distribution of
the target resource across its immediate children will be controlled.
Consider the following sub-hierarchy.  The enabled controllers are
listed in parentheses::

  A(cpu,memory) - B(memory) - C()
                            \ D()

As A has "cpu" and "memory" enabled, A will control the distribution
of CPU cycles and memory to its children, in this case, B.  As B has
"memory" enabled but not "CPU", C and D will compete freely on CPU
cycles but their division of memory available to B will be controlled.

As a controller regulates the distribution of the target resource to
the cgroup's children, enabling it creates the controller's interface
files in the child cgroups.  In the above example, enabling "cpu" on B
would create the "cpu." prefixed controller interface files in C and
D.  Likewise, disabling "memory" from B would remove the "memory."
prefixed controller interface files from C and D.  This means that the
controller interface files - anything which doesn't start with
"cgroup." are owned by the parent rather than the cgroup itself.

Top-down Constraint
~~~~~~~~~~~~~~~~~~~

Resources are distributed top-down and a cgroup can further distribute
a resource only if the resource has been distributed to it from the
parent.  This means that all non-root "cgroup.subtree_control" files
can only contain controllers which are enabled in the parent's
"cgroup.subtree_control" file.  A controller can be enabled only if
the parent has the controller enabled and a controller can't be
disabled if one or more children have it enabled.

No Internal Process Constraint
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Non-root cgroups can distribute domain resources to their children
only when they don't have any processes of their own.  In other words,
only domain cgroups which don't contain any processes can have domain
controllers enabled in their "cgroup.subtree_control" files.

This guarantees that, when a domain controller is looking at the part
of the hierarchy which has it enabled, processes are always only on
the leaves.  This rules out situations where child cgroups compete
against internal processes of the parent.

The root cgroup is exempt from this restriction.  Root contains
processes and anonymous resource consumption which can't be associated
with any other cgroups and requires special treatment from most
controllers.  How resource consumption in the root cgroup is governed
is up to each controller (for more information on this topic please
refer to the Non-normative information section in the Controllers
chapter).

Note that the restriction doesn't get in the way if there is no
enabled controller in the cgroup's "cgroup.subtree_control".  This is
important as otherwise it wouldn't be possible to create children of a
populated cgroup.  To control resource distribution of a cgroup, the
cgroup must create children and transfer all its processes to the
children before enabling controllers in its "cgroup.subtree_control"
file.

Delegation
----------

Model of Delegation
~~~~~~~~~~~~~~~~~~~

A cgroup can be delegated in two ways.  First, to a less privileged
user by granting write access of the directory and its "cgroup.procs",
"cgroup.threads" and "cgroup.subtree_control" files to the user.
Second, if the "nsdelegate" mount option is set, automatically to a
cgroup namespace on namespace creation.

Because the resource control interface files in a given directory
control the distribution of the parent's resources, the delegatee
shouldn't be allowed to write to them.  For the first method, this is
achieved by not granting access to these files.  For the second, files
outside the namespace should be hidden from the delegatee by the means
of at least mount namespacing, and the kernel rejects writes to all
files on a namespace root from inside the cgroup namespace, except for
those files listed in "/sys/kernel/cgroup/delegate" (including
"cgroup.procs", "cgroup.threads", "cgroup.subtree_control", etc.).

The end results are equivalent for both delegation types.  Once
delegated, the user can build sub-hierarchy under the directory,
organize processes inside it as it sees fit and further distribute the
resources it received from the parent.  The limits and other settings
of all resource controllers are hierarchical and regardless of what
happens in the delegated sub-hierarchy, nothing can escape the
resource restrictions imposed by the parent.

Currently, cgroup doesn't impose any restrictions on the number of
cgroups in or nesting depth of a delegated sub-hierarchy; however,
this may be limited explicitly in the future.

Delegation Containment
~~~~~~~~~~~~~~~~~~~~~~

A delegated sub-hierarchy is contained in the sense that processes
can't be moved into or out of the sub-hierarchy by the delegatee.

For delegations to a less privileged user, this is achieved by
requiring the following conditions for a process with a non-root euid
to migrate a target process into a cgroup by writing its PID to the
"cgroup.procs" file.

- The writer must have write access to the "cgroup.procs" file.

- The writer must have write access to the "cgroup.procs" file of the
  common ancestor of the source and destination cgroups.

The above two constraints ensure that while a delegatee may migrate
processes around freely in the delegated sub-hierarchy it can't pull
in from or push out to outside the sub-hierarchy.

For an example, let's assume cgroups C0 and C1 have been delegated to
user U0 who created C00, C01 under C0 and C10 under C1 as follows and
all processes under C0 and C1 belong to U0::

  ~~~~~~~~~~~~~ - C0 - C00
  ~ cgroup    ~      \ C01
  ~ hierarchy ~
  ~~~~~~~~~~~~~ - C1 - C10

Let's also say U0 wants to write the PID of a process which is
currently in C10 into "C00/cgroup.procs".  U0 has write access to the
file; however, the common ancestor of the source cgroup C10 and the
destination cgroup C00 is above the points of delegation and U0 would
not have write access to its "cgroup.procs" files and thus the write
will be denied with -EACCES.

For delegations to namespaces, containment is achieved by requiring
that both the source and destination cgroups are reachable from the
namespace of the process which is attempting the migration.  If either
is not reachable, the migration is rejected with -ENOENT.

Guidelines
----------

Organize Once and Control
~~~~~~~~~~~~~~~~~~~~~~~~~

Migrating a process across cgroups is a relatively expensive operation
and stateful resources such as memory are not moved together with the
process.  This is an explicit design decision as there often exist
inherent trade-offs between migration and various hot paths in terms
of synchronization cost.

As such, migrating processes across cgroups frequently as a means to
apply different resource restrictions is discouraged.  A workload
should be assigned to a cgroup according to the system's logical and
resource structure once on start-up.  Dynamic adjustments to resource
distribution can be made by changing controller configuration through
the interface files.

Avoid Name Collisions
~~~~~~~~~~~~~~~~~~~~~

Interface files for a cgroup and its children cgroups occupy the same
directory and it is possible to create children cgroups which collide
with interface files.

All cgroup core interface files are prefixed with "cgroup." and each
controller's interface files are prefixed with the controller name and
a dot.  A controller's name is composed of lower case alphabets and
'_'s but never begins with an '_' so it can be used as the prefix
character for collision avoidance.  Also, interface file names won't
start or end with terms which are often used in categorizing workloads
such as job, service, slice, unit or workload.

cgroup doesn't do anything to prevent name collisions and it's the
user's responsibility to avoid them.

Resource Distribution Models
============================

cgroup controllers implement several resource distribution schemes
depending on the resource type and expected use cases.  This section
describes major schemes in use along with their expected behaviors.

Weights
-------

A parent's resource is distributed by adding up the weights of all
active children and giving each the fraction matching the ratio of its
weight against the sum.  As only children which can make use of the
resource at the moment participate in the distribution, this is
work-conserving.  Due to the dynamic nature, this model is usually
used for stateless resources.

All weights are in the range [1, 10000] with the default at 100.  This
allows symmetric multiplicative biases in both directions at fine
enough granularity while staying in the intuitive range.

As long as the weight is in range, all configuration combinations are
valid and there is no reason to reject configuration changes or
process migrations.

"cpu.weight" proportionally distributes CPU cycles to active children
and is an example of this type.

.. _cgroupv2-limits-distributor:

Limits
------

A child can only consume up to the configured amount of the resource.
Limits can be over-committed - the sum of the limits of children can
exceed the amount of resource available to the parent.

Limits are in the range [0, max] and defaults to "max", which is noop.

As limits can be over-committed, all configuration combinations are
valid and there is no reason to reject configuration changes or
process migrations.

"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
on an IO device and is an example of this type.

.. _cgroupv2-protections-distributor:

Protections
-----------

A cgroup is protected up to the configured amount of the resource
as long as the usages of all its ancestors are under their
protected levels.  Protections can be hard guarantees or best effort
soft boundaries.  Protections can also be over-committed in which case
only up to the amount available to the parent is protected among
children.

Protections are in the range [0, max] and defaults to 0, which is
noop.

As protections can be over-committed, all configuration combinations
are valid and there is no reason to reject configuration changes or
process migrations.

"memory.low" implements best-effort memory protection and is an
example of this type.

Allocations
-----------

A cgroup is exclusively allocated a certain amount of a finite
resource.  Allocations can't be over-committed - the sum of the
allocations of children can not exceed the amount of resource
available to the parent.

Allocations are in the range [0, max] and defaults to 0, which is no
resource.

As allocations can't be over-committed, some configuration
combinations are invalid and should be rejected.  Also, if the
resource is mandatory for execution of processes, process migrations
may be rejected.

"cpu.rt.max" hard-allocates realtime slices and is an example of this
type.

Interface Files
===============

Format
------

All interface files should be in one of the following formats whenever
possible::

  New-line separated values
  (when only one value can be written at once)

	VAL0\n
	VAL1\n
	...

  Space separated values
  (when read-only or multiple values can be written at once)

	VAL0 VAL1 ...\n

  Flat keyed

	KEY0 VAL0\n
	KEY1 VAL1\n
	...

  Nested keyed

	KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
	KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
	...

For a writable file, the format for writing should generally match
reading; however, controllers may allow omitting later fields or
implement restricted shortcuts for most common use cases.

For both flat and nested keyed files, only the values for a single key
can be written at a time.  For nested keyed files, the sub key pairs
may be specified in any order and not all pairs have to be specified.

Conventions
-----------

- Settings for a single feature should be contained in a single file.

- The root cgroup should be exempt from resource control and thus
  shouldn't have resource control interface files.

- The default time unit is microseconds.  If a different unit is ever
  used, an explicit unit suffix must be present.

- A parts-per quantity should use a percentage decimal with at least
  two digit fractional part - e.g. 13.40.

- If a controller implements weight based resource distribution, its
  interface file should be named "weight" and have the range [1,
  10000] with 100 as the default.  The values are chosen to allow
  enough and symmetric bias in both directions while keeping it
  intuitive (the default is 100%).

- If a controller implements an absolute resource guarantee and/or
  limit, the interface files should be named "min" and "max"
  respectively.  If a controller implements best effort resource
  guarantee and/or limit, the interface files should be named "low"
  and "high" respectively.

  In the above four control files, the special token "max" should be
  used to represent upward infinity for both reading and writing.

- If a setting has a configurable default value and keyed specific
  overrides, the default entry should be keyed with "default" and
  appear as the first entry in the file.

  The default value can be updated by writing either "default $VAL" or
  "$VAL".

  When writing to update a specific override, "default" can be used as
  the value to indicate removal of the override.  Override entries
  with "default" as the value must not appear when read.

  For example, a setting which is keyed by major:minor device numbers
  with integer values may look like the following::

    # cat cgroup-example-interface-file
    default 150
    8:0 300

  The default value can be updated by::

    # echo 125 > cgroup-example-interface-file

  or::

    # echo "default 125" > cgroup-example-interface-file

  An override can be set by::

    # echo "8:16 170" > cgroup-example-interface-file

  and cleared by::

    # echo "8:0 default" > cgroup-example-interface-file
    # cat cgroup-example-interface-file
    default 125
    8:16 170

- For events which are not very high frequency, an interface file
  "events" should be created which lists event key value pairs.
  Whenever a notifiable event happens, file modified event should be
  generated on the file.

Core Interface Files
--------------------

All cgroup core files are prefixed with "cgroup."

  cgroup.type
	A read-write single value file which exists on non-root
	cgroups.

	When read, it indicates the current type of the cgroup, which
	can be one of the following values.

	- "domain" : A normal valid domain cgroup.

	- "domain threaded" : A threaded domain cgroup which is
          serving as the root of a threaded subtree.

	- "domain invalid" : A cgroup which is in an invalid state.
	  It can't be populated or have controllers enabled.  It may
	  be allowed to become a threaded cgroup.

	- "threaded" : A threaded cgroup which is a member of a
          threaded subtree.

	A cgroup can be turned into a threaded cgroup by writing
	"threaded" to this file.

  cgroup.procs
	A read-write new-line separated values file which exists on
	all cgroups.

	When read, it lists the PIDs of all processes which belong to
	the cgroup one-per-line.  The PIDs are not ordered and the
	same PID may show up more than once if the process got moved
	to another cgroup and then back or the PID got recycled while
	reading.

	A PID can be written to migrate the process associated with
	the PID to the cgroup.  The writer should match all of the
	following conditions.

	- It must have write access to the "cgroup.procs" file.

	- It must have write access to the "cgroup.procs" file of the
	  common ancestor of the source and destination cgroups.

	When delegating a sub-hierarchy, write access to this file
	should be granted along with the containing directory.

	In a threaded cgroup, reading this file fails with EOPNOTSUPP
	as all the processes belong to the thread root.  Writing is
	supported and moves every thread of the process to the cgroup.

  cgroup.threads
	A read-write new-line separated values file which exists on
	all cgroups.

	When read, it lists the TIDs of all threads which belong to
	the cgroup one-per-line.  The TIDs are not ordered and the
	same TID may show up more than once if the thread got moved to
	another cgroup and then back or the TID got recycled while
	reading.

	A TID can be written to migrate the thread associated with the
	TID to the cgroup.  The writer should match all of the
	following conditions.

	- It must have write access to the "cgroup.threads" file.

	- The cgroup that the thread is currently in must be in the
          same resource domain as the destination cgroup.

	- It must have write access to the "cgroup.procs" file of the
	  common ancestor of the source and destination cgroups.

	When delegating a sub-hierarchy, write access to this file
	should be granted along with the containing directory.

  cgroup.controllers
	A read-only space separated values file which exists on all
	cgroups.

	It shows space separated list of all controllers available to
	the cgroup.  The controllers are not ordered.

  cgroup.subtree_control
	A read-write space separated values file which exists on all
	cgroups.  Starts out empty.

	When read, it shows space separated list of the controllers
	which are enabled to control resource distribution from the
	cgroup to its children.

	Space separated list of controllers prefixed with '+' or '-'
	can be written to enable or disable controllers.  A controller
	name prefixed with '+' enables the controller and '-'
	disables.  If a controller appears more than once on the list,
	the last one is effective.  When multiple enable and disable
	operations are specified, either all succeed or all fail.

  cgroup.events
	A read-only flat-keyed file which exists on non-root cgroups.
	The following entries are defined.  Unless specified
	otherwise, a value change in this file generates a file
	modified event.

	  populated
		1 if the cgroup or its descendants contains any live
		processes; otherwise, 0.
	  frozen
		1 if the cgroup is frozen; otherwise, 0.

  cgroup.max.descendants
	A read-write single value files.  The default is "max".

	Maximum allowed number of descent cgroups.
	If the actual number of descendants is equal or larger,
	an attempt to create a new cgroup in the hierarchy will fail.

  cgroup.max.depth
	A read-write single value files.  The default is "max".

	Maximum allowed descent depth below the current cgroup.
	If the actual descent depth is equal or larger,
	an attempt to create a new child cgroup will fail.

  cgroup.stat
	A read-only flat-keyed file with the following entries:

	  nr_descendants
		Total number of visible descendant cgroups.

	  nr_dying_descendants
		Total number of dying descendant cgroups. A cgroup becomes
		dying after being deleted by a user. The cgroup will remain
		in dying state for some time undefined time (which can depend
		on system load) before being completely destroyed.

		A process can't enter a dying cgroup under any circumstances,
		a dying cgroup can't revive.

		A dying cgroup can consume system resources not exceeding
		limits, which were active at the moment of cgroup deletion.

	  nr_subsys_<cgroup_subsys>
		Total number of live cgroup subsystems (e.g memory
		cgroup) at and beneath the current cgroup.

	  nr_dying_subsys_<cgroup_subsys>
		Total number of dying cgroup subsystems (e.g. memory
		cgroup) at and beneath the current cgroup.

  cgroup.freeze
	A read-write single value file which exists on non-root cgroups.
	Allowed values are "0" and "1". The default is "0".

	Writing "1" to the file causes freezing of the cgroup and all
	descendant cgroups. This means that all belonging processes will
	be stopped and will not run until the cgroup will be explicitly
	unfrozen. Freezing of the cgroup may take some time; when this action
	is completed, the "frozen" value in the cgroup.events control file
	will be updated to "1" and the corresponding notification will be
	issued.

	A cgroup can be frozen either by its own settings, or by settings
	of any ancestor cgroups. If any of ancestor cgroups is frozen, the
	cgroup will remain frozen.

	Processes in the frozen cgroup can be killed by a fatal signal.
	They also can enter and leave a frozen cgroup: either by an explicit
	move by a user, or if freezing of the cgroup races with fork().
	If a process is moved to a frozen cgroup, it stops. If a process is
	moved out of a frozen cgroup, it becomes running.

	Frozen status of a cgroup doesn't affect any cgroup tree operations:
	it's possible to delete a frozen (and empty) cgroup, as well as
	create new sub-cgroups.

  cgroup.kill
	A write-only single value file which exists in non-root cgroups.
	The only allowed value is "1".

	Writing "1" to the file causes the cgroup and all descendant cgroups to
	be killed. This means that all processes located in the affected cgroup
	tree will be killed via SIGKILL.

	Killing a cgroup tree will deal with concurrent forks appropriately and
	is protected against migrations.

	In a threaded cgroup, writing this file fails with EOPNOTSUPP as
	killing cgroups is a process directed operation, i.e. it affects
	the whole thread-group.

  cgroup.pressure
	A read-write single value file that allowed values are "0" and "1".
	The default is "1".

	Writing "0" to the file will disable the cgroup PSI accounting.
	Writing "1" to the file will re-enable the cgroup PSI accounting.

	This control attribute is not hierarchical, so disable or enable PSI
	accounting in a cgroup does not affect PSI accounting in descendants
	and doesn't need pass enablement via ancestors from root.

	The reason this control attribute exists is that PSI accounts stalls for
	each cgroup separately and aggregates it at each level of the hierarchy.
	This may cause non-negligible overhead for some workloads when under
	deep level of the hierarchy, in which case this control attribute can
	be used to disable PSI accounting in the non-leaf cgroups.

  irq.pressure
	A read-write nested-keyed file.

	Shows pressure stall information for IRQ/SOFTIRQ. See
	:ref:`Documentation/accounting/psi.rst <psi>` for details.

Controllers
===========

.. _cgroup-v2-cpu:

CPU
---

The "cpu" controllers regulates distribution of CPU cycles.  This
controller implements weight and absolute bandwidth limit models for
normal scheduling policy and absolute bandwidth allocation model for
realtime scheduling policy.

In all the above models, cycles distribution is defined only on a temporal
base and it does not account for the frequency at which tasks are executed.
The (optional) utilization clamping support allows to hint the schedutil
cpufreq governor about the minimum desired frequency which should always be
provided by a CPU, as well as the maximum desired frequency, which should not
be exceeded by a CPU.

WARNING: cgroup2 doesn't yet support control of realtime processes. For
a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group
scheduling of realtime processes, the cpu controller can only be enabled
when all RT processes are in the root cgroup.  This limitation does
not apply if CONFIG_RT_GROUP_SCHED is disabled.  Be aware that system
management software may already have placed RT processes into nonroot
cgroups during the system boot process, and these processes may need
to be moved to the root cgroup before the cpu controller can be enabled
with a CONFIG_RT_GROUP_SCHED enabled kernel.

CPU Interface Files
~~~~~~~~~~~~~~~~~~~

All time durations are in microseconds.

  cpu.stat
	A read-only flat-keyed file.
	This file exists whether the controller is enabled or not.

	It always reports the following three stats:

	- usage_usec
	- user_usec
	- system_usec

	and the following five when the controller is enabled:

	- nr_periods
	- nr_throttled
	- throttled_usec
	- nr_bursts
	- burst_usec

  cpu.weight
	A read-write single value file which exists on non-root
	cgroups.  The default is "100".

	For non idle groups (cpu.idle = 0), the weight is in the
	range [1, 10000].

	If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1),
	then the weight will show as a 0.

  cpu.weight.nice
	A read-write single value file which exists on non-root
	cgroups.  The default is "0".

	The nice value is in the range [-20, 19].

	This interface file is an alternative interface for
	"cpu.weight" and allows reading and setting weight using the
	same values used by nice(2).  Because the range is smaller and
	granularity is coarser for the nice values, the read value is
	the closest approximation of the current weight.

  cpu.max
	A read-write two value file which exists on non-root cgroups.
	The default is "max 100000".

	The maximum bandwidth limit.  It's in the following format::

	  $MAX $PERIOD

	which indicates that the group may consume up to $MAX in each
	$PERIOD duration.  "max" for $MAX indicates no limit.  If only
	one number is written, $MAX is updated.

  cpu.max.burst
	A read-write single value file which exists on non-root
	cgroups.  The default is "0".

	The burst in the range [0, $MAX].

  cpu.pressure
	A read-write nested-keyed file.

	Shows pressure stall information for CPU. See
	:ref:`Documentation/accounting/psi.rst <psi>` for details.

  cpu.uclamp.min
        A read-write single value file which exists on non-root cgroups.
        The default is "0", i.e. no utilization boosting.

        The requested minimum utilization (protection) as a percentage
        rational number, e.g. 12.34 for 12.34%.

        This interface allows reading and setting minimum utilization clamp
        values similar to the sched_setattr(2). This minimum utilization
        value is used to clamp the task specific minimum utilization clamp.

        The requested minimum utilization (protection) is always capped by
        the current value for the maximum utilization (limit), i.e.
        `cpu.uclamp.max`.

  cpu.uclamp.max
        A read-write single value file which exists on non-root cgroups.
        The default is "max". i.e. no utilization capping

        The requested maximum utilization (limit) as a percentage rational
        number, e.g. 98.76 for 98.76%.

        This interface allows reading and setting maximum utilization clamp
        values similar to the sched_setattr(2). This maximum utilization
        value is used to clamp the task specific maximum utilization clamp.

  cpu.idle
	A read-write single value file which exists on non-root cgroups.
	The default is 0.

	This is the cgroup analog of the per-task SCHED_IDLE sched policy.
	Setting this value to a 1 will make the scheduling policy of the
	cgroup SCHED_IDLE. The threads inside the cgroup will retain their
	own relative priorities, but the cgroup itself will be treated as
	very low priority relative to its peers.

Memory
------

The "memory" controller regulates distribution of memory.  Memory is
stateful and implements both limit and protection models.  Due to the
intertwining between memory usage and reclaim pressure and the
stateful nature of memory, the distribution model is relatively
complex.

While not completely water-tight, all major memory usages by a given
cgroup are tracked so that the total memory consumption can be
accounted and controlled to a reasonable extent.  Currently, the
following types of memory usages are tracked.

- Userland memory - page cache and anonymous memory.

- Kernel data structures such as dentries and inodes.

- TCP socket buffers.

The above list may expand in the future for better coverage.

Memory Interface Files
~~~~~~~~~~~~~~~~~~~~~~

All memory amounts are in bytes.  If a value which is not aligned to
PAGE_SIZE is written, the value may be rounded up to the closest
PAGE_SIZE multiple when read back.

  memory.current
	A read-only single value file which exists on non-root
	cgroups.

	The total amount of memory currently being used by the cgroup
	and its descendants.

  memory.min
	A read-write single value file which exists on non-root
	cgroups.  The default is "0".

	Hard memory protection.  If the memory usage of a cgroup
	is within its effective min boundary, the cgroup's memory
	won't be reclaimed under any conditions. If there is no
	unprotected reclaimable memory available, OOM killer
	is invoked. Above the effective min boundary (or
	effective low boundary if it is higher), pages are reclaimed
	proportionally to the overage, reducing reclaim pressure for
	smaller overages.

	Effective min boundary is limited by memory.min values of
	all ancestor cgroups. If there is memory.min overcommitment
	(child cgroup or cgroups are requiring more protected memory
	than parent will allow), then each child cgroup will get
	the part of parent's protection proportional to its
	actual memory usage below memory.min.

	Putting more memory than generally available under this
	protection is discouraged and may lead to constant OOMs.

	If a memory cgroup is not populated with processes,
	its memory.min is ignored.

  memory.low
	A read-write single value file which exists on non-root
	cgroups.  The default is "0".

	Best-effort memory protection.  If the memory usage of a
	cgroup is within its effective low boundary, the cgroup's
	memory won't be reclaimed unless there is no reclaimable
	memory available in unprotected cgroups.
	Above the effective low	boundary (or
	effective min boundary if it is higher), pages are reclaimed
	proportionally to the overage, reducing reclaim pressure for
	smaller overages.

	Effective low boundary is limited by memory.low values of
	all ancestor cgroups. If there is memory.low overcommitment
	(child cgroup or cgroups are requiring more protected memory
	than parent will allow), then each child cgroup will get
	the part of parent's protection proportional to its
	actual memory usage below memory.low.

	Putting more memory than generally available under this
	protection is discouraged.

  memory.high
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Memory usage throttle limit.  If a cgroup's usage goes
	over the high boundary, the processes of the cgroup are
	throttled and put under heavy reclaim pressure.

	Going over the high limit never invokes the OOM killer and
	under extreme conditions the limit may be breached. The high
	limit should be used in scenarios where an external process
	monitors the limited cgroup to alleviate heavy reclaim
	pressure.

  memory.max
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Memory usage hard limit.  This is the main mechanism to limit
	memory usage of a cgroup.  If a cgroup's memory usage reaches
	this limit and can't be reduced, the OOM killer is invoked in
	the cgroup. Under certain circumstances, the usage may go
	over the limit temporarily.

	In default configuration regular 0-order allocations always
	succeed unless OOM killer chooses current task as a victim.

	Some kinds of allocations don't invoke the OOM killer.
	Caller could retry them differently, return into userspace
	as -ENOMEM or silently ignore in cases like disk readahead.

  memory.reclaim
	A write-only nested-keyed file which exists for all cgroups.

	This is a simple interface to trigger memory reclaim in the
	target cgroup.

	Example::

	  echo "1G" > memory.reclaim

	Please note that the kernel can over or under reclaim from
	the target cgroup. If less bytes are reclaimed than the
	specified amount, -EAGAIN is returned.

	Please note that the proactive reclaim (triggered by this
	interface) is not meant to indicate memory pressure on the
	memory cgroup. Therefore socket memory balancing triggered by
	the memory reclaim normally is not exercised in this case.
	This means that the networking layer will not adapt based on
	reclaim induced by memory.reclaim.

The following nested keys are defined.

	  ==========            ================================
	  swappiness            Swappiness value to reclaim with
	  ==========            ================================

	Specifying a swappiness value instructs the kernel to perform
	the reclaim with that swappiness value. Note that this has the
	same semantics as vm.swappiness applied to memcg reclaim with
	all the existing limitations and potential future extensions.

  memory.peak
	A read-write single value file which exists on non-root cgroups.

	The max memory usage recorded for the cgroup and its descendants since
	either the creation of the cgroup or the most recent reset for that FD.

	A write of any non-empty string to this file resets it to the
	current memory usage for subsequent reads through the same
	file descriptor.

  memory.oom.group
	A read-write single value file which exists on non-root
	cgroups.  The default value is "0".

	Determines whether the cgroup should be treated as
	an indivisible workload by the OOM killer. If set,
	all tasks belonging to the cgroup or to its descendants
	(if the memory cgroup is not a leaf cgroup) are killed
	together or not at all. This can be used to avoid
	partial kills to guarantee workload integrity.

	Tasks with the OOM protection (oom_score_adj set to -1000)
	are treated as an exception and are never killed.

	If the OOM killer is invoked in a cgroup, it's not going
	to kill any tasks outside of this cgroup, regardless
	memory.oom.group values of ancestor cgroups.

  memory.events
	A read-only flat-keyed file which exists on non-root cgroups.
	The following entries are defined.  Unless specified
	otherwise, a value change in this file generates a file
	modified event.

	Note that all fields in this file are hierarchical and the
	file modified event can be generated due to an event down the
	hierarchy. For the local events at the cgroup level see
	memory.events.local.

	  low
		The number of times the cgroup is reclaimed due to
		high memory pressure even though its usage is under
		the low boundary.  This usually indicates that the low
		boundary is over-committed.

	  high
		The number of times processes of the cgroup are
		throttled and routed to perform direct memory reclaim
		because the high memory boundary was exceeded.  For a
		cgroup whose memory usage is capped by the high limit
		rather than global memory pressure, this event's
		occurrences are expected.

	  max
		The number of times the cgroup's memory usage was
		about to go over the max boundary.  If direct reclaim
		fails to bring it down, the cgroup goes to OOM state.

	  oom
		The number of time the cgroup's memory usage was
		reached the limit and allocation was about to fail.

		This event is not raised if the OOM killer is not
		considered as an option, e.g. for failed high-order
		allocations or if caller asked to not retry attempts.

	  oom_kill
		The number of processes belonging to this cgroup
		killed by any kind of OOM killer.

          oom_group_kill
                The number of times a group OOM has occurred.

  memory.events.local
	Similar to memory.events but the fields in the file are local
	to the cgroup i.e. not hierarchical. The file modified event
	generated on this file reflects only the local events.

  memory.stat
	A read-only flat-keyed file which exists on non-root cgroups.

	This breaks down the cgroup's memory footprint into different
	types of memory, type-specific details, and other information
	on the state and past events of the memory management system.

	All memory amounts are in bytes.

	The entries are ordered to be human readable, and new entries
	can show up in the middle. Don't rely on items remaining in a
	fixed position; use the keys to look up specific values!

	If the entry has no per-node counter (or not show in the
	memory.numa_stat). We use 'npn' (non-per-node) as the tag
	to indicate that it will not show in the memory.numa_stat.

	  anon
		Amount of memory used in anonymous mappings such as
		brk(), sbrk(), and mmap(MAP_ANONYMOUS)

	  file
		Amount of memory used to cache filesystem data,
		including tmpfs and shared memory.

	  kernel (npn)
		Amount of total kernel memory, including
		(kernel_stack, pagetables, percpu, vmalloc, slab) in
		addition to other kernel memory use cases.

	  kernel_stack
		Amount of memory allocated to kernel stacks.

	  pagetables
                Amount of memory allocated for page tables.

	  sec_pagetables
		Amount of memory allocated for secondary page tables,
		this currently includes KVM mmu allocations on x86
		and arm64 and IOMMU page tables.

	  percpu (npn)
		Amount of memory used for storing per-cpu kernel
		data structures.

	  sock (npn)
		Amount of memory used in network transmission buffers

	  vmalloc (npn)
		Amount of memory used for vmap backed memory.

	  shmem
		Amount of cached filesystem data that is swap-backed,
		such as tmpfs, shm segments, shared anonymous mmap()s

	  zswap
		Amount of memory consumed by the zswap compression backend.

	  zswapped
		Amount of application memory swapped out to zswap.

	  file_mapped
		Amount of cached filesystem data mapped with mmap()

	  file_dirty
		Amount of cached filesystem data that was modified but
		not yet written back to disk

	  file_writeback
		Amount of cached filesystem data that was modified and
		is currently being written back to disk

	  swapcached
		Amount of swap cached in memory. The swapcache is accounted
		against both memory and swap usage.

	  anon_thp
		Amount of memory used in anonymous mappings backed by
		transparent hugepages

	  file_thp
		Amount of cached filesystem data backed by transparent
		hugepages

	  shmem_thp
		Amount of shm, tmpfs, shared anonymous mmap()s backed by
		transparent hugepages

	  inactive_anon, active_anon, inactive_file, active_file, unevictable
		Amount of memory, swap-backed and filesystem-backed,
		on the internal memory management lists used by the
		page reclaim algorithm.

		As these represent internal list state (eg. shmem pages are on anon
		memory management lists), inactive_foo + active_foo may not be equal to
		the value for the foo counter, since the foo counter is type-based, not
		list-based.

	  slab_reclaimable
		Part of "slab" that might be reclaimed, such as
		dentries and inodes.

	  slab_unreclaimable
		Part of "slab" that cannot be reclaimed on memory
		pressure.

	  slab (npn)
		Amount of memory used for storing in-kernel data
		structures.

	  workingset_refault_anon
		Number of refaults of previously evicted anonymous pages.

	  workingset_refault_file
		Number of refaults of previously evicted file pages.

	  workingset_activate_anon
		Number of refaulted anonymous pages that were immediately
		activated.

	  workingset_activate_file
		Number of refaulted file pages that were immediately activated.

	  workingset_restore_anon
		Number of restored anonymous pages which have been detected as
		an active workingset before they got reclaimed.

	  workingset_restore_file
		Number of restored file pages which have been detected as an
		active workingset before they got reclaimed.

	  workingset_nodereclaim
		Number of times a shadow node has been reclaimed

	  pgscan (npn)
		Amount of scanned pages (in an inactive LRU list)

	  pgsteal (npn)
		Amount of reclaimed pages

	  pgscan_kswapd (npn)
		Amount of scanned pages by kswapd (in an inactive LRU list)

	  pgscan_direct (npn)
		Amount of scanned pages directly  (in an inactive LRU list)

	  pgscan_khugepaged (npn)
		Amount of scanned pages by khugepaged  (in an inactive LRU list)

	  pgsteal_kswapd (npn)
		Amount of reclaimed pages by kswapd

	  pgsteal_direct (npn)
		Amount of reclaimed pages directly

	  pgsteal_khugepaged (npn)
		Amount of reclaimed pages by khugepaged

	  pgfault (npn)
		Total number of page faults incurred

	  pgmajfault (npn)
		Number of major page faults incurred

	  pgrefill (npn)
		Amount of scanned pages (in an active LRU list)

	  pgactivate (npn)
		Amount of pages moved to the active LRU list

	  pgdeactivate (npn)
		Amount of pages moved to the inactive LRU list

	  pglazyfree (npn)
		Amount of pages postponed to be freed under memory pressure

	  pglazyfreed (npn)
		Amount of reclaimed lazyfree pages

	  swpin_zero
		Number of pages swapped into memory and filled with zero, where I/O
		was optimized out because the page content was detected to be zero
		during swapout.

	  swpout_zero
		Number of zero-filled pages swapped out with I/O skipped due to the
		content being detected as zero.

	  zswpin
		Number of pages moved in to memory from zswap.

	  zswpout
		Number of pages moved out of memory to zswap.

	  zswpwb
		Number of pages written from zswap to swap.

	  thp_fault_alloc (npn)
		Number of transparent hugepages which were allocated to satisfy
		a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE
                is not set.

	  thp_collapse_alloc (npn)
		Number of transparent hugepages which were allocated to allow
		collapsing an existing range of pages. This counter is not
		present when CONFIG_TRANSPARENT_HUGEPAGE is not set.

	  thp_swpout (npn)
		Number of transparent hugepages which are swapout in one piece
		without splitting.

	  thp_swpout_fallback (npn)
		Number of transparent hugepages which were split before swapout.
		Usually because failed to allocate some continuous swap space
		for the huge page.

	  numa_pages_migrated (npn)
		Number of pages migrated by NUMA balancing.

	  numa_pte_updates (npn)
		Number of pages whose page table entries are modified by
		NUMA balancing to produce NUMA hinting faults on access.

	  numa_hint_faults (npn)
		Number of NUMA hinting faults.

	  pgdemote_kswapd
		Number of pages demoted by kswapd.

	  pgdemote_direct
		Number of pages demoted directly.

	  pgdemote_khugepaged
		Number of pages demoted by khugepaged.

	  hugetlb
		Amount of memory used by hugetlb pages. This metric only shows
		up if hugetlb usage is accounted for in memory.current (i.e.
		cgroup is mounted with the memory_hugetlb_accounting option).

  memory.numa_stat
	A read-only nested-keyed file which exists on non-root cgroups.

	This breaks down the cgroup's memory footprint into different
	types of memory, type-specific details, and other information
	per node on the state of the memory management system.

	This is useful for providing visibility into the NUMA locality
	information within an memcg since the pages are allowed to be
	allocated from any physical node. One of the use case is evaluating
	application performance by combining this information with the
	application's CPU allocation.

	All memory amounts are in bytes.

	The output format of memory.numa_stat is::

	  type N0=<bytes in node 0> N1=<bytes in node 1> ...

	The entries are ordered to be human readable, and new entries
	can show up in the middle. Don't rely on items remaining in a
	fixed position; use the keys to look up specific values!

	The entries can refer to the memory.stat.

  memory.swap.current
	A read-only single value file which exists on non-root
	cgroups.

	The total amount of swap currently being used by the cgroup
	and its descendants.

  memory.swap.high
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Swap usage throttle limit.  If a cgroup's swap usage exceeds
	this limit, all its further allocations will be throttled to
	allow userspace to implement custom out-of-memory procedures.

	This limit marks a point of no return for the cgroup. It is NOT
	designed to manage the amount of swapping a workload does
	during regular operation. Compare to memory.swap.max, which
	prohibits swapping past a set amount, but lets the cgroup
	continue unimpeded as long as other memory can be reclaimed.

	Healthy workloads are not expected to reach this limit.

  memory.swap.peak
	A read-write single value file which exists on non-root cgroups.

	The max swap usage recorded for the cgroup and its descendants since
	the creation of the cgroup or the most recent reset for that FD.

	A write of any non-empty string to this file resets it to the
	current memory usage for subsequent reads through the same
	file descriptor.

  memory.swap.max
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Swap usage hard limit.  If a cgroup's swap usage reaches this
	limit, anonymous memory of the cgroup will not be swapped out.

  memory.swap.events
	A read-only flat-keyed file which exists on non-root cgroups.
	The following entries are defined.  Unless specified
	otherwise, a value change in this file generates a file
	modified event.

	  high
		The number of times the cgroup's swap usage was over
		the high threshold.

	  max
		The number of times the cgroup's swap usage was about
		to go over the max boundary and swap allocation
		failed.

	  fail
		The number of times swap allocation failed either
		because of running out of swap system-wide or max
		limit.

	When reduced under the current usage, the existing swap
	entries are reclaimed gradually and the swap usage may stay
	higher than the limit for an extended period of time.  This
	reduces the impact on the workload and memory management.

  memory.zswap.current
	A read-only single value file which exists on non-root
	cgroups.

	The total amount of memory consumed by the zswap compression
	backend.

  memory.zswap.max
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Zswap usage hard limit. If a cgroup's zswap pool reaches this
	limit, it will refuse to take any more stores before existing
	entries fault back in or are written out to disk.

  memory.zswap.writeback
	A read-write single value file. The default value is "1".
	Note that this setting is hierarchical, i.e. the writeback would be
	implicitly disabled for child cgroups if the upper hierarchy
	does so.

	When this is set to 0, all swapping attempts to swapping devices
	are disabled. This included both zswap writebacks, and swapping due
	to zswap store failures. If the zswap store failures are recurring
	(for e.g if the pages are incompressible), users can observe
	reclaim inefficiency after disabling writeback (because the same
	pages might be rejected again and again).

	Note that this is subtly different from setting memory.swap.max to
	0, as it still allows for pages to be written to the zswap pool.
	This setting has no effect if zswap is disabled, and swapping
	is allowed unless memory.swap.max is set to 0.

  memory.pressure
	A read-only nested-keyed file.

	Shows pressure stall information for memory. See
	:ref:`Documentation/accounting/psi.rst <psi>` for details.

Usage Guidelines
~~~~~~~~~~~~~~~~

"memory.high" is the main mechanism to control memory usage.
Over-committing on high limit (sum of high limits > available memory)
and letting global memory pressure to distribute memory according to
usage is a viable strategy.

Because breach of the high limit doesn't trigger the OOM killer but
throttles the offending cgroup, a management agent has ample
opportunities to monitor and take appropriate actions such as granting
more memory or terminating the workload.

Determining whether a cgroup has enough memory is not trivial as
memory usage doesn't indicate whether the workload can benefit from
more memory.  For example, a workload which writes data received from
network to a file can use all available memory but can also operate as
performant with a small amount of memory.  A measure of memory
pressure - how much the workload is being impacted due to lack of
memory - is necessary to determine whether a workload needs more
memory; unfortunately, memory pressure monitoring mechanism isn't
implemented yet.

Memory Ownership
~~~~~~~~~~~~~~~~

A memory area is charged to the cgroup which instantiated it and stays
charged to the cgroup until the area is released.  Migrating a process
to a different cgroup doesn't move the memory usages that it
instantiated while in the previous cgroup to the new cgroup.

A memory area may be used by processes belonging to different cgroups.
To which cgroup the area will be charged is in-deterministic; however,
over time, the memory area is likely to end up in a cgroup which has
enough memory allowance to avoid high reclaim pressure.

If a cgroup sweeps a considerable amount of memory which is expected
to be accessed repeatedly by other cgroups, it may make sense to use
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
belonging to the affected files to ensure correct memory ownership.

IO
--

The "io" controller regulates the distribution of IO resources.  This
controller implements both weight based and absolute bandwidth or IOPS
limit distribution; however, weight based distribution is available
only if cfq-iosched is in use and neither scheme is available for
blk-mq devices.

IO Interface Files
~~~~~~~~~~~~~~~~~~

  io.stat
	A read-only nested-keyed file.

	Lines are keyed by $MAJ:$MIN device numbers and not ordered.
	The following nested keys are defined.

	  ======	=====================
	  rbytes	Bytes read
	  wbytes	Bytes written
	  rios		Number of read IOs
	  wios		Number of write IOs
	  dbytes	Bytes discarded
	  dios		Number of discard IOs
	  ======	=====================

	An example read output follows::

	  8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0
	  8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021

  io.cost.qos
	A read-write nested-keyed file which exists only on the root
	cgroup.

	This file configures the Quality of Service of the IO cost
	model based controller (CONFIG_BLK_CGROUP_IOCOST) which
	currently implements "io.weight" proportional control.  Lines
	are keyed by $MAJ:$MIN device numbers and not ordered.  The
	line for a given device is populated on the first write for
	the device on "io.cost.qos" or "io.cost.model".  The following
	nested keys are defined.

	  ======	=====================================
	  enable	Weight-based control enable
	  ctrl		"auto" or "user"
	  rpct		Read latency percentile    [0, 100]
	  rlat		Read latency threshold
	  wpct		Write latency percentile   [0, 100]
	  wlat		Write latency threshold
	  min		Minimum scaling percentage [1, 10000]
	  max		Maximum scaling percentage [1, 10000]
	  ======	=====================================

	The controller is disabled by default and can be enabled by
	setting "enable" to 1.  "rpct" and "wpct" parameters default
	to zero and the controller uses internal device saturation
	state to adjust the overall IO rate between "min" and "max".

	When a better control quality is needed, latency QoS
	parameters can be configured.  For example::

	  8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0

	shows that on sdb, the controller is enabled, will consider
	the device saturated if the 95th percentile of read completion
	latencies is above 75ms or write 150ms, and adjust the overall
	IO issue rate between 50% and 150% accordingly.

	The lower the saturation point, the better the latency QoS at
	the cost of aggregate bandwidth.  The narrower the allowed
	adjustment range between "min" and "max", the more conformant
	to the cost model the IO behavior.  Note that the IO issue
	base rate may be far off from 100% and setting "min" and "max"
	blindly can lead to a significant loss of device capacity or
	control quality.  "min" and "max" are useful for regulating
	devices which show wide temporary behavior changes - e.g. a
	ssd which accepts writes at the line speed for a while and
	then completely stalls for multiple seconds.

	When "ctrl" is "auto", the parameters are controlled by the
	kernel and may change automatically.  Setting "ctrl" to "user"
	or setting any of the percentile and latency parameters puts
	it into "user" mode and disables the automatic changes.  The
	automatic mode can be restored by setting "ctrl" to "auto".

  io.cost.model
	A read-write nested-keyed file which exists only on the root
	cgroup.

	This file configures the cost model of the IO cost model based
	controller (CONFIG_BLK_CGROUP_IOCOST) which currently
	implements "io.weight" proportional control.  Lines are keyed
	by $MAJ:$MIN device numbers and not ordered.  The line for a
	given device is populated on the first write for the device on
	"io.cost.qos" or "io.cost.model".  The following nested keys
	are defined.

	  =====		================================
	  ctrl		"auto" or "user"
	  model		The cost model in use - "linear"
	  =====		================================

	When "ctrl" is "auto", the kernel may change all parameters
	dynamically.  When "ctrl" is set to "user" or any other
	parameters are written to, "ctrl" become "user" and the
	automatic changes are disabled.

	When "model" is "linear", the following model parameters are
	defined.

	  =============	========================================
	  [r|w]bps	The maximum sequential IO throughput
	  [r|w]seqiops	The maximum 4k sequential IOs per second
	  [r|w]randiops	The maximum 4k random IOs per second
	  =============	========================================

	From the above, the builtin linear model determines the base
	costs of a sequential and random IO and the cost coefficient
	for the IO size.  While simple, this model can cover most
	common device classes acceptably.

	The IO cost model isn't expected to be accurate in absolute
	sense and is scaled to the device behavior dynamically.

	If needed, tools/cgroup/iocost_coef_gen.py can be used to
	generate device-specific coefficients.

  io.weight
	A read-write flat-keyed file which exists on non-root cgroups.
	The default is "default 100".

	The first line is the default weight applied to devices
	without specific override.  The rest are overrides keyed by
	$MAJ:$MIN device numbers and not ordered.  The weights are in
	the range [1, 10000] and specifies the relative amount IO time
	the cgroup can use in relation to its siblings.

	The default weight can be updated by writing either "default
	$WEIGHT" or simply "$WEIGHT".  Overrides can be set by writing
	"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".

	An example read output follows::

	  default 100
	  8:16 200
	  8:0 50

  io.max
	A read-write nested-keyed file which exists on non-root
	cgroups.

	BPS and IOPS based IO limit.  Lines are keyed by $MAJ:$MIN
	device numbers and not ordered.  The following nested keys are
	defined.

	  =====		==================================
	  rbps		Max read bytes per second
	  wbps		Max write bytes per second
	  riops		Max read IO operations per second
	  wiops		Max write IO operations per second
	  =====		==================================

	When writing, any number of nested key-value pairs can be
	specified in any order.  "max" can be specified as the value
	to remove a specific limit.  If the same key is specified
	multiple times, the outcome is undefined.

	BPS and IOPS are measured in each IO direction and IOs are
	delayed if limit is reached.  Temporary bursts are allowed.

	Setting read limit at 2M BPS and write at 120 IOPS for 8:16::

	  echo "8:16 rbps=2097152 wiops=120" > io.max

	Reading returns the following::

	  8:16 rbps=2097152 wbps=max riops=max wiops=120

	Write IOPS limit can be removed by writing the following::

	  echo "8:16 wiops=max" > io.max

	Reading now returns the following::

	  8:16 rbps=2097152 wbps=max riops=max wiops=max

  io.pressure
	A read-only nested-keyed file.

	Shows pressure stall information for IO. See
	:ref:`Documentation/accounting/psi.rst <psi>` for details.

Writeback
~~~~~~~~~

Page cache is dirtied through buffered writes and shared mmaps and
written asynchronously to the backing filesystem by the writeback
mechanism.  Writeback sits between the memory and IO domains and
regulates the proportion of dirty memory by balancing dirtying and
write IOs.

The io controller, in conjunction with the memory controller,
implements control of page cache writeback IOs.  The memory controller
defines the memory domain that dirty memory ratio is calculated and
maintained for and the io controller defines the io domain which
writes out dirty pages for the memory domain.  Both system-wide and
per-cgroup dirty memory states are examined and the more restrictive
of the two is enforced.

cgroup writeback requires explicit support from the underlying
filesystem.  Currently, cgroup writeback is implemented on ext2, ext4,
btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are
attributed to the root cgroup.

There are inherent differences in memory and writeback management
which affects how cgroup ownership is tracked.  Memory is tracked per
page while writeback per inode.  For the purpose of writeback, an
inode is assigned to a cgroup and all IO requests to write dirty pages
from the inode are attributed to that cgroup.

As cgroup ownership for memory is tracked per page, there can be pages
which are associated with different cgroups than the one the inode is
associated with.  These are called foreign pages.  The writeback
constantly keeps track of foreign pages and, if a particular foreign
cgroup becomes the majority over a certain period of time, switches
the ownership of the inode to that cgroup.

While this model is enough for most use cases where a given inode is
mostly dirtied by a single cgroup even when the main writing cgroup
changes over time, use cases where multiple cgroups write to a single
inode simultaneously are not supported well.  In such circumstances, a
significant portion of IOs are likely to be attributed incorrectly.
As memory controller assigns page ownership on the first use and
doesn't update it until the page is released, even if writeback
strictly follows page ownership, multiple cgroups dirtying overlapping
areas wouldn't work as expected.  It's recommended to avoid such usage
patterns.

The sysctl knobs which affect writeback behavior are applied to cgroup
writeback as follows.

  vm.dirty_background_ratio, vm.dirty_ratio
	These ratios apply the same to cgroup writeback with the
	amount of available memory capped by limits imposed by the
	memory controller and system-wide clean memory.

  vm.dirty_background_bytes, vm.dirty_bytes
	For cgroup writeback, this is calculated into ratio against
	total available memory and applied the same way as
	vm.dirty[_background]_ratio.

IO Latency
~~~~~~~~~~

This is a cgroup v2 controller for IO workload protection.  You provide a group
with a latency target, and if the average latency exceeds that target the
controller will throttle any peers that have a lower latency target than the
protected workload.

The limits are only applied at the peer level in the hierarchy.  This means that
in the diagram below, only groups A, B, and C will influence each other, and
groups D and F will influence each other.  Group G will influence nobody::

			[root]
		/	   |		\
		A	   B		C
	       /  \        |
	      D    F	   G

So the ideal way to configure this is to set io.latency in groups A, B, and C.
Generally you do not want to set a value lower than the latency your device
supports.  Experiment to find the value that works best for your workload.
Start at higher than the expected latency for your device and watch the
avg_lat value in io.stat for your workload group to get an idea of the
latency you see during normal operation.  Use the avg_lat value as a basis for
your real setting, setting at 10-15% higher than the value in io.stat.

How IO Latency Throttling Works
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

io.latency is work conserving; so as long as everybody is meeting their latency
target the controller doesn't do anything.  Once a group starts missing its
target it begins throttling any peer group that has a higher target than itself.
This throttling takes 2 forms:

- Queue depth throttling.  This is the number of outstanding IO's a group is
  allowed to have.  We will clamp down relatively quickly, starting at no limit
  and going all the way down to 1 IO at a time.

- Artificial delay induction.  There are certain types of IO that cannot be
  throttled without possibly adversely affecting higher priority groups.  This
  includes swapping and metadata IO.  These types of IO are allowed to occur
  normally, however they are "charged" to the originating group.  If the
  originating group is being throttled you will see the use_delay and delay
  fields in io.stat increase.  The delay value is how many microseconds that are
  being added to any process that runs in this group.  Because this number can
  grow quite large if there is a lot of swapping or metadata IO occurring we
  limit the individual delay events to 1 second at a time.

Once the victimized group starts meeting its latency target again it will start
unthrottling any peer groups that were throttled previously.  If the victimized
group simply stops doing IO the global counter will unthrottle appropriately.

IO Latency Interface Files
~~~~~~~~~~~~~~~~~~~~~~~~~~

  io.latency
	This takes a similar format as the other controllers.

		"MAJOR:MINOR target=<target time in microseconds>"

  io.stat
	If the controller is enabled you will see extra stats in io.stat in
	addition to the normal ones.

	  depth
		This is the current queue depth for the group.

	  avg_lat
		This is an exponential moving average with a decay rate of 1/exp
		bound by the sampling interval.  The decay rate interval can be
		calculated by multiplying the win value in io.stat by the
		corresponding number of samples based on the win value.

	  win
		The sampling window size in milliseconds.  This is the minimum
		duration of time between evaluation events.  Windows only elapse
		with IO activity.  Idle periods extend the most recent window.

IO Priority
~~~~~~~~~~~

A single attribute controls the behavior of the I/O priority cgroup policy,
namely the io.prio.class attribute. The following values are accepted for
that attribute:

  no-change
	Do not modify the I/O priority class.

  promote-to-rt
	For requests that have a non-RT I/O priority class, change it into RT.
	Also change the priority level of these requests to 4. Do not modify
	the I/O priority of requests that have priority class RT.

  restrict-to-be
	For requests that do not have an I/O priority class or that have I/O
	priority class RT, change it into BE. Also change the priority level
	of these requests to 0. Do not modify the I/O priority class of
	requests that have priority class IDLE.

  idle
	Change the I/O priority class of all requests into IDLE, the lowest
	I/O priority class.

  none-to-rt
	Deprecated. Just an alias for promote-to-rt.

The following numerical values are associated with the I/O priority policies:

+----------------+---+
| no-change      | 0 |
+----------------+---+
| promote-to-rt  | 1 |
+----------------+---+
| restrict-to-be | 2 |
+----------------+---+
| idle           | 3 |
+----------------+---+

The numerical value that corresponds to each I/O priority class is as follows:

+-------------------------------+---+
| IOPRIO_CLASS_NONE             | 0 |
+-------------------------------+---+
| IOPRIO_CLASS_RT (real-time)   | 1 |
+-------------------------------+---+
| IOPRIO_CLASS_BE (best effort) | 2 |
+-------------------------------+---+
| IOPRIO_CLASS_IDLE             | 3 |
+-------------------------------+---+

The algorithm to set the I/O priority class for a request is as follows:

- If I/O priority class policy is promote-to-rt, change the request I/O
  priority class to IOPRIO_CLASS_RT and change the request I/O priority
  level to 4.
- If I/O priority class policy is not promote-to-rt, translate the I/O priority
  class policy into a number, then change the request I/O priority class
  into the maximum of the I/O priority class policy number and the numerical
  I/O priority class.

PID
---

The process number controller is used to allow a cgroup to stop any
new tasks from being fork()'d or clone()'d after a specified limit is
reached.

The number of tasks in a cgroup can be exhausted in ways which other
controllers cannot prevent, thus warranting its own controller.  For
example, a fork bomb is likely to exhaust the number of tasks before
hitting memory restrictions.

Note that PIDs used in this controller refer to TIDs, process IDs as
used by the kernel.

PID Interface Files
~~~~~~~~~~~~~~~~~~~

  pids.max
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Hard limit of number of processes.

  pids.current
	A read-only single value file which exists on non-root cgroups.

	The number of processes currently in the cgroup and its
	descendants.

  pids.peak
	A read-only single value file which exists on non-root cgroups.

	The maximum value that the number of processes in the cgroup and its
	descendants has ever reached.

  pids.events
	A read-only flat-keyed file which exists on non-root cgroups. Unless
	specified otherwise, a value change in this file generates a file
	modified event. The following entries are defined.

	  max
		The number of times the cgroup's total number of processes hit the pids.max
		limit (see also pids_localevents).

  pids.events.local
	Similar to pids.events but the fields in the file are local
	to the cgroup i.e. not hierarchical. The file modified event
	generated on this file reflects only the local events.

Organisational operations are not blocked by cgroup policies, so it is
possible to have pids.current > pids.max.  This can be done by either
setting the limit to be smaller than pids.current, or attaching enough
processes to the cgroup such that pids.current is larger than
pids.max.  However, it is not possible to violate a cgroup PID policy
through fork() or clone(). These will return -EAGAIN if the creation
of a new process would cause a cgroup policy to be violated.

Cpuset
------

The "cpuset" controller provides a mechanism for constraining
the CPU and memory node placement of tasks to only the resources
specified in the cpuset interface files in a task's current cgroup.
This is especially valuable on large NUMA systems where placing jobs
on properly sized subsets of the systems with careful processor and
memory placement to reduce cross-node memory access and contention
can improve overall system performance.

The "cpuset" controller is hierarchical.  That means the controller
cannot use CPUs or memory nodes not allowed in its parent.

Cpuset Interface Files
~~~~~~~~~~~~~~~~~~~~~~

  cpuset.cpus
	A read-write multiple values file which exists on non-root
	cpuset-enabled cgroups.

	It lists the requested CPUs to be used by tasks within this
	cgroup.  The actual list of CPUs to be granted, however, is
	subjected to constraints imposed by its parent and can differ
	from the requested CPUs.

	The CPU numbers are comma-separated numbers or ranges.
	For example::

	  # cat cpuset.cpus
	  0-4,6,8-10

	An empty value indicates that the cgroup is using the same
	setting as the nearest cgroup ancestor with a non-empty
	"cpuset.cpus" or all the available CPUs if none is found.

	The value of "cpuset.cpus" stays constant until the next update
	and won't be affected by any CPU hotplug events.

  cpuset.cpus.effective
	A read-only multiple values file which exists on all
	cpuset-enabled cgroups.

	It lists the onlined CPUs that are actually granted to this
	cgroup by its parent.  These CPUs are allowed to be used by
	tasks within the current cgroup.

	If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
	all the CPUs from the parent cgroup that can be available to
	be used by this cgroup.  Otherwise, it should be a subset of
	"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
	can be granted.  In this case, it will be treated just like an
	empty "cpuset.cpus".

	Its value will be affected by CPU hotplug events.

  cpuset.mems
	A read-write multiple values file which exists on non-root
	cpuset-enabled cgroups.

	It lists the requested memory nodes to be used by tasks within
	this cgroup.  The actual list of memory nodes granted, however,
	is subjected to constraints imposed by its parent and can differ
	from the requested memory nodes.

	The memory node numbers are comma-separated numbers or ranges.
	For example::

	  # cat cpuset.mems
	  0-1,3

	An empty value indicates that the cgroup is using the same
	setting as the nearest cgroup ancestor with a non-empty
	"cpuset.mems" or all the available memory nodes if none
	is found.

	The value of "cpuset.mems" stays constant until the next update
	and won't be affected by any memory nodes hotplug events.

	Setting a non-empty value to "cpuset.mems" causes memory of
	tasks within the cgroup to be migrated to the designated nodes if
	they are currently using memory outside of the designated nodes.

	There is a cost for this memory migration.  The migration
	may not be complete and some memory pages may be left behind.
	So it is recommended that "cpuset.mems" should be set properly
	before spawning new tasks into the cpuset.  Even if there is
	a need to change "cpuset.mems" with active tasks, it shouldn't
	be done frequently.

  cpuset.mems.effective
	A read-only multiple values file which exists on all
	cpuset-enabled cgroups.

	It lists the onlined memory nodes that are actually granted to
	this cgroup by its parent. These memory nodes are allowed to
	be used by tasks within the current cgroup.

	If "cpuset.mems" is empty, it shows all the memory nodes from the
	parent cgroup that will be available to be used by this cgroup.
	Otherwise, it should be a subset of "cpuset.mems" unless none of
	the memory nodes listed in "cpuset.mems" can be granted.  In this
	case, it will be treated just like an empty "cpuset.mems".

	Its value will be affected by memory nodes hotplug events.

  cpuset.cpus.exclusive
	A read-write multiple values file which exists on non-root
	cpuset-enabled cgroups.

	It lists all the exclusive CPUs that are allowed to be used
	to create a new cpuset partition.  Its value is not used
	unless the cgroup becomes a valid partition root.  See the
	"cpuset.cpus.partition" section below for a description of what
	a cpuset partition is.

	When the cgroup becomes a partition root, the actual exclusive
	CPUs that are allocated to that partition are listed in
	"cpuset.cpus.exclusive.effective" which may be different
	from "cpuset.cpus.exclusive".  If "cpuset.cpus.exclusive"
	has previously been set, "cpuset.cpus.exclusive.effective"
	is always a subset of it.

	Users can manually set it to a value that is different from
	"cpuset.cpus".	One constraint in setting it is that the list of
	CPUs must be exclusive with respect to "cpuset.cpus.exclusive"
	of its sibling.  If "cpuset.cpus.exclusive" of a sibling cgroup
	isn't set, its "cpuset.cpus" value, if set, cannot be a subset
	of it to leave at least one CPU available when the exclusive
	CPUs are taken away.

	For a parent cgroup, any one of its exclusive CPUs can only
	be distributed to at most one of its child cgroups.  Having an
	exclusive CPU appearing in two or more of its child cgroups is
	not allowed (the exclusivity rule).  A value that violates the
	exclusivity rule will be rejected with a write error.

	The root cgroup is a partition root and all its available CPUs
	are in its exclusive CPU set.

  cpuset.cpus.exclusive.effective
	A read-only multiple values file which exists on all non-root
	cpuset-enabled cgroups.

	This file shows the effective set of exclusive CPUs that
	can be used to create a partition root.  The content
	of this file will always be a subset of its parent's
	"cpuset.cpus.exclusive.effective" if its parent is not the root
	cgroup.  It will also be a subset of "cpuset.cpus.exclusive"
	if it is set.  If "cpuset.cpus.exclusive" is not set, it is
	treated to have an implicit value of "cpuset.cpus" in the
	formation of local partition.

  cpuset.cpus.isolated
	A read-only and root cgroup only multiple values file.

	This file shows the set of all isolated CPUs used in existing
	isolated partitions. It will be empty if no isolated partition
	is created.

  cpuset.cpus.partition
	A read-write single value file which exists on non-root
	cpuset-enabled cgroups.  This flag is owned by the parent cgroup
	and is not delegatable.

	It accepts only the following input values when written to.

	  ==========	=====================================
	  "member"	Non-root member of a partition
	  "root"	Partition root
	  "isolated"	Partition root without load balancing
	  ==========	=====================================

	A cpuset partition is a collection of cpuset-enabled cgroups with
	a partition root at the top of the hierarchy and its descendants
	except those that are separate partition roots themselves and
	their descendants.  A partition has exclusive access to the
	set of exclusive CPUs allocated to it.	Other cgroups outside
	of that partition cannot use any CPUs in that set.

	There are two types of partitions - local and remote.  A local
	partition is one whose parent cgroup is also a valid partition
	root.  A remote partition is one whose parent cgroup is not a
	valid partition root itself.  Writing to "cpuset.cpus.exclusive"
	is optional for the creation of a local partition as its
	"cpuset.cpus.exclusive" file will assume an implicit value that
	is the same as "cpuset.cpus" if it is not set.	Writing the
	proper "cpuset.cpus.exclusive" values down the cgroup hierarchy
	before the target partition root is mandatory for the creation
	of a remote partition.

	Currently, a remote partition cannot be created under a local
	partition.  All the ancestors of a remote partition root except
	the root cgroup cannot be a partition root.

	The root cgroup is always a partition root and its state cannot
	be changed.  All other non-root cgroups start out as "member".

	When set to "root", the current cgroup is the root of a new
	partition or scheduling domain.  The set of exclusive CPUs is
	determined by the value of its "cpuset.cpus.exclusive.effective".

	When set to "isolated", the CPUs in that partition will be in
	an isolated state without any load balancing from the scheduler
	and excluded from the unbound workqueues.  Tasks placed in such
	a partition with multiple CPUs should be carefully distributed
	and bound to each of the individual CPUs for optimal performance.

	A partition root ("root" or "isolated") can be in one of the
	two possible states - valid or invalid.  An invalid partition
	root is in a degraded state where some state information may
	be retained, but behaves more like a "member".

	All possible state transitions among "member", "root" and
	"isolated" are allowed.

	On read, the "cpuset.cpus.partition" file can show the following
	values.

	  =============================	=====================================
	  "member"			Non-root member of a partition
	  "root"			Partition root
	  "isolated"			Partition root without load balancing
	  "root invalid (<reason>)"	Invalid partition root
	  "isolated invalid (<reason>)"	Invalid isolated partition root
	  =============================	=====================================

	In the case of an invalid partition root, a descriptive string on
	why the partition is invalid is included within parentheses.

	For a local partition root to be valid, the following conditions
	must be met.

	1) The parent cgroup is a valid partition root.
	2) The "cpuset.cpus.exclusive.effective" file cannot be empty,
	   though it may contain offline CPUs.
	3) The "cpuset.cpus.effective" cannot be empty unless there is
	   no task associated with this partition.

	For a remote partition root to be valid, all the above conditions
	except the first one must be met.

	External events like hotplug or changes to "cpuset.cpus" or
	"cpuset.cpus.exclusive" can cause a valid partition root to
	become invalid and vice versa.	Note that a task cannot be
	moved to a cgroup with empty "cpuset.cpus.effective".

	A valid non-root parent partition may distribute out all its CPUs
	to its child local partitions when there is no task associated
	with it.

	Care must be taken to change a valid partition root to "member"
	as all its child local partitions, if present, will become
	invalid causing disruption to tasks running in those child
	partitions. These inactivated partitions could be recovered if
	their parent is switched back to a partition root with a proper
	value in "cpuset.cpus" or "cpuset.cpus.exclusive".

	Poll and inotify events are triggered whenever the state of
	"cpuset.cpus.partition" changes.  That includes changes caused
	by write to "cpuset.cpus.partition", cpu hotplug or other
	changes that modify the validity status of the partition.
	This will allow user space agents to monitor unexpected changes
	to "cpuset.cpus.partition" without the need to do continuous
	polling.

	A user can pre-configure certain CPUs to an isolated state
	with load balancing disabled at boot time with the "isolcpus"
	kernel boot command line option.  If those CPUs are to be put
	into a partition, they have to be used in an isolated partition.

Device controller
-----------------

Device controller manages access to device files. It includes both
creation of new device files (using mknod), and access to the
existing device files.

Cgroup v2 device controller has no interface files and is implemented
on top of cgroup BPF. To control access to device files, a user may
create bpf programs of type BPF_PROG_TYPE_CGROUP_DEVICE and attach
them to cgroups with BPF_CGROUP_DEVICE flag. On an attempt to access a
device file, corresponding BPF programs will be executed, and depending
on the return value the attempt will succeed or fail with -EPERM.

A BPF_PROG_TYPE_CGROUP_DEVICE program takes a pointer to the
bpf_cgroup_dev_ctx structure, which describes the device access attempt:
access type (mknod/read/write) and device (type, major and minor numbers).
If the program returns 0, the attempt fails with -EPERM, otherwise it
succeeds.

An example of BPF_PROG_TYPE_CGROUP_DEVICE program may be found in
tools/testing/selftests/bpf/progs/dev_cgroup.c in the kernel source tree.

RDMA
----

The "rdma" controller regulates the distribution and accounting of
RDMA resources.

RDMA Interface Files
~~~~~~~~~~~~~~~~~~~~

  rdma.max
	A readwrite nested-keyed file that exists for all the cgroups
	except root that describes current configured resource limit
	for a RDMA/IB device.

	Lines are keyed by device name and are not ordered.
	Each line contains space separated resource name and its configured
	limit that can be distributed.

	The following nested keys are defined.

	  ==========	=============================
	  hca_handle	Maximum number of HCA Handles
	  hca_object 	Maximum number of HCA Objects
	  ==========	=============================

	An example for mlx4 and ocrdma device follows::

	  mlx4_0 hca_handle=2 hca_object=2000
	  ocrdma1 hca_handle=3 hca_object=max

  rdma.current
	A read-only file that describes current resource usage.
	It exists for all the cgroup except root.

	An example for mlx4 and ocrdma device follows::

	  mlx4_0 hca_handle=1 hca_object=20
	  ocrdma1 hca_handle=1 hca_object=23

HugeTLB
-------

The HugeTLB controller allows to limit the HugeTLB usage per control group and
enforces the controller limit during page fault.

HugeTLB Interface Files
~~~~~~~~~~~~~~~~~~~~~~~

  hugetlb.<hugepagesize>.current
	Show current usage for "hugepagesize" hugetlb.  It exists for all
	the cgroup except root.

  hugetlb.<hugepagesize>.max
	Set/show the hard limit of "hugepagesize" hugetlb usage.
	The default value is "max".  It exists for all the cgroup except root.

  hugetlb.<hugepagesize>.events
	A read-only flat-keyed file which exists on non-root cgroups.

	  max
		The number of allocation failure due to HugeTLB limit

  hugetlb.<hugepagesize>.events.local
	Similar to hugetlb.<hugepagesize>.events but the fields in the file
	are local to the cgroup i.e. not hierarchical. The file modified event
	generated on this file reflects only the local events.

  hugetlb.<hugepagesize>.numa_stat
	Similar to memory.numa_stat, it shows the numa information of the
        hugetlb pages of <hugepagesize> in this cgroup.  Only active in
        use hugetlb pages are included.  The per-node values are in bytes.

Misc
----

The Miscellaneous cgroup provides the resource limiting and tracking
mechanism for the scalar resources which cannot be abstracted like the other
cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config
option.

A resource can be added to the controller via enum misc_res_type{} in the
include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[]
in the kernel/cgroup/misc.c file. Provider of the resource must set its
capacity prior to using the resource by calling misc_cg_set_capacity().

Once a capacity is set then the resource usage can be updated using charge and
uncharge APIs. All of the APIs to interact with misc controller are in
include/linux/misc_cgroup.h.

Misc Interface Files
~~~~~~~~~~~~~~~~~~~~

Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then:

  misc.capacity
        A read-only flat-keyed file shown only in the root cgroup.  It shows
        miscellaneous scalar resources available on the platform along with
        their quantities::

	  $ cat misc.capacity
	  res_a 50
	  res_b 10

  misc.current
        A read-only flat-keyed file shown in the all cgroups.  It shows
        the current usage of the resources in the cgroup and its children.::

	  $ cat misc.current
	  res_a 3
	  res_b 0

  misc.peak
        A read-only flat-keyed file shown in all cgroups.  It shows the
        historical maximum usage of the resources in the cgroup and its
        children.::

	  $ cat misc.peak
	  res_a 10
	  res_b 8

  misc.max
        A read-write flat-keyed file shown in the non root cgroups. Allowed
        maximum usage of the resources in the cgroup and its children.::

	  $ cat misc.max
	  res_a max
	  res_b 4

	Limit can be set by::

	  # echo res_a 1 > misc.max

	Limit can be set to max by::

	  # echo res_a max > misc.max

        Limits can be set higher than the capacity value in the misc.capacity
        file.

  misc.events
	A read-only flat-keyed file which exists on non-root cgroups. The
	following entries are defined. Unless specified otherwise, a value
	change in this file generates a file modified event. All fields in
	this file are hierarchical.

	  max
		The number of times the cgroup's resource usage was
		about to go over the max boundary.

  misc.events.local
        Similar to misc.events but the fields in the file are local to the
        cgroup i.e. not hierarchical. The file modified event generated on
        this file reflects only the local events.

Migration and Ownership
~~~~~~~~~~~~~~~~~~~~~~~

A miscellaneous scalar resource is charged to the cgroup in which it is used
first, and stays charged to that cgroup until that resource is freed. Migrating
a process to a different cgroup does not move the charge to the destination
cgroup where the process has moved.

Others
------

perf_event
~~~~~~~~~~

perf_event controller, if not mounted on a legacy hierarchy, is
automatically enabled on the v2 hierarchy so that perf events can
always be filtered by cgroup v2 path.  The controller can still be
moved to a legacy hierarchy after v2 hierarchy is populated.

Non-normative information
-------------------------

This section contains information that isn't considered to be a part of
the stable kernel API and so is subject to change.

CPU controller root cgroup process behaviour
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When distributing CPU cycles in the root cgroup each thread in this
cgroup is treated as if it was hosted in a separate child cgroup of the
root cgroup. This child cgroup weight is dependent on its thread nice
level.

For details of this mapping see sched_prio_to_weight array in
kernel/sched/core.c file (values from this array should be scaled
appropriately so the neutral - nice 0 - value is 100 instead of 1024).

IO controller root cgroup process behaviour
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Root cgroup processes are hosted in an implicit leaf child node.
When distributing IO resources this implicit child node is taken into
account as if it was a normal child cgroup of the root cgroup with a
weight value of 200.

Namespace
=========

Basics
------

cgroup namespace provides a mechanism to virtualize the view of the
"/proc/$PID/cgroup" file and cgroup mounts.  The CLONE_NEWCGROUP clone
flag can be used with clone(2) and unshare(2) to create a new cgroup
namespace.  The process running inside the cgroup namespace will have
its "/proc/$PID/cgroup" output restricted to cgroupns root.  The
cgroupns root is the cgroup of the process at the time of creation of
the cgroup namespace.

Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
complete path of the cgroup of a process.  In a container setup where
a set of cgroups and namespaces are intended to isolate processes the
"/proc/$PID/cgroup" file may leak potential system level information
to the isolated processes.  For example::

  # cat /proc/self/cgroup
  0::/batchjobs/container_id1

The path '/batchjobs/container_id1' can be considered as system-data
and undesirable to expose to the isolated processes.  cgroup namespace
can be used to restrict visibility of this path.  For example, before
creating a cgroup namespace, one would see::

  # ls -l /proc/self/ns/cgroup
  lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
  # cat /proc/self/cgroup
  0::/batchjobs/container_id1

After unsharing a new namespace, the view changes::

  # ls -l /proc/self/ns/cgroup
  lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
  # cat /proc/self/cgroup
  0::/

When some thread from a multi-threaded process unshares its cgroup
namespace, the new cgroupns gets applied to the entire process (all
the threads).  This is natural for the v2 hierarchy; however, for the
legacy hierarchies, this may be unexpected.

A cgroup namespace is alive as long as there are processes inside or
mounts pinning it.  When the last usage goes away, the cgroup
namespace is destroyed.  The cgroupns root and the actual cgroups
remain.

The Root and Views
------------------

The 'cgroupns root' for a cgroup namespace is the cgroup in which the
process calling unshare(2) is running.  For example, if a process in
/batchjobs/container_id1 cgroup calls unshare, cgroup
/batchjobs/container_id1 becomes the cgroupns root.  For the
init_cgroup_ns, this is the real root ('/') cgroup.

The cgroupns root cgroup does not change even if the namespace creator
process later moves to a different cgroup::

  # ~/unshare -c # unshare cgroupns in some cgroup
  # cat /proc/self/cgroup
  0::/
  # mkdir sub_cgrp_1
  # echo 0 > sub_cgrp_1/cgroup.procs
  # cat /proc/self/cgroup
  0::/sub_cgrp_1

Each process gets its namespace-specific view of "/proc/$PID/cgroup"

Processes running inside the cgroup namespace will be able to see
cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
From within an unshared cgroupns::

  # sleep 100000 &
  [1] 7353
  # echo 7353 > sub_cgrp_1/cgroup.procs
  # cat /proc/7353/cgroup
  0::/sub_cgrp_1

From the initial cgroup namespace, the real cgroup path will be
visible::

  $ cat /proc/7353/cgroup
  0::/batchjobs/container_id1/sub_cgrp_1

From a sibling cgroup namespace (that is, a namespace rooted at a
different cgroup), the cgroup path relative to its own cgroup
namespace root will be shown.  For instance, if PID 7353's cgroup
namespace root is at '/batchjobs/container_id2', then it will see::

  # cat /proc/7353/cgroup
  0::/../container_id2/sub_cgrp_1

Note that the relative path always starts with '/' to indicate that
its relative to the cgroup namespace root of the caller.

Migration and setns(2)
----------------------

Processes inside a cgroup namespace can move into and out of the
namespace root if they have proper access to external cgroups.  For
example, from inside a namespace with cgroupns root at
/batchjobs/container_id1, and assuming that the global hierarchy is
still accessible inside cgroupns::

  # cat /proc/7353/cgroup
  0::/sub_cgrp_1
  # echo 7353 > batchjobs/container_id2/cgroup.procs
  # cat /proc/7353/cgroup
  0::/../container_id2

Note that this kind of setup is not encouraged.  A task inside cgroup
namespace should only be exposed to its own cgroupns hierarchy.

setns(2) to another cgroup namespace is allowed when:

(a) the process has CAP_SYS_ADMIN against its current user namespace
(b) the process has CAP_SYS_ADMIN against the target cgroup
    namespace's userns

No implicit cgroup changes happen with attaching to another cgroup
namespace.  It is expected that the someone moves the attaching
process under the target cgroup namespace root.

Interaction with Other Namespaces
---------------------------------

Namespace specific cgroup hierarchy can be mounted by a process
running inside a non-init cgroup namespace::

  # mount -t cgroup2 none $MOUNT_POINT

This will mount the unified cgroup hierarchy with cgroupns root as the
filesystem root.  The process needs CAP_SYS_ADMIN against its user and
mount namespaces.

The virtualization of /proc/self/cgroup file combined with restricting
the view of cgroup hierarchy by namespace-private cgroupfs mount
provides a properly isolated cgroup view inside the container.

Information on Kernel Programming
=================================

This section contains kernel programming information in the areas
where interacting with cgroup is necessary.  cgroup core and
controllers are not covered.

Filesystem Support for Writeback
--------------------------------

A filesystem can support cgroup writeback by updating
address_space_operations->writepage[s]() to annotate bio's using the
following two functions.

  wbc_init_bio(@wbc, @bio)
	Should be called for each bio carrying writeback data and
	associates the bio with the inode's owner cgroup and the
	corresponding request queue.  This must be called after
	a queue (device) has been associated with the bio and
	before submission.

  wbc_account_cgroup_owner(@wbc, @folio, @bytes)
	Should be called for each data segment being written out.
	While this function doesn't care exactly when it's called
	during the writeback session, it's the easiest and most
	natural to call it as data segments are added to a bio.

With writeback bio's annotated, cgroup support can be enabled per
super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
selective disabling of cgroup writeback support which is helpful when
certain filesystem features, e.g. journaled data mode, are
incompatible.

wbc_init_bio() binds the specified bio to its cgroup.  Depending on
the configuration, the bio may be executed at a lower priority and if
the writeback session is holding shared resources, e.g. a journal
entry, may lead to priority inversion.  There is no one easy solution
for the problem.  Filesystems can try to work around specific problem
cases by skipping wbc_init_bio() and using bio_associate_blkg()
directly.

Deprecated v1 Core Features
===========================

- Multiple hierarchies including named ones are not supported.

- All v1 mount options are not supported.

- The "tasks" file is removed and "cgroup.procs" is not sorted.

- "cgroup.clone_children" is removed.

- /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" or
  "cgroup.stat" files at the root instead.

Issues with v1 and Rationales for v2
====================================

Multiple Hierarchies
--------------------

cgroup v1 allowed an arbitrary number of hierarchies and each
hierarchy could host any number of controllers.  While this seemed to
provide a high level of flexibility, it wasn't useful in practice.

For example, as there is only one instance of each controller, utility
type controllers such as freezer which can be useful in all
hierarchies could only be used in one.  The issue is exacerbated by
the fact that controllers couldn't be moved to another hierarchy once
hierarchies were populated.  Another issue was that all controllers
bound to a hierarchy were forced to have exactly the same view of the
hierarchy.  It wasn't possible to vary the granularity depending on
the specific controller.

In practice, these issues heavily limited which controllers could be
put on the same hierarchy and most configurations resorted to putting
each controller on its own hierarchy.  Only closely related ones, such
as the cpu and cpuacct controllers, made sense to be put on the same
hierarchy.  This often meant that userland ended up managing multiple
similar hierarchies repeating the same steps on each hierarchy
whenever a hierarchy management operation was necessary.

Furthermore, support for multiple hierarchies came at a steep cost.
It greatly complicated cgroup core implementation but more importantly
the support for multiple hierarchies restricted how cgroup could be
used in general and what controllers was able to do.

There was no limit on how many hierarchies there might be, which meant
that a thread's cgroup membership couldn't be described in finite
length.  The key might contain any number of entries and was unlimited
in length, which made it highly awkward to manipulate and led to
addition of controllers which existed only to identify membership,
which in turn exacerbated the original problem of proliferating number
of hierarchies.

Also, as a controller couldn't have any expectation regarding the
topologies of hierarchies other controllers might be on, each
controller had to assume that all other controllers were attached to
completely orthogonal hierarchies.  This made it impossible, or at
least very cumbersome, for controllers to cooperate with each other.

In most use cases, putting controllers on hierarchies which are
completely orthogonal to each other isn't necessary.  What usually is
called for is the ability to have differing levels of granularity
depending on the specific controller.  In other words, hierarchy may
be collapsed from leaf towards root when viewed from specific
controllers.  For example, a given configuration might not care about
how memory is distributed beyond a certain level while still wanting
to control how CPU cycles are distributed.

Thread Granularity
------------------

cgroup v1 allowed threads of a process to belong to different cgroups.
This didn't make sense for some controllers and those controllers
ended up implementing different ways to ignore such situations but
much more importantly it blurred the line between API exposed to
individual applications and system management interface.

Generally, in-process knowledge is available only to the process
itself; thus, unlike service-level organization of processes,
categorizing threads of a process requires active participation from
the application which owns the target process.

cgroup v1 had an ambiguously defined delegation model which got abused
in combination with thread granularity.  cgroups were delegated to
individual applications so that they can create and manage their own
sub-hierarchies and control resource distributions along them.  This
effectively raised cgroup to the status of a syscall-like API exposed
to lay programs.

First of all, cgroup has a fundamentally inadequate interface to be
exposed this way.  For a process to access its own knobs, it has to
extract the path on the target hierarchy from /proc/self/cgroup,
construct the path by appending the name of the knob to the path, open
and then read and/or write to it.  This is not only extremely clunky
and unusual but also inherently racy.  There is no conventional way to
define transaction across the required steps and nothing can guarantee
that the process would actually be operating on its own sub-hierarchy.

cgroup controllers implemented a number of knobs which would never be
accepted as public APIs because they were just adding control knobs to
system-management pseudo filesystem.  cgroup ended up with interface
knobs which were not properly abstracted or refined and directly
revealed kernel internal details.  These knobs got exposed to
individual applications through the ill-defined delegation mechanism
effectively abusing cgroup as a shortcut to implementing public APIs
without going through the required scrutiny.

This was painful for both userland and kernel.  Userland ended up with
misbehaving and poorly abstracted interfaces and kernel exposing and
locked into constructs inadvertently.

Competition Between Inner Nodes and Threads
-------------------------------------------

cgroup v1 allowed threads to be in any cgroups which created an
interesting problem where threads belonging to a parent cgroup and its
children cgroups competed for resources.  This was nasty as two
different types of entities competed and there was no obvious way to
settle it.  Different controllers did different things.

The cpu controller considered threads and cgroups as equivalents and
mapped nice levels to cgroup weights.  This worked for some cases but
fell flat when children wanted to be allocated specific ratios of CPU
cycles and the number of internal threads fluctuated - the ratios
constantly changed as the number of competing entities fluctuated.
There also were other issues.  The mapping from nice level to weight
wasn't obvious or universal, and there were various other knobs which
simply weren't available for threads.

The io controller implicitly created a hidden leaf node for each
cgroup to host the threads.  The hidden leaf had its own copies of all
the knobs with ``leaf_`` prefixed.  While this allowed equivalent
control over internal threads, it was with serious drawbacks.  It
always added an extra layer of nesting which wouldn't be necessary
otherwise, made the interface messy and significantly complicated the
implementation.

The memory controller didn't have a way to control what happened
between internal tasks and child cgroups and the behavior was not
clearly defined.  There were attempts to add ad-hoc behaviors and
knobs to tailor the behavior to specific workloads which would have
led to problems extremely difficult to resolve in the long term.

Multiple controllers struggled with internal tasks and came up with
different ways to deal with it; unfortunately, all the approaches were
severely flawed and, furthermore, the widely different behaviors
made cgroup as a whole highly inconsistent.

This clearly is a problem which needs to be addressed from cgroup core
in a uniform way.

Other Interface Issues
----------------------

cgroup v1 grew without oversight and developed a large number of
idiosyncrasies and inconsistencies.  One issue on the cgroup core side
was how an empty cgroup was notified - a userland helper binary was
forked and executed for each event.  The event delivery wasn't
recursive or delegatable.  The limitations of the mechanism also led
to in-kernel event delivery filtering mechanism further complicating
the interface.

Controller interfaces were problematic too.  An extreme example is
controllers completely ignoring hierarchical organization and treating
all cgroups as if they were all located directly under the root
cgroup.  Some controllers exposed a large amount of inconsistent
implementation details to userland.

There also was no consistency across controllers.  When a new cgroup
was created, some controllers defaulted to not imposing extra
restrictions while others disallowed any resource usage until
explicitly configured.  Configuration knobs for the same type of
control used widely differing naming schemes and formats.  Statistics
and information knobs were named arbitrarily and used different
formats and units even in the same controller.

cgroup v2 establishes common conventions where appropriate and updates
controllers so that they expose minimal and consistent interfaces.

Controller Issues and Remedies
------------------------------

Memory
~~~~~~

The original lower boundary, the soft limit, is defined as a limit
that is per default unset.  As a result, the set of cgroups that
global reclaim prefers is opt-in, rather than opt-out.  The costs for
optimizing these mostly negative lookups are so high that the
implementation, despite its enormous size, does not even provide the
basic desirable behavior.  First off, the soft limit has no
hierarchical meaning.  All configured groups are organized in a global
rbtree and treated like equal peers, regardless where they are located
in the hierarchy.  This makes subtree delegation impossible.  Second,
the soft limit reclaim pass is so aggressive that it not just
introduces high allocation latencies into the system, but also impacts
system performance due to overreclaim, to the point where the feature
becomes self-defeating.

The memory.low boundary on the other hand is a top-down allocated
reserve.  A cgroup enjoys reclaim protection when it's within its
effective low, which makes delegation of subtrees possible. It also
enjoys having reclaim pressure proportional to its overage when
above its effective low.

The original high boundary, the hard limit, is defined as a strict
limit that can not budge, even if the OOM killer has to be called.
But this generally goes against the goal of making the most out of the
available memory.  The memory consumption of workloads varies during
runtime, and that requires users to overcommit.  But doing that with a
strict upper limit requires either a fairly accurate prediction of the
working set size or adding slack to the limit.  Since working set size
estimation is hard and error prone, and getting it wrong results in
OOM kills, most users tend to err on the side of a looser limit and
end up wasting precious resources.

The memory.high boundary on the other hand can be set much more
conservatively.  When hit, it throttles allocations by forcing them
into direct reclaim to work off the excess, but it never invokes the
OOM killer.  As a result, a high boundary that is chosen too
aggressively will not terminate the processes, but instead it will
lead to gradual performance degradation.  The user can monitor this
and make corrections until the minimal memory footprint that still
gives acceptable performance is found.

In extreme cases, with many concurrent allocations and a complete
breakdown of reclaim progress within the group, the high boundary can
be exceeded.  But even then it's mostly better to satisfy the
allocation from the slack available in other groups or the rest of the
system than killing the group.  Otherwise, memory.max is there to
limit this type of spillover and ultimately contain buggy or even
malicious applications.

Setting the original memory.limit_in_bytes below the current usage was
subject to a race condition, where concurrent charges could cause the
limit setting to fail. memory.max on the other hand will first set the
limit to prevent new charges, and then reclaim and OOM kill until the
new limit is met - or the task writing to memory.max is killed.

The combined memory+swap accounting and limiting is replaced by real
control over swap space.

The main argument for a combined memory+swap facility in the original
cgroup design was that global or parental pressure would always be
able to swap all anonymous memory of a child group, regardless of the
child's own (possibly untrusted) configuration.  However, untrusted
groups can sabotage swapping by other means - such as referencing its
anonymous memory in a tight loop - and an admin can not assume full
swappability when overcommitting untrusted jobs.

For trusted jobs, on the other hand, a combined counter is not an
intuitive userspace interface, and it flies in the face of the idea
that cgroup controllers should account and limit specific physical
resources.  Swap space is a resource like all others in the system,
and that's why unified hierarchy allows distributing it separately.

cgroup v2                    Control Group v2                    Date  October  2015  Author  Tejun Heo  tj kernel org   This is the authoritative documentation on the design  interface and conventions of cgroup v2   It describes all userland visible aspects of cgroup including core and specific controller behaviors   All future changes must be reflected in this document   Documentation for v1 is available under  ref  Documentation admin guide cgroup v1 index rst  cgroup v1        CONTENTS     1  Introduction      1 1  Terminology      1 2  What is cgroup     2  Basic Operations      2 1  Mounting      2 2  Organizing Processes and Threads        2 2 1  Processes        2 2 2  Threads      2 3   Un populated Notification      2 4  Controlling Controllers        2 4 1  Enabling and Disabling        2 4 2  Top down Constraint        2 4 3  No Internal Process Constraint      2 5  Delegation        2 5 1  Model of Delegation        2 5 2  Delegation Containment      2 6  Guidelines        2 6 1  Organize Once and Control        2 6 2  Avoid Name Collisions    3  Resource Distribution Models      3 1  Weights      3 2  Limits      3 3  Protections      3 4  Allocations    4  Interface Files      4 1  Format      4 2  Conventions      4 3  Core Interface Files    5  Controllers      5 1  CPU        5 1 1  CPU Interface Files      5 2  Memory        5 2 1  Memory Interface Files        5 2 2  Usage Guidelines        5 2 3  Memory Ownership      5 3  IO        5 3 1  IO Interface Files        5 3 2  Writeback        5 3 3  IO Latency          5 3 3 1  How IO Latency Throttling Works          5 3 3 2  IO Latency Interface Files        5 3 4  IO Priority      5 4  PID        5 4 1  PID Interface Files      5 5  Cpuset        5 5 1  Cpuset Interface Files      5 6  Device      5 7  RDMA        5 7 1  RDMA Interface Files      5 8  HugeTLB        5 8 1  HugeTLB Interface Files      5 9  Misc        5 9 1 Miscellaneous cgroup Interface Files        5 9 2 Migration and Ownership      5 10  Others        5 10 1  perf event      5 N  Non normative information        5 N 1  CPU controller root cgroup process behaviour        5 N 2  IO controller root cgroup process behaviour    6  Namespace      6 1  Basics      6 2  The Root and Views      6 3  Migration and setns 2       6 4  Interaction with Other Namespaces    P  Information on Kernel Programming      P 1  Filesystem Support for Writeback    D  Deprecated v1 Core Features    R  Issues with v1 and Rationales for v2      R 1  Multiple Hierarchies      R 2  Thread Granularity      R 3  Competition Between Inner Nodes and Threads      R 4  Other Interface Issues      R 5  Controller Issues and Remedies        R 5 1  Memory   Introduction               Terminology               cgroup  stands for  control group  and is never capitalized   The singular form is used to designate the whole feature and also as a qualifier as in  cgroup controllers    When explicitly referring to multiple individual control groups  the plural form  cgroups  is used    What is cgroup                   cgroup is a mechanism to organize processes hierarchically and distribute system resources along the hierarchy in a controlled and configurable manner   cgroup is largely composed of two parts   the core and controllers  cgroup core is primarily responsible for hierarchically organizing processes   A cgroup controller is usually responsible for distributing a specific type of system resource along the hierarchy although there are utility controllers which serve purposes other than resource distribution   cgroups form a tree structure and every process in the system belongs to one and only one cgroup   All threads of a process belong to the same cgroup   On creation  all processes are put in the cgroup that the parent process belongs to at the time   A process can be migrated to another cgroup   Migration of a process doesn t affect already existing descendant processes   Following certain structural constraints  controllers may be enabled or disabled selectively on a cgroup   All controller behaviors are hierarchical   if a controller is enabled on a cgroup  it affects all processes which belong to the cgroups consisting the inclusive sub hierarchy of the cgroup   When a controller is enabled on a nested cgroup  it always restricts the resource distribution further   The restrictions set closer to the root in the hierarchy can not be overridden from further away    Basic Operations                   Mounting           Unlike v1  cgroup v2 has only single hierarchy   The cgroup v2 hierarchy can be mounted with the following mount command        mount  t cgroup2 none  MOUNT POINT  cgroup2 filesystem has the magic number 0x63677270   cgrp     All controllers which support v2 and are not bound to a v1 hierarchy are automatically bound to the v2 hierarchy and show up at the root  Controllers which are not in active use in the v2 hierarchy can be bound to other hierarchies   This allows mixing v2 hierarchy with the legacy v1 multiple hierarchies in a fully backward compatible way   A controller can be moved across hierarchies only after the controller is no longer referenced in its current hierarchy   Because per cgroup controller states are destroyed asynchronously and controllers may have lingering references  a controller may not show up immediately on the v2 hierarchy after the final umount of the previous hierarchy  Similarly  a controller should be fully disabled to be moved out of the unified hierarchy and it may take some time for the disabled controller to become available for other hierarchies  furthermore  due to inter controller dependencies  other controllers may need to be disabled too   While useful for development and manual configurations  moving controllers dynamically between the v2 and other hierarchies is strongly discouraged for production use   It is recommended to decide the hierarchies and controller associations before starting using the controllers after system boot   During transition to v2  system management software might still automount the v1 cgroup filesystem and so hijack all controllers during boot  before manual intervention is possible  To make testing and experimenting easier  the kernel parameter cgroup no v1  allows disabling controllers in v1 and make them always available in v2   cgroup v2 currently supports the following mount options     nsdelegate  Consider cgroup namespaces as delegation boundaries   This  option is system wide and can only be set on mount or modified  through remount from the init namespace   The mount option is  ignored on non init namespace mounts   Please refer to the  Delegation section for details     favordynmods         Reduce the latencies of dynamic cgroup modifications such as         task migrations and controller on offs at the cost of making         hot path operations such as forks and exits more expensive          The static usage pattern of creating a cgroup  enabling         controllers  and then seeding it with CLONE INTO CGROUP is         not affected by this option     memory localevents         Only populate memory events with data for the current cgroup          and not any subtrees  This is legacy behaviour  the default         behaviour without this option is to include subtree counts          This option is system wide and can only be set on mount or         modified through remount from the init namespace  The mount         option is ignored on non init namespace mounts     memory recursiveprot         Recursively apply memory min and memory low protection to         entire subtrees  without requiring explicit downward         propagation into leaf cgroups   This allows protecting entire         subtrees from one another  while retaining free competition         within those subtrees   This should have been the default         behavior but is a mount option to avoid regressing setups         relying on the original semantics  e g  specifying bogusly         high  bypass  protection values at higher tree levels      memory hugetlb accounting         Count HugeTLB memory usage towards the cgroup s overall         memory usage for the memory controller  for the purpose of         statistics reporting and memory protetion   This is a new         behavior that could regress existing setups  so it must be         explicitly opted in with this mount option           A few caveats to keep in mind             There is no HugeTLB pool management involved in the memory           controller  The pre allocated pool does not belong to anyone            Specifically  when a new HugeTLB folio is allocated to           the pool  it is not accounted for from the perspective of the           memory controller  It is only charged to a cgroup when it is           actually used  for e g at page fault time   Host memory           overcommit management has to consider this when configuring           hard limits  In general  HugeTLB pool management should be           done via other mechanisms  such as the HugeTLB controller             Failure to charge a HugeTLB folio to the memory controller           results in SIGBUS  This could happen even if the HugeTLB pool           still has pages available  but the cgroup limit is hit and           reclaim attempt fails             Charging HugeTLB memory towards the memory controller affects           memory protection and reclaim dynamics  Any userspace tuning            of low  min limits for e g  needs to take this into account            HugeTLB pages utilized while this option is not selected           will not be tracked by the memory controller  even if cgroup           v2 is remounted later on      pids localevents         The option restores v1 like behavior of pids events max  that is only         local  inside cgroup proper  fork failures are counted  Without this         option pids events max represents any pids max enforcemnt across         cgroup s subtree     Organizing Processes and Threads                                   Processes            Initially  only the root cgroup exists to which all processes belong  A child cgroup can be created by creating a sub directory        mkdir  CGROUP NAME  A given cgroup may have multiple child cgroups forming a tree structure   Each cgroup has a read writable interface file  cgroup procs    When read  it lists the PIDs of all processes which belong to the cgroup one per line   The PIDs are not ordered and the same PID may show up more than once if the process got moved to another cgroup and then back or the PID got recycled while reading   A process can be migrated into a cgroup by writing its PID to the target cgroup s  cgroup procs  file   Only one process can be migrated on a single write 2  call   If a process is composed of multiple threads  writing the PID of any thread migrates all threads of the process   When a process forks a child process  the new process is born into the cgroup that the forking process belongs to at the time of the operation   After exit  a process stays associated with the cgroup that it belonged to at the time of exit until it s reaped  however  a zombie process does not appear in  cgroup procs  and thus can t be moved to another cgroup   A cgroup which doesn t have any children or live processes can be destroyed by removing the directory   Note that a cgroup which doesn t have any children and is associated only with zombie processes is considered empty and can be removed        rmdir  CGROUP NAME    proc  PID cgroup  lists a process s cgroup membership   If legacy cgroup is in use in the system  this file may contain multiple lines  one for each hierarchy   The entry for cgroup v2 is always in the format  0   PATH         cat  proc 842 cgroup         0   test cgroup test cgroup nested  If the process becomes a zombie and the cgroup it was associated with is removed subsequently     deleted   is appended to the path        cat  proc 842 cgroup         0   test cgroup test cgroup nested  deleted    Threads          cgroup v2 supports thread granularity for a subset of controllers to support use cases requiring hierarchical resource distribution across the threads of a group of processes   By default  all threads of a process belong to the same cgroup  which also serves as the resource domain to host resource consumptions which are not specific to a process or thread   The thread mode allows threads to be spread across a subtree while still maintaining the common resource domain for them   Controllers which support thread mode are called threaded controllers  The ones which don t are called domain controllers   Marking a cgroup threaded makes it join the resource domain of its parent as a threaded cgroup   The parent may be another threaded cgroup whose resource domain is further up in the hierarchy   The root of a threaded subtree  that is  the nearest ancestor which is not threaded  is called threaded domain or thread root interchangeably and serves as the resource domain for the entire subtree   Inside a threaded subtree  threads of a process can be put in different cgroups and are not subject to the no internal process constraint   threaded controllers can be enabled on non leaf cgroups whether they have threads in them or not   As the threaded domain cgroup hosts all the domain resource consumptions of the subtree  it is considered to have internal resource consumptions whether there are processes in it or not and can t have populated child cgroups which aren t threaded   Because the root cgroup is not subject to no internal process constraint  it can serve both as a threaded domain and a parent to domain cgroups   The current operation mode or type of the cgroup is shown in the  cgroup type  file which indicates whether the cgroup is a normal domain  a domain which is serving as the domain of a threaded subtree  or a threaded cgroup   On creation  a cgroup is always a domain cgroup and can be made threaded by writing  threaded  to the  cgroup type  file   The operation is single direction        echo threaded   cgroup type  Once threaded  the cgroup can t be made a domain again   To enable the thread mode  the following conditions must be met     As the cgroup will join the parent s resource domain   The parent   must either be a valid  threaded  domain or a threaded cgroup     When the parent is an unthreaded domain  it must not have any domain   controllers enabled or populated domain children   The root is   exempt from this requirement   Topology wise  a cgroup can be in an invalid state   Please consider the following topology      A  threaded domain    B  threaded    C  domain  just created   C is created as a domain but isn t connected to a parent which can host child domains   C can t be used until it is turned into a threaded cgroup    cgroup type  file will report  domain  invalid   in these cases   Operations which fail due to invalid topology use EOPNOTSUPP as the errno   A domain cgroup is turned into a threaded domain when one of its child cgroup becomes threaded or threaded controllers are enabled in the  cgroup subtree control  file while there are processes in the cgroup  A threaded domain reverts to a normal domain when the conditions clear   When read   cgroup threads  contains the list of the thread IDs of all threads in the cgroup   Except that the operations are per thread instead of per process   cgroup threads  has the same format and behaves the same way as  cgroup procs    While  cgroup threads  can be written to in any cgroup  as it can only move threads inside the same threaded domain  its operations are confined inside each threaded subtree   The threaded domain cgroup serves as the resource domain for the whole subtree  and  while the threads can be scattered across the subtree  all the processes are considered to be in the threaded domain cgroup   cgroup procs  in a threaded domain cgroup contains the PIDs of all processes in the subtree and is not readable in the subtree proper  However   cgroup procs  can be written to from anywhere in the subtree to migrate all threads of the matching process to the cgroup   Only threaded controllers can be enabled in a threaded subtree   When a threaded controller is enabled inside a threaded subtree  it only accounts for and controls resource consumptions associated with the threads in the cgroup and its descendants   All consumptions which aren t tied to a specific thread belong to the threaded domain cgroup   Because a threaded subtree is exempt from no internal process constraint  a threaded controller must be able to handle competition between threads in a non leaf cgroup and its child cgroups   Each threaded controller defines how such competitions are handled   Currently  the following controllers are threaded and can be enabled in a threaded cgroup      cpu   cpuset   perf event   pids   Un populated Notification                             Each non root cgroup has a  cgroup events  file which contains  populated  field indicating whether the cgroup s sub hierarchy has live processes in it   Its value is 0 if there is no live process in the cgroup and its descendants  otherwise  1   poll and  id notify events are triggered when the value changes   This can be used  for example  to start a clean up operation after all processes of a given sub hierarchy have exited   The populated state updates and notifications are recursive   Consider the following sub hierarchy where the numbers in the parentheses represent the numbers of processes in each cgroup      A 4    B 0    C 1                  D 0   A  B and C s  populated  fields would be 1 while D s 0   After the one process in C exits  B and C s  populated  fields would flip to  0  and file modified events will be generated on the  cgroup events  files of both cgroups    Controlling Controllers                          Enabling and Disabling                         Each cgroup has a  cgroup controllers  file which lists all controllers available for the cgroup to enable        cat cgroup controllers   cpu io memory  No controller is enabled by default   Controllers can be enabled and disabled by writing to the  cgroup subtree control  file        echo   cpu  memory  io    cgroup subtree control  Only controllers which are listed in  cgroup controllers  can be enabled   When multiple operations are specified as above  either they all succeed or fail   If multiple operations on the same controller are specified  the last one is effective   Enabling a controller in a cgroup indicates that the distribution of the target resource across its immediate children will be controlled  Consider the following sub hierarchy   The enabled controllers are listed in parentheses      A cpu memory    B memory    C                                 D    As A has  cpu  and  memory  enabled  A will control the distribution of CPU cycles and memory to its children  in this case  B   As B has  memory  enabled but not  CPU   C and D will compete freely on CPU cycles but their division of memory available to B will be controlled   As a controller regulates the distribution of the target resource to the cgroup s children  enabling it creates the controller s interface files in the child cgroups   In the above example  enabling  cpu  on B would create the  cpu   prefixed controller interface files in C and D   Likewise  disabling  memory  from B would remove the  memory   prefixed controller interface files from C and D   This means that the controller interface files   anything which doesn t start with  cgroup   are owned by the parent rather than the cgroup itself    Top down Constraint                      Resources are distributed top down and a cgroup can further distribute a resource only if the resource has been distributed to it from the parent   This means that all non root  cgroup subtree control  files can only contain controllers which are enabled in the parent s  cgroup subtree control  file   A controller can be enabled only if the parent has the controller enabled and a controller can t be disabled if one or more children have it enabled    No Internal Process Constraint                                 Non root cgroups can distribute domain resources to their children only when they don t have any processes of their own   In other words  only domain cgroups which don t contain any processes can have domain controllers enabled in their  cgroup subtree control  files   This guarantees that  when a domain controller is looking at the part of the hierarchy which has it enabled  processes are always only on the leaves   This rules out situations where child cgroups compete against internal processes of the parent   The root cgroup is exempt from this restriction   Root contains processes and anonymous resource consumption which can t be associated with any other cgroups and requires special treatment from most controllers   How resource consumption in the root cgroup is governed is up to each controller  for more information on this topic please refer to the Non normative information section in the Controllers chapter    Note that the restriction doesn t get in the way if there is no enabled controller in the cgroup s  cgroup subtree control    This is important as otherwise it wouldn t be possible to create children of a populated cgroup   To control resource distribution of a cgroup  the cgroup must create children and transfer all its processes to the children before enabling controllers in its  cgroup subtree control  file    Delegation             Model of Delegation                      A cgroup can be delegated in two ways   First  to a less privileged user by granting write access of the directory and its  cgroup procs    cgroup threads  and  cgroup subtree control  files to the user  Second  if the  nsdelegate  mount option is set  automatically to a cgroup namespace on namespace creation   Because the resource control interface files in a given directory control the distribution of the parent s resources  the delegatee shouldn t be allowed to write to them   For the first method  this is achieved by not granting access to these files   For the second  files outside the namespace should be hidden from the delegatee by the means of at least mount namespacing  and the kernel rejects writes to all files on a namespace root from inside the cgroup namespace  except for those files listed in   sys kernel cgroup delegate   including  cgroup procs    cgroup threads    cgroup subtree control   etc     The end results are equivalent for both delegation types   Once delegated  the user can build sub hierarchy under the directory  organize processes inside it as it sees fit and further distribute the resources it received from the parent   The limits and other settings of all resource controllers are hierarchical and regardless of what happens in the delegated sub hierarchy  nothing can escape the resource restrictions imposed by the parent   Currently  cgroup doesn t impose any restrictions on the number of cgroups in or nesting depth of a delegated sub hierarchy  however  this may be limited explicitly in the future    Delegation Containment                         A delegated sub hierarchy is contained in the sense that processes can t be moved into or out of the sub hierarchy by the delegatee   For delegations to a less privileged user  this is achieved by requiring the following conditions for a process with a non root euid to migrate a target process into a cgroup by writing its PID to the  cgroup procs  file     The writer must have write access to the  cgroup procs  file     The writer must have write access to the  cgroup procs  file of the   common ancestor of the source and destination cgroups   The above two constraints ensure that while a delegatee may migrate processes around freely in the delegated sub hierarchy it can t pull in from or push out to outside the sub hierarchy   For an example  let s assume cgroups C0 and C1 have been delegated to user U0 who created C00  C01 under C0 and C10 under C1 as follows and all processes under C0 and C1 belong to U0                      C0   C00     cgroup             C01     hierarchy                     C1   C10  Let s also say U0 wants to write the PID of a process which is currently in C10 into  C00 cgroup procs    U0 has write access to the file  however  the common ancestor of the source cgroup C10 and the destination cgroup C00 is above the points of delegation and U0 would not have write access to its  cgroup procs  files and thus the write will be denied with  EACCES   For delegations to namespaces  containment is achieved by requiring that both the source and destination cgroups are reachable from the namespace of the process which is attempting the migration   If either is not reachable  the migration is rejected with  ENOENT    Guidelines             Organize Once and Control                            Migrating a process across cgroups is a relatively expensive operation and stateful resources such as memory are not moved together with the process   This is an explicit design decision as there often exist inherent trade offs between migration and various hot paths in terms of synchronization cost   As such  migrating processes across cgroups frequently as a means to apply different resource restrictions is discouraged   A workload should be assigned to a cgroup according to the system s logical and resource structure once on start up   Dynamic adjustments to resource distribution can be made by changing controller configuration through the interface files    Avoid Name Collisions                        Interface files for a cgroup and its children cgroups occupy the same directory and it is possible to create children cgroups which collide with interface files   All cgroup core interface files are prefixed with  cgroup   and each controller s interface files are prefixed with the controller name and a dot   A controller s name is composed of lower case alphabets and    s but never begins with an     so it can be used as the prefix character for collision avoidance   Also  interface file names won t start or end with terms which are often used in categorizing workloads such as job  service  slice  unit or workload   cgroup doesn t do anything to prevent name collisions and it s the user s responsibility to avoid them    Resource Distribution Models                               cgroup controllers implement several resource distribution schemes depending on the resource type and expected use cases   This section describes major schemes in use along with their expected behaviors    Weights          A parent s resource is distributed by adding up the weights of all active children and giving each the fraction matching the ratio of its weight against the sum   As only children which can make use of the resource at the moment participate in the distribution  this is work conserving   Due to the dynamic nature  this model is usually used for stateless resources   All weights are in the range  1  10000  with the default at 100   This allows symmetric multiplicative biases in both directions at fine enough granularity while staying in the intuitive range   As long as the weight is in range  all configuration combinations are valid and there is no reason to reject configuration changes or process migrations    cpu weight  proportionally distributes CPU cycles to active children and is an example of this type        cgroupv2 limits distributor   Limits         A child can only consume up to the configured amount of the resource  Limits can be over committed   the sum of the limits of children can exceed the amount of resource available to the parent   Limits are in the range  0  max  and defaults to  max   which is noop   As limits can be over committed  all configuration combinations are valid and there is no reason to reject configuration changes or process migrations    io max  limits the maximum BPS and or IOPS that a cgroup can consume on an IO device and is an example of this type       cgroupv2 protections distributor   Protections              A cgroup is protected up to the configured amount of the resource as long as the usages of all its ancestors are under their protected levels   Protections can be hard guarantees or best effort soft boundaries   Protections can also be over committed in which case only up to the amount available to the parent is protected among children   Protections are in the range  0  max  and defaults to 0  which is noop   As protections can be over committed  all configuration combinations are valid and there is no reason to reject configuration changes or process migrations    memory low  implements best effort memory protection and is an example of this type    Allocations              A cgroup is exclusively allocated a certain amount of a finite resource   Allocations can t be over committed   the sum of the allocations of children can not exceed the amount of resource available to the parent   Allocations are in the range  0  max  and defaults to 0  which is no resource   As allocations can t be over committed  some configuration combinations are invalid and should be rejected   Also  if the resource is mandatory for execution of processes  process migrations may be rejected    cpu rt max  hard allocates realtime slices and is an example of this type    Interface Files                  Format         All interface files should be in one of the following formats whenever possible      New line separated values    when only one value can be written at once    VAL0 n  VAL1 n         Space separated values    when read only or multiple values can be written at once    VAL0 VAL1     n    Flat keyed   KEY0 VAL0 n  KEY1 VAL1 n         Nested keyed   KEY0 SUB KEY0 VAL00 SUB KEY1 VAL01     KEY1 SUB KEY0 VAL10 SUB KEY1 VAL11          For a writable file  the format for writing should generally match reading  however  controllers may allow omitting later fields or implement restricted shortcuts for most common use cases   For both flat and nested keyed files  only the values for a single key can be written at a time   For nested keyed files  the sub key pairs may be specified in any order and not all pairs have to be specified    Conventions                Settings for a single feature should be contained in a single file     The root cgroup should be exempt from resource control and thus   shouldn t have resource control interface files     The default time unit is microseconds   If a different unit is ever   used  an explicit unit suffix must be present     A parts per quantity should use a percentage decimal with at least   two digit fractional part   e g  13 40     If a controller implements weight based resource distribution  its   interface file should be named  weight  and have the range  1    10000  with 100 as the default   The values are chosen to allow   enough and symmetric bias in both directions while keeping it   intuitive  the default is 100       If a controller implements an absolute resource guarantee and or   limit  the interface files should be named  min  and  max    respectively   If a controller implements best effort resource   guarantee and or limit  the interface files should be named  low    and  high  respectively     In the above four control files  the special token  max  should be   used to represent upward infinity for both reading and writing     If a setting has a configurable default value and keyed specific   overrides  the default entry should be keyed with  default  and   appear as the first entry in the file     The default value can be updated by writing either  default  VAL  or     VAL      When writing to update a specific override   default  can be used as   the value to indicate removal of the override   Override entries   with  default  as the value must not appear when read     For example  a setting which is keyed by major minor device numbers   with integer values may look like the following          cat cgroup example interface file     default 150     8 0 300    The default value can be updated by          echo 125   cgroup example interface file    or          echo  default 125    cgroup example interface file    An override can be set by          echo  8 16 170    cgroup example interface file    and cleared by          echo  8 0 default    cgroup example interface file       cat cgroup example interface file     default 125     8 16 170    For events which are not very high frequency  an interface file    events  should be created which lists event key value pairs    Whenever a notifiable event happens  file modified event should be   generated on the file    Core Interface Files                       All cgroup core files are prefixed with  cgroup      cgroup type  A read write single value file which exists on non root  cgroups    When read  it indicates the current type of the cgroup  which  can be one of the following values       domain    A normal valid domain cgroup       domain threaded    A threaded domain cgroup which is           serving as the root of a threaded subtree       domain invalid    A cgroup which is in an invalid state     It can t be populated or have controllers enabled   It may    be allowed to become a threaded cgroup       threaded    A threaded cgroup which is a member of a           threaded subtree    A cgroup can be turned into a threaded cgroup by writing   threaded  to this file     cgroup procs  A read write new line separated values file which exists on  all cgroups    When read  it lists the PIDs of all processes which belong to  the cgroup one per line   The PIDs are not ordered and the  same PID may show up more than once if the process got moved  to another cgroup and then back or the PID got recycled while  reading    A PID can be written to migrate the process associated with  the PID to the cgroup   The writer should match all of the  following conditions      It must have write access to the  cgroup procs  file      It must have write access to the  cgroup procs  file of the    common ancestor of the source and destination cgroups    When delegating a sub hierarchy  write access to this file  should be granted along with the containing directory    In a threaded cgroup  reading this file fails with EOPNOTSUPP  as all the processes belong to the thread root   Writing is  supported and moves every thread of the process to the cgroup     cgroup threads  A read write new line separated values file which exists on  all cgroups    When read  it lists the TIDs of all threads which belong to  the cgroup one per line   The TIDs are not ordered and the  same TID may show up more than once if the thread got moved to  another cgroup and then back or the TID got recycled while  reading    A TID can be written to migrate the thread associated with the  TID to the cgroup   The writer should match all of the  following conditions      It must have write access to the  cgroup threads  file      The cgroup that the thread is currently in must be in the           same resource domain as the destination cgroup      It must have write access to the  cgroup procs  file of the    common ancestor of the source and destination cgroups    When delegating a sub hierarchy  write access to this file  should be granted along with the containing directory     cgroup controllers  A read only space separated values file which exists on all  cgroups    It shows space separated list of all controllers available to  the cgroup   The controllers are not ordered     cgroup subtree control  A read write space separated values file which exists on all  cgroups   Starts out empty    When read  it shows space separated list of the controllers  which are enabled to control resource distribution from the  cgroup to its children    Space separated list of controllers prefixed with     or      can be written to enable or disable controllers   A controller  name prefixed with     enables the controller and      disables   If a controller appears more than once on the list   the last one is effective   When multiple enable and disable  operations are specified  either all succeed or all fail     cgroup events  A read only flat keyed file which exists on non root cgroups   The following entries are defined   Unless specified  otherwise  a value change in this file generates a file  modified event      populated   1 if the cgroup or its descendants contains any live   processes  otherwise  0     frozen   1 if the cgroup is frozen  otherwise  0     cgroup max descendants  A read write single value files   The default is  max     Maximum allowed number of descent cgroups   If the actual number of descendants is equal or larger   an attempt to create a new cgroup in the hierarchy will fail     cgroup max depth  A read write single value files   The default is  max     Maximum allowed descent depth below the current cgroup   If the actual descent depth is equal or larger   an attempt to create a new child cgroup will fail     cgroup stat  A read only flat keyed file with the following entries      nr descendants   Total number of visible descendant cgroups      nr dying descendants   Total number of dying descendant cgroups  A cgroup becomes   dying after being deleted by a user  The cgroup will remain   in dying state for some time undefined time  which can depend   on system load  before being completely destroyed     A process can t enter a dying cgroup under any circumstances    a dying cgroup can t revive     A dying cgroup can consume system resources not exceeding   limits  which were active at the moment of cgroup deletion      nr subsys  cgroup subsys    Total number of live cgroup subsystems  e g memory   cgroup  at and beneath the current cgroup      nr dying subsys  cgroup subsys    Total number of dying cgroup subsystems  e g  memory   cgroup  at and beneath the current cgroup     cgroup freeze  A read write single value file which exists on non root cgroups   Allowed values are  0  and  1   The default is  0     Writing  1  to the file causes freezing of the cgroup and all  descendant cgroups  This means that all belonging processes will  be stopped and will not run until the cgroup will be explicitly  unfrozen  Freezing of the cgroup may take some time  when this action  is completed  the  frozen  value in the cgroup events control file  will be updated to  1  and the corresponding notification will be  issued    A cgroup can be frozen either by its own settings  or by settings  of any ancestor cgroups  If any of ancestor cgroups is frozen  the  cgroup will remain frozen    Processes in the frozen cgroup can be killed by a fatal signal   They also can enter and leave a frozen cgroup  either by an explicit  move by a user  or if freezing of the cgroup races with fork     If a process is moved to a frozen cgroup  it stops  If a process is  moved out of a frozen cgroup  it becomes running    Frozen status of a cgroup doesn t affect any cgroup tree operations   it s possible to delete a frozen  and empty  cgroup  as well as  create new sub cgroups     cgroup kill  A write only single value file which exists in non root cgroups   The only allowed value is  1     Writing  1  to the file causes the cgroup and all descendant cgroups to  be killed  This means that all processes located in the affected cgroup  tree will be killed via SIGKILL    Killing a cgroup tree will deal with concurrent forks appropriately and  is protected against migrations    In a threaded cgroup  writing this file fails with EOPNOTSUPP as  killing cgroups is a process directed operation  i e  it affects  the whole thread group     cgroup pressure  A read write single value file that allowed values are  0  and  1    The default is  1     Writing  0  to the file will disable the cgroup PSI accounting   Writing  1  to the file will re enable the cgroup PSI accounting    This control attribute is not hierarchical  so disable or enable PSI  accounting in a cgroup does not affect PSI accounting in descendants  and doesn t need pass enablement via ancestors from root    The reason this control attribute exists is that PSI accounts stalls for  each cgroup separately and aggregates it at each level of the hierarchy   This may cause non negligible overhead for some workloads when under  deep level of the hierarchy  in which case this control attribute can  be used to disable PSI accounting in the non leaf cgroups     irq pressure  A read write nested keyed file    Shows pressure stall information for IRQ SOFTIRQ  See   ref  Documentation accounting psi rst  psi   for details   Controllers                  cgroup v2 cpu   CPU      The  cpu  controllers regulates distribution of CPU cycles   This controller implements weight and absolute bandwidth limit models for normal scheduling policy and absolute bandwidth allocation model for realtime scheduling policy   In all the above models  cycles distribution is defined only on a temporal base and it does not account for the frequency at which tasks are executed  The  optional  utilization clamping support allows to hint the schedutil cpufreq governor about the minimum desired frequency which should always be provided by a CPU  as well as the maximum desired frequency  which should not be exceeded by a CPU   WARNING  cgroup2 doesn t yet support control of realtime processes  For a kernel built with the CONFIG RT GROUP SCHED option enabled for group scheduling of realtime processes  the cpu controller can only be enabled when all RT processes are in the root cgroup   This limitation does not apply if CONFIG RT GROUP SCHED is disabled   Be aware that system management software may already have placed RT processes into nonroot cgroups during the system boot process  and these processes may need to be moved to the root cgroup before the cpu controller can be enabled with a CONFIG RT GROUP SCHED enabled kernel    CPU Interface Files                      All time durations are in microseconds     cpu stat  A read only flat keyed file   This file exists whether the controller is enabled or not    It always reports the following three stats      usage usec    user usec    system usec   and the following five when the controller is enabled      nr periods    nr throttled    throttled usec    nr bursts    burst usec    cpu weight  A read write single value file which exists on non root  cgroups   The default is  100     For non idle groups  cpu idle   0   the weight is in the  range  1  10000     If the cgroup has been configured to be SCHED IDLE  cpu idle   1    then the weight will show as a 0     cpu weight nice  A read write single value file which exists on non root  cgroups   The default is  0     The nice value is in the range   20  19     This interface file is an alternative interface for   cpu weight  and allows reading and setting weight using the  same values used by nice 2    Because the range is smaller and  granularity is coarser for the nice values  the read value is  the closest approximation of the current weight     cpu max  A read write two value file which exists on non root cgroups   The default is  max 100000     The maximum bandwidth limit   It s in the following format        MAX  PERIOD   which indicates that the group may consume up to  MAX in each   PERIOD duration    max  for  MAX indicates no limit   If only  one number is written   MAX is updated     cpu max burst  A read write single value file which exists on non root  cgroups   The default is  0     The burst in the range  0   MAX      cpu pressure  A read write nested keyed file    Shows pressure stall information for CPU  See   ref  Documentation accounting psi rst  psi   for details     cpu uclamp min         A read write single value file which exists on non root cgroups          The default is  0   i e  no utilization boosting           The requested minimum utilization  protection  as a percentage         rational number  e g  12 34 for 12 34            This interface allows reading and setting minimum utilization clamp         values similar to the sched setattr 2   This minimum utilization         value is used to clamp the task specific minimum utilization clamp           The requested minimum utilization  protection  is always capped by         the current value for the maximum utilization  limit   i e           cpu uclamp max      cpu uclamp max         A read write single value file which exists on non root cgroups          The default is  max   i e  no utilization capping          The requested maximum utilization  limit  as a percentage rational         number  e g  98 76 for 98 76            This interface allows reading and setting maximum utilization clamp         values similar to the sched setattr 2   This maximum utilization         value is used to clamp the task specific maximum utilization clamp     cpu idle  A read write single value file which exists on non root cgroups   The default is 0    This is the cgroup analog of the per task SCHED IDLE sched policy   Setting this value to a 1 will make the scheduling policy of the  cgroup SCHED IDLE  The threads inside the cgroup will retain their  own relative priorities  but the cgroup itself will be treated as  very low priority relative to its peers     Memory         The  memory  controller regulates distribution of memory   Memory is stateful and implements both limit and protection models   Due to the intertwining between memory usage and reclaim pressure and the stateful nature of memory  the distribution model is relatively complex   While not completely water tight  all major memory usages by a given cgroup are tracked so that the total memory consumption can be accounted and controlled to a reasonable extent   Currently  the following types of memory usages are tracked     Userland memory   page cache and anonymous memory     Kernel data structures such as dentries and inodes     TCP socket buffers   The above list may expand in the future for better coverage    Memory Interface Files                         All memory amounts are in bytes   If a value which is not aligned to PAGE SIZE is written  the value may be rounded up to the closest PAGE SIZE multiple when read back     memory current  A read only single value file which exists on non root  cgroups    The total amount of memory currently being used by the cgroup  and its descendants     memory min  A read write single value file which exists on non root  cgroups   The default is  0     Hard memory protection   If the memory usage of a cgroup  is within its effective min boundary  the cgroup s memory  won t be reclaimed under any conditions  If there is no  unprotected reclaimable memory available  OOM killer  is invoked  Above the effective min boundary  or  effective low boundary if it is higher   pages are reclaimed  proportionally to the overage  reducing reclaim pressure for  smaller overages    Effective min boundary is limited by memory min values of  all ancestor cgroups  If there is memory min overcommitment   child cgroup or cgroups are requiring more protected memory  than parent will allow   then each child cgroup will get  the part of parent s protection proportional to its  actual memory usage below memory min    Putting more memory than generally available under this  protection is discouraged and may lead to constant OOMs    If a memory cgroup is not populated with processes   its memory min is ignored     memory low  A read write single value file which exists on non root  cgroups   The default is  0     Best effort memory protection   If the memory usage of a  cgroup is within its effective low boundary  the cgroup s  memory won t be reclaimed unless there is no reclaimable  memory available in unprotected cgroups   Above the effective low boundary  or   effective min boundary if it is higher   pages are reclaimed  proportionally to the overage  reducing reclaim pressure for  smaller overages    Effective low boundary is limited by memory low values of  all ancestor cgroups  If there is memory low overcommitment   child cgroup or cgroups are requiring more protected memory  than parent will allow   then each child cgroup will get  the part of parent s protection proportional to its  actual memory usage below memory low    Putting more memory than generally available under this  protection is discouraged     memory high  A read write single value file which exists on non root  cgroups   The default is  max     Memory usage throttle limit   If a cgroup s usage goes  over the high boundary  the processes of the cgroup are  throttled and put under heavy reclaim pressure    Going over the high limit never invokes the OOM killer and  under extreme conditions the limit may be breached  The high  limit should be used in scenarios where an external process  monitors the limited cgroup to alleviate heavy reclaim  pressure     memory max  A read write single value file which exists on non root  cgroups   The default is  max     Memory usage hard limit   This is the main mechanism to limit  memory usage of a cgroup   If a cgroup s memory usage reaches  this limit and can t be reduced  the OOM killer is invoked in  the cgroup  Under certain circumstances  the usage may go  over the limit temporarily    In default configuration regular 0 order allocations always  succeed unless OOM killer chooses current task as a victim    Some kinds of allocations don t invoke the OOM killer   Caller could retry them differently  return into userspace  as  ENOMEM or silently ignore in cases like disk readahead     memory reclaim  A write only nested keyed file which exists for all cgroups    This is a simple interface to trigger memory reclaim in the  target cgroup    Example       echo  1G    memory reclaim   Please note that the kernel can over or under reclaim from  the target cgroup  If less bytes are reclaimed than the  specified amount   EAGAIN is returned    Please note that the proactive reclaim  triggered by this  interface  is not meant to indicate memory pressure on the  memory cgroup  Therefore socket memory balancing triggered by  the memory reclaim normally is not exercised in this case   This means that the networking layer will not adapt based on  reclaim induced by memory reclaim   The following nested keys are defined                                                                swappiness            Swappiness value to reclaim with                                                             Specifying a swappiness value instructs the kernel to perform  the reclaim with that swappiness value  Note that this has the  same semantics as vm swappiness applied to memcg reclaim with  all the existing limitations and potential future extensions     memory peak  A read write single value file which exists on non root cgroups    The max memory usage recorded for the cgroup and its descendants since  either the creation of the cgroup or the most recent reset for that FD    A write of any non empty string to this file resets it to the  current memory usage for subsequent reads through the same  file descriptor     memory oom group  A read write single value file which exists on non root  cgroups   The default value is  0     Determines whether the cgroup should be treated as  an indivisible workload by the OOM killer  If set   all tasks belonging to the cgroup or to its descendants   if the memory cgroup is not a leaf cgroup  are killed  together or not at all  This can be used to avoid  partial kills to guarantee workload integrity    Tasks with the OOM protection  oom score adj set to  1000   are treated as an exception and are never killed    If the OOM killer is invoked in a cgroup  it s not going  to kill any tasks outside of this cgroup  regardless  memory oom group values of ancestor cgroups     memory events  A read only flat keyed file which exists on non root cgroups   The following entries are defined   Unless specified  otherwise  a value change in this file generates a file  modified event    Note that all fields in this file are hierarchical and the  file modified event can be generated due to an event down the  hierarchy  For the local events at the cgroup level see  memory events local      low   The number of times the cgroup is reclaimed due to   high memory pressure even though its usage is under   the low boundary   This usually indicates that the low   boundary is over committed      high   The number of times processes of the cgroup are   throttled and routed to perform direct memory reclaim   because the high memory boundary was exceeded   For a   cgroup whose memory usage is capped by the high limit   rather than global memory pressure  this event s   occurrences are expected      max   The number of times the cgroup s memory usage was   about to go over the max boundary   If direct reclaim   fails to bring it down  the cgroup goes to OOM state      oom   The number of time the cgroup s memory usage was   reached the limit and allocation was about to fail     This event is not raised if the OOM killer is not   considered as an option  e g  for failed high order   allocations or if caller asked to not retry attempts      oom kill   The number of processes belonging to this cgroup   killed by any kind of OOM killer             oom group kill                 The number of times a group OOM has occurred     memory events local  Similar to memory events but the fields in the file are local  to the cgroup i e  not hierarchical  The file modified event  generated on this file reflects only the local events     memory stat  A read only flat keyed file which exists on non root cgroups    This breaks down the cgroup s memory footprint into different  types of memory  type specific details  and other information  on the state and past events of the memory management system    All memory amounts are in bytes    The entries are ordered to be human readable  and new entries  can show up in the middle  Don t rely on items remaining in a  fixed position  use the keys to look up specific values    If the entry has no per node counter  or not show in the  memory numa stat   We use  npn   non per node  as the tag  to indicate that it will not show in the memory numa stat      anon   Amount of memory used in anonymous mappings such as   brk    sbrk    and mmap MAP ANONYMOUS      file   Amount of memory used to cache filesystem data    including tmpfs and shared memory      kernel  npn    Amount of total kernel memory  including    kernel stack  pagetables  percpu  vmalloc  slab  in   addition to other kernel memory use cases      kernel stack   Amount of memory allocated to kernel stacks      pagetables                 Amount of memory allocated for page tables      sec pagetables   Amount of memory allocated for secondary page tables    this currently includes KVM mmu allocations on x86   and arm64 and IOMMU page tables      percpu  npn    Amount of memory used for storing per cpu kernel   data structures      sock  npn    Amount of memory used in network transmission buffers     vmalloc  npn    Amount of memory used for vmap backed memory      shmem   Amount of cached filesystem data that is swap backed    such as tmpfs  shm segments  shared anonymous mmap  s     zswap   Amount of memory consumed by the zswap compression backend      zswapped   Amount of application memory swapped out to zswap      file mapped   Amount of cached filesystem data mapped with mmap       file dirty   Amount of cached filesystem data that was modified but   not yet written back to disk     file writeback   Amount of cached filesystem data that was modified and   is currently being written back to disk     swapcached   Amount of swap cached in memory  The swapcache is accounted   against both memory and swap usage      anon thp   Amount of memory used in anonymous mappings backed by   transparent hugepages     file thp   Amount of cached filesystem data backed by transparent   hugepages     shmem thp   Amount of shm  tmpfs  shared anonymous mmap  s backed by   transparent hugepages     inactive anon  active anon  inactive file  active file  unevictable   Amount of memory  swap backed and filesystem backed    on the internal memory management lists used by the   page reclaim algorithm     As these represent internal list state  eg  shmem pages are on anon   memory management lists   inactive foo   active foo may not be equal to   the value for the foo counter  since the foo counter is type based  not   list based      slab reclaimable   Part of  slab  that might be reclaimed  such as   dentries and inodes      slab unreclaimable   Part of  slab  that cannot be reclaimed on memory   pressure      slab  npn    Amount of memory used for storing in kernel data   structures      workingset refault anon   Number of refaults of previously evicted anonymous pages      workingset refault file   Number of refaults of previously evicted file pages      workingset activate anon   Number of refaulted anonymous pages that were immediately   activated      workingset activate file   Number of refaulted file pages that were immediately activated      workingset restore anon   Number of restored anonymous pages which have been detected as   an active workingset before they got reclaimed      workingset restore file   Number of restored file pages which have been detected as an   active workingset before they got reclaimed      workingset nodereclaim   Number of times a shadow node has been reclaimed     pgscan  npn    Amount of scanned pages  in an inactive LRU list      pgsteal  npn    Amount of reclaimed pages     pgscan kswapd  npn    Amount of scanned pages by kswapd  in an inactive LRU list      pgscan direct  npn    Amount of scanned pages directly   in an inactive LRU list      pgscan khugepaged  npn    Amount of scanned pages by khugepaged   in an inactive LRU list      pgsteal kswapd  npn    Amount of reclaimed pages by kswapd     pgsteal direct  npn    Amount of reclaimed pages directly     pgsteal khugepaged  npn    Amount of reclaimed pages by khugepaged     pgfault  npn    Total number of page faults incurred     pgmajfault  npn    Number of major page faults incurred     pgrefill  npn    Amount of scanned pages  in an active LRU list      pgactivate  npn    Amount of pages moved to the active LRU list     pgdeactivate  npn    Amount of pages moved to the inactive LRU list     pglazyfree  npn    Amount of pages postponed to be freed under memory pressure     pglazyfreed  npn    Amount of reclaimed lazyfree pages     swpin zero   Number of pages swapped into memory and filled with zero  where I O   was optimized out because the page content was detected to be zero   during swapout      swpout zero   Number of zero filled pages swapped out with I O skipped due to the   content being detected as zero      zswpin   Number of pages moved in to memory from zswap      zswpout   Number of pages moved out of memory to zswap      zswpwb   Number of pages written from zswap to swap      thp fault alloc  npn    Number of transparent hugepages which were allocated to satisfy   a page fault  This counter is not present when CONFIG TRANSPARENT HUGEPAGE                 is not set      thp collapse alloc  npn    Number of transparent hugepages which were allocated to allow   collapsing an existing range of pages  This counter is not   present when CONFIG TRANSPARENT HUGEPAGE is not set      thp swpout  npn    Number of transparent hugepages which are swapout in one piece   without splitting      thp swpout fallback  npn    Number of transparent hugepages which were split before swapout    Usually because failed to allocate some continuous swap space   for the huge page      numa pages migrated  npn    Number of pages migrated by NUMA balancing      numa pte updates  npn    Number of pages whose page table entries are modified by   NUMA balancing to produce NUMA hinting faults on access      numa hint faults  npn    Number of NUMA hinting faults      pgdemote kswapd   Number of pages demoted by kswapd      pgdemote direct   Number of pages demoted directly      pgdemote khugepaged   Number of pages demoted by khugepaged      hugetlb   Amount of memory used by hugetlb pages  This metric only shows   up if hugetlb usage is accounted for in memory current  i e    cgroup is mounted with the memory hugetlb accounting option      memory numa stat  A read only nested keyed file which exists on non root cgroups    This breaks down the cgroup s memory footprint into different  types of memory  type specific details  and other information  per node on the state of the memory management system    This is useful for providing visibility into the NUMA locality  information within an memcg since the pages are allowed to be  allocated from any physical node  One of the use case is evaluating  application performance by combining this information with the  application s CPU allocation    All memory amounts are in bytes    The output format of memory numa stat is       type N0  bytes in node 0  N1  bytes in node 1        The entries are ordered to be human readable  and new entries  can show up in the middle  Don t rely on items remaining in a  fixed position  use the keys to look up specific values    The entries can refer to the memory stat     memory swap current  A read only single value file which exists on non root  cgroups    The total amount of swap currently being used by the cgroup  and its descendants     memory swap high  A read write single value file which exists on non root  cgroups   The default is  max     Swap usage throttle limit   If a cgroup s swap usage exceeds  this limit  all its further allocations will be throttled to  allow userspace to implement custom out of memory procedures    This limit marks a point of no return for the cgroup  It is NOT  designed to manage the amount of swapping a workload does  during regular operation  Compare to memory swap max  which  prohibits swapping past a set amount  but lets the cgroup  continue unimpeded as long as other memory can be reclaimed    Healthy workloads are not expected to reach this limit     memory swap peak  A read write single value file which exists on non root cgroups    The max swap usage recorded for the cgroup and its descendants since  the creation of the cgroup or the most recent reset for that FD    A write of any non empty string to this file resets it to the  current memory usage for subsequent reads through the same  file descriptor     memory swap max  A read write single value file which exists on non root  cgroups   The default is  max     Swap usage hard limit   If a cgroup s swap usage reaches this  limit  anonymous memory of the cgroup will not be swapped out     memory swap events  A read only flat keyed file which exists on non root cgroups   The following entries are defined   Unless specified  otherwise  a value change in this file generates a file  modified event      high   The number of times the cgroup s swap usage was over   the high threshold      max   The number of times the cgroup s swap usage was about   to go over the max boundary and swap allocation   failed      fail   The number of times swap allocation failed either   because of running out of swap system wide or max   limit    When reduced under the current usage  the existing swap  entries are reclaimed gradually and the swap usage may stay  higher than the limit for an extended period of time   This  reduces the impact on the workload and memory management     memory zswap current  A read only single value file which exists on non root  cgroups    The total amount of memory consumed by the zswap compression  backend     memory zswap max  A read write single value file which exists on non root  cgroups   The default is  max     Zswap usage hard limit  If a cgroup s zswap pool reaches this  limit  it will refuse to take any more stores before existing  entries fault back in or are written out to disk     memory zswap writeback  A read write single value file  The default value is  1    Note that this setting is hierarchical  i e  the writeback would be  implicitly disabled for child cgroups if the upper hierarchy  does so    When this is set to 0  all swapping attempts to swapping devices  are disabled  This included both zswap writebacks  and swapping due  to zswap store failures  If the zswap store failures are recurring   for e g if the pages are incompressible   users can observe  reclaim inefficiency after disabling writeback  because the same  pages might be rejected again and again     Note that this is subtly different from setting memory swap max to  0  as it still allows for pages to be written to the zswap pool   This setting has no effect if zswap is disabled  and swapping  is allowed unless memory swap max is set to 0     memory pressure  A read only nested keyed file    Shows pressure stall information for memory  See   ref  Documentation accounting psi rst  psi   for details    Usage Guidelines                    memory high  is the main mechanism to control memory usage  Over committing on high limit  sum of high limits   available memory  and letting global memory pressure to distribute memory according to usage is a viable strategy   Because breach of the high limit doesn t trigger the OOM killer but throttles the offending cgroup  a management agent has ample opportunities to monitor and take appropriate actions such as granting more memory or terminating the workload   Determining whether a cgroup has enough memory is not trivial as memory usage doesn t indicate whether the workload can benefit from more memory   For example  a workload which writes data received from network to a file can use all available memory but can also operate as performant with a small amount of memory   A measure of memory pressure   how much the workload is being impacted due to lack of memory   is necessary to determine whether a workload needs more memory  unfortunately  memory pressure monitoring mechanism isn t implemented yet    Memory Ownership                   A memory area is charged to the cgroup which instantiated it and stays charged to the cgroup until the area is released   Migrating a process to a different cgroup doesn t move the memory usages that it instantiated while in the previous cgroup to the new cgroup   A memory area may be used by processes belonging to different cgroups  To which cgroup the area will be charged is in deterministic  however  over time  the memory area is likely to end up in a cgroup which has enough memory allowance to avoid high reclaim pressure   If a cgroup sweeps a considerable amount of memory which is expected to be accessed repeatedly by other cgroups  it may make sense to use POSIX FADV DONTNEED to relinquish the ownership of memory areas belonging to the affected files to ensure correct memory ownership    IO     The  io  controller regulates the distribution of IO resources   This controller implements both weight based and absolute bandwidth or IOPS limit distribution  however  weight based distribution is available only if cfq iosched is in use and neither scheme is available for blk mq devices    IO Interface Files                       io stat  A read only nested keyed file    Lines are keyed by  MAJ  MIN device numbers and not ordered   The following nested keys are defined                                      rbytes Bytes read    wbytes Bytes written    rios  Number of read IOs    wios  Number of write IOs    dbytes Bytes discarded    dios  Number of discard IOs                                   An example read output follows       8 16 rbytes 1459200 wbytes 314773504 rios 192 wios 353 dbytes 0 dios 0    8 0 rbytes 90430464 wbytes 299008000 rios 8950 wios 1252 dbytes 50331648 dios 3021    io cost qos  A read write nested keyed file which exists only on the root  cgroup    This file configures the Quality of Service of the IO cost  model based controller  CONFIG BLK CGROUP IOCOST  which  currently implements  io weight  proportional control   Lines  are keyed by  MAJ  MIN device numbers and not ordered   The  line for a given device is populated on the first write for  the device on  io cost qos  or  io cost model    The following  nested keys are defined                                                      enable Weight based control enable    ctrl   auto  or  user     rpct  Read latency percentile     0  100     rlat  Read latency threshold    wpct  Write latency percentile    0  100     wlat  Write latency threshold    min  Minimum scaling percentage  1  10000     max  Maximum scaling percentage  1  10000                                                    The controller is disabled by default and can be enabled by  setting  enable  to 1    rpct  and  wpct  parameters default  to zero and the controller uses internal device saturation  state to adjust the overall IO rate between  min  and  max     When a better control quality is needed  latency QoS  parameters can be configured   For example       8 16 enable 1 ctrl auto rpct 95 00 rlat 75000 wpct 95 00 wlat 150000 min 50 00 max 150 0   shows that on sdb  the controller is enabled  will consider  the device saturated if the 95th percentile of read completion  latencies is above 75ms or write 150ms  and adjust the overall  IO issue rate between 50  and 150  accordingly    The lower the saturation point  the better the latency QoS at  the cost of aggregate bandwidth   The narrower the allowed  adjustment range between  min  and  max   the more conformant  to the cost model the IO behavior   Note that the IO issue  base rate may be far off from 100  and setting  min  and  max   blindly can lead to a significant loss of device capacity or  control quality    min  and  max  are useful for regulating  devices which show wide temporary behavior changes   e g  a  ssd which accepts writes at the line speed for a while and  then completely stalls for multiple seconds    When  ctrl  is  auto   the parameters are controlled by the  kernel and may change automatically   Setting  ctrl  to  user   or setting any of the percentile and latency parameters puts  it into  user  mode and disables the automatic changes   The  automatic mode can be restored by setting  ctrl  to  auto      io cost model  A read write nested keyed file which exists only on the root  cgroup    This file configures the cost model of the IO cost model based  controller  CONFIG BLK CGROUP IOCOST  which currently  implements  io weight  proportional control   Lines are keyed  by  MAJ  MIN device numbers and not ordered   The line for a  given device is populated on the first write for the device on   io cost qos  or  io cost model    The following nested keys  are defined                                                 ctrl   auto  or  user     model  The cost model in use    linear                                               When  ctrl  is  auto   the kernel may change all parameters  dynamically   When  ctrl  is set to  user  or any other  parameters are written to   ctrl  become  user  and the  automatic changes are disabled    When  model  is  linear   the following model parameters are  defined                                                                 r w bps The maximum sequential IO throughput     r w seqiops The maximum 4k sequential IOs per second     r w randiops The maximum 4k random IOs per second                                                             From the above  the builtin linear model determines the base  costs of a sequential and random IO and the cost coefficient  for the IO size   While simple  this model can cover most  common device classes acceptably    The IO cost model isn t expected to be accurate in absolute  sense and is scaled to the device behavior dynamically    If needed  tools cgroup iocost coef gen py can be used to  generate device specific coefficients     io weight  A read write flat keyed file which exists on non root cgroups   The default is  default 100     The first line is the default weight applied to devices  without specific override   The rest are overrides keyed by   MAJ  MIN device numbers and not ordered   The weights are in  the range  1  10000  and specifies the relative amount IO time  the cgroup can use in relation to its siblings    The default weight can be updated by writing either  default   WEIGHT  or simply   WEIGHT    Overrides can be set by writing    MAJ  MIN  WEIGHT  and unset by writing   MAJ  MIN default     An example read output follows       default 100    8 16 200    8 0 50    io max  A read write nested keyed file which exists on non root  cgroups    BPS and IOPS based IO limit   Lines are keyed by  MAJ  MIN  device numbers and not ordered   The following nested keys are  defined                                                   rbps  Max read bytes per second    wbps  Max write bytes per second    riops  Max read IO operations per second    wiops  Max write IO operations per second                                                When writing  any number of nested key value pairs can be  specified in any order    max  can be specified as the value  to remove a specific limit   If the same key is specified  multiple times  the outcome is undefined    BPS and IOPS are measured in each IO direction and IOs are  delayed if limit is reached   Temporary bursts are allowed    Setting read limit at 2M BPS and write at 120 IOPS for 8 16       echo  8 16 rbps 2097152 wiops 120    io max   Reading returns the following       8 16 rbps 2097152 wbps max riops max wiops 120   Write IOPS limit can be removed by writing the following       echo  8 16 wiops max    io max   Reading now returns the following       8 16 rbps 2097152 wbps max riops max wiops max    io pressure  A read only nested keyed file    Shows pressure stall information for IO  See   ref  Documentation accounting psi rst  psi   for details    Writeback            Page cache is dirtied through buffered writes and shared mmaps and written asynchronously to the backing filesystem by the writeback mechanism   Writeback sits between the memory and IO domains and regulates the proportion of dirty memory by balancing dirtying and write IOs   The io controller  in conjunction with the memory controller  implements control of page cache writeback IOs   The memory controller defines the memory domain that dirty memory ratio is calculated and maintained for and the io controller defines the io domain which writes out dirty pages for the memory domain   Both system wide and per cgroup dirty memory states are examined and the more restrictive of the two is enforced   cgroup writeback requires explicit support from the underlying filesystem   Currently  cgroup writeback is implemented on ext2  ext4  btrfs  f2fs  and xfs   On other filesystems  all writeback IOs are  attributed to the root cgroup   There are inherent differences in memory and writeback management which affects how cgroup ownership is tracked   Memory is tracked per page while writeback per inode   For the purpose of writeback  an inode is assigned to a cgroup and all IO requests to write dirty pages from the inode are attributed to that cgroup   As cgroup ownership for memory is tracked per page  there can be pages which are associated with different cgroups than the one the inode is associated with   These are called foreign pages   The writeback constantly keeps track of foreign pages and  if a particular foreign cgroup becomes the majority over a certain period of time  switches the ownership of the inode to that cgroup   While this model is enough for most use cases where a given inode is mostly dirtied by a single cgroup even when the main writing cgroup changes over time  use cases where multiple cgroups write to a single inode simultaneously are not supported well   In such circumstances  a significant portion of IOs are likely to be attributed incorrectly  As memory controller assigns page ownership on the first use and doesn t update it until the page is released  even if writeback strictly follows page ownership  multiple cgroups dirtying overlapping areas wouldn t work as expected   It s recommended to avoid such usage patterns   The sysctl knobs which affect writeback behavior are applied to cgroup writeback as follows     vm dirty background ratio  vm dirty ratio  These ratios apply the same to cgroup writeback with the  amount of available memory capped by limits imposed by the  memory controller and system wide clean memory     vm dirty background bytes  vm dirty bytes  For cgroup writeback  this is calculated into ratio against  total available memory and applied the same way as  vm dirty  background  ratio    IO Latency             This is a cgroup v2 controller for IO workload protection   You provide a group with a latency target  and if the average latency exceeds that target the controller will throttle any peers that have a lower latency target than the protected workload   The limits are only applied at the peer level in the hierarchy   This means that in the diagram below  only groups A  B  and C will influence each other  and groups D and F will influence each other   Group G will influence nobody        root                A    B  C                              D    F    G   So the ideal way to configure this is to set io latency in groups A  B  and C  Generally you do not want to set a value lower than the latency your device supports   Experiment to find the value that works best for your workload  Start at higher than the expected latency for your device and watch the avg lat value in io stat for your workload group to get an idea of the latency you see during normal operation   Use the avg lat value as a basis for your real setting  setting at 10 15  higher than the value in io stat   How IO Latency Throttling Works                                  io latency is work conserving  so as long as everybody is meeting their latency target the controller doesn t do anything   Once a group starts missing its target it begins throttling any peer group that has a higher target than itself  This throttling takes 2 forms     Queue depth throttling   This is the number of outstanding IO s a group is   allowed to have   We will clamp down relatively quickly  starting at no limit   and going all the way down to 1 IO at a time     Artificial delay induction   There are certain types of IO that cannot be   throttled without possibly adversely affecting higher priority groups   This   includes swapping and metadata IO   These types of IO are allowed to occur   normally  however they are  charged  to the originating group   If the   originating group is being throttled you will see the use delay and delay   fields in io stat increase   The delay value is how many microseconds that are   being added to any process that runs in this group   Because this number can   grow quite large if there is a lot of swapping or metadata IO occurring we   limit the individual delay events to 1 second at a time   Once the victimized group starts meeting its latency target again it will start unthrottling any peer groups that were throttled previously   If the victimized group simply stops doing IO the global counter will unthrottle appropriately   IO Latency Interface Files                               io latency  This takes a similar format as the other controllers      MAJOR MINOR target  target time in microseconds      io stat  If the controller is enabled you will see extra stats in io stat in  addition to the normal ones      depth   This is the current queue depth for the group      avg lat   This is an exponential moving average with a decay rate of 1 exp   bound by the sampling interval   The decay rate interval can be   calculated by multiplying the win value in io stat by the   corresponding number of samples based on the win value      win   The sampling window size in milliseconds   This is the minimum   duration of time between evaluation events   Windows only elapse   with IO activity   Idle periods extend the most recent window   IO Priority              A single attribute controls the behavior of the I O priority cgroup policy  namely the io prio class attribute  The following values are accepted for that attribute     no change  Do not modify the I O priority class     promote to rt  For requests that have a non RT I O priority class  change it into RT   Also change the priority level of these requests to 4  Do not modify  the I O priority of requests that have priority class RT     restrict to be  For requests that do not have an I O priority class or that have I O  priority class RT  change it into BE  Also change the priority level  of these requests to 0  Do not modify the I O priority class of  requests that have priority class IDLE     idle  Change the I O priority class of all requests into IDLE  the lowest  I O priority class     none to rt  Deprecated  Just an alias for promote to rt   The following numerical values are associated with the I O priority policies                            no change        0                            promote to rt    1                            restrict to be   2                            idle             3                           The numerical value that corresponds to each I O priority class is as follows                                           IOPRIO CLASS NONE               0                                           IOPRIO CLASS RT  real time      1                                           IOPRIO CLASS BE  best effort    2                                           IOPRIO CLASS IDLE               3                                          The algorithm to set the I O priority class for a request is as follows     If I O priority class policy is promote to rt  change the request I O   priority class to IOPRIO CLASS RT and change the request I O priority   level to 4    If I O priority class policy is not promote to rt  translate the I O priority   class policy into a number  then change the request I O priority class   into the maximum of the I O priority class policy number and the numerical   I O priority class   PID      The process number controller is used to allow a cgroup to stop any new tasks from being fork   d or clone   d after a specified limit is reached   The number of tasks in a cgroup can be exhausted in ways which other controllers cannot prevent  thus warranting its own controller   For example  a fork bomb is likely to exhaust the number of tasks before hitting memory restrictions   Note that PIDs used in this controller refer to TIDs  process IDs as used by the kernel    PID Interface Files                        pids max  A read write single value file which exists on non root  cgroups   The default is  max     Hard limit of number of processes     pids current  A read only single value file which exists on non root cgroups    The number of processes currently in the cgroup and its  descendants     pids peak  A read only single value file which exists on non root cgroups    The maximum value that the number of processes in the cgroup and its  descendants has ever reached     pids events  A read only flat keyed file which exists on non root cgroups  Unless  specified otherwise  a value change in this file generates a file  modified event  The following entries are defined      max   The number of times the cgroup s total number of processes hit the pids max   limit  see also pids localevents      pids events local  Similar to pids events but the fields in the file are local  to the cgroup i e  not hierarchical  The file modified event  generated on this file reflects only the local events   Organisational operations are not blocked by cgroup policies  so it is possible to have pids current   pids max   This can be done by either setting the limit to be smaller than pids current  or attaching enough processes to the cgroup such that pids current is larger than pids max   However  it is not possible to violate a cgroup PID policy through fork   or clone    These will return  EAGAIN if the creation of a new process would cause a cgroup policy to be violated    Cpuset         The  cpuset  controller provides a mechanism for constraining the CPU and memory node placement of tasks to only the resources specified in the cpuset interface files in a task s current cgroup  This is especially valuable on large NUMA systems where placing jobs on properly sized subsets of the systems with careful processor and memory placement to reduce cross node memory access and contention can improve overall system performance   The  cpuset  controller is hierarchical   That means the controller cannot use CPUs or memory nodes not allowed in its parent    Cpuset Interface Files                           cpuset cpus  A read write multiple values file which exists on non root  cpuset enabled cgroups    It lists the requested CPUs to be used by tasks within this  cgroup   The actual list of CPUs to be granted  however  is  subjected to constraints imposed by its parent and can differ  from the requested CPUs    The CPU numbers are comma separated numbers or ranges   For example         cat cpuset cpus    0 4 6 8 10   An empty value indicates that the cgroup is using the same  setting as the nearest cgroup ancestor with a non empty   cpuset cpus  or all the available CPUs if none is found    The value of  cpuset cpus  stays constant until the next update  and won t be affected by any CPU hotplug events     cpuset cpus effective  A read only multiple values file which exists on all  cpuset enabled cgroups    It lists the onlined CPUs that are actually granted to this  cgroup by its parent   These CPUs are allowed to be used by  tasks within the current cgroup    If  cpuset cpus  is empty  the  cpuset cpus effective  file shows  all the CPUs from the parent cgroup that can be available to  be used by this cgroup   Otherwise  it should be a subset of   cpuset cpus  unless none of the CPUs listed in  cpuset cpus   can be granted   In this case  it will be treated just like an  empty  cpuset cpus     Its value will be affected by CPU hotplug events     cpuset mems  A read write multiple values file which exists on non root  cpuset enabled cgroups    It lists the requested memory nodes to be used by tasks within  this cgroup   The actual list of memory nodes granted  however   is subjected to constraints imposed by its parent and can differ  from the requested memory nodes    The memory node numbers are comma separated numbers or ranges   For example         cat cpuset mems    0 1 3   An empty value indicates that the cgroup is using the same  setting as the nearest cgroup ancestor with a non empty   cpuset mems  or all the available memory nodes if none  is found    The value of  cpuset mems  stays constant until the next update  and won t be affected by any memory nodes hotplug events    Setting a non empty value to  cpuset mems  causes memory of  tasks within the cgroup to be migrated to the designated nodes if  they are currently using memory outside of the designated nodes    There is a cost for this memory migration   The migration  may not be complete and some memory pages may be left behind   So it is recommended that  cpuset mems  should be set properly  before spawning new tasks into the cpuset   Even if there is  a need to change  cpuset mems  with active tasks  it shouldn t  be done frequently     cpuset mems effective  A read only multiple values file which exists on all  cpuset enabled cgroups    It lists the onlined memory nodes that are actually granted to  this cgroup by its parent  These memory nodes are allowed to  be used by tasks within the current cgroup    If  cpuset mems  is empty  it shows all the memory nodes from the  parent cgroup that will be available to be used by this cgroup   Otherwise  it should be a subset of  cpuset mems  unless none of  the memory nodes listed in  cpuset mems  can be granted   In this  case  it will be treated just like an empty  cpuset mems     Its value will be affected by memory nodes hotplug events     cpuset cpus exclusive  A read write multiple values file which exists on non root  cpuset enabled cgroups    It lists all the exclusive CPUs that are allowed to be used  to create a new cpuset partition   Its value is not used  unless the cgroup becomes a valid partition root   See the   cpuset cpus partition  section below for a description of what  a cpuset partition is    When the cgroup becomes a partition root  the actual exclusive  CPUs that are allocated to that partition are listed in   cpuset cpus exclusive effective  which may be different  from  cpuset cpus exclusive    If  cpuset cpus exclusive   has previously been set   cpuset cpus exclusive effective   is always a subset of it    Users can manually set it to a value that is different from   cpuset cpus   One constraint in setting it is that the list of  CPUs must be exclusive with respect to  cpuset cpus exclusive   of its sibling   If  cpuset cpus exclusive  of a sibling cgroup  isn t set  its  cpuset cpus  value  if set  cannot be a subset  of it to leave at least one CPU available when the exclusive  CPUs are taken away    For a parent cgroup  any one of its exclusive CPUs can only  be distributed to at most one of its child cgroups   Having an  exclusive CPU appearing in two or more of its child cgroups is  not allowed  the exclusivity rule    A value that violates the  exclusivity rule will be rejected with a write error    The root cgroup is a partition root and all its available CPUs  are in its exclusive CPU set     cpuset cpus exclusive effective  A read only multiple values file which exists on all non root  cpuset enabled cgroups    This file shows the effective set of exclusive CPUs that  can be used to create a partition root   The content  of this file will always be a subset of its parent s   cpuset cpus exclusive effective  if its parent is not the root  cgroup   It will also be a subset of  cpuset cpus exclusive   if it is set   If  cpuset cpus exclusive  is not set  it is  treated to have an implicit value of  cpuset cpus  in the  formation of local partition     cpuset cpus isolated  A read only and root cgroup only multiple values file    This file shows the set of all isolated CPUs used in existing  isolated partitions  It will be empty if no isolated partition  is created     cpuset cpus partition  A read write single value file which exists on non root  cpuset enabled cgroups   This flag is owned by the parent cgroup  and is not delegatable    It accepts only the following input values when written to                                                           member  Non root member of a partition     root  Partition root     isolated  Partition root without load balancing                                                       A cpuset partition is a collection of cpuset enabled cgroups with  a partition root at the top of the hierarchy and its descendants  except those that are separate partition roots themselves and  their descendants   A partition has exclusive access to the  set of exclusive CPUs allocated to it  Other cgroups outside  of that partition cannot use any CPUs in that set    There are two types of partitions   local and remote   A local  partition is one whose parent cgroup is also a valid partition  root   A remote partition is one whose parent cgroup is not a  valid partition root itself   Writing to  cpuset cpus exclusive   is optional for the creation of a local partition as its   cpuset cpus exclusive  file will assume an implicit value that  is the same as  cpuset cpus  if it is not set  Writing the  proper  cpuset cpus exclusive  values down the cgroup hierarchy  before the target partition root is mandatory for the creation  of a remote partition    Currently  a remote partition cannot be created under a local  partition   All the ancestors of a remote partition root except  the root cgroup cannot be a partition root    The root cgroup is always a partition root and its state cannot  be changed   All other non root cgroups start out as  member     When set to  root   the current cgroup is the root of a new  partition or scheduling domain   The set of exclusive CPUs is  determined by the value of its  cpuset cpus exclusive effective     When set to  isolated   the CPUs in that partition will be in  an isolated state without any load balancing from the scheduler  and excluded from the unbound workqueues   Tasks placed in such  a partition with multiple CPUs should be carefully distributed  and bound to each of the individual CPUs for optimal performance    A partition root   root  or  isolated   can be in one of the  two possible states   valid or invalid   An invalid partition  root is in a degraded state where some state information may  be retained  but behaves more like a  member     All possible state transitions among  member    root  and   isolated  are allowed    On read  the  cpuset cpus partition  file can show the following  values                                                                              member    Non root member of a partition     root    Partition root     isolated    Partition root without load balancing     root invalid   reason    Invalid partition root     isolated invalid   reason    Invalid isolated partition root                                                                          In the case of an invalid partition root  a descriptive string on  why the partition is invalid is included within parentheses    For a local partition root to be valid  the following conditions  must be met    1  The parent cgroup is a valid partition root   2  The  cpuset cpus exclusive effective  file cannot be empty      though it may contain offline CPUs   3  The  cpuset cpus effective  cannot be empty unless there is     no task associated with this partition    For a remote partition root to be valid  all the above conditions  except the first one must be met    External events like hotplug or changes to  cpuset cpus  or   cpuset cpus exclusive  can cause a valid partition root to  become invalid and vice versa  Note that a task cannot be  moved to a cgroup with empty  cpuset cpus effective     A valid non root parent partition may distribute out all its CPUs  to its child local partitions when there is no task associated  with it    Care must be taken to change a valid partition root to  member   as all its child local partitions  if present  will become  invalid causing disruption to tasks running in those child  partitions  These inactivated partitions could be recovered if  their parent is switched back to a partition root with a proper  value in  cpuset cpus  or  cpuset cpus exclusive     Poll and inotify events are triggered whenever the state of   cpuset cpus partition  changes   That includes changes caused  by write to  cpuset cpus partition   cpu hotplug or other  changes that modify the validity status of the partition   This will allow user space agents to monitor unexpected changes  to  cpuset cpus partition  without the need to do continuous  polling    A user can pre configure certain CPUs to an isolated state  with load balancing disabled at boot time with the  isolcpus   kernel boot command line option   If those CPUs are to be put  into a partition  they have to be used in an isolated partition    Device controller                    Device controller manages access to device files  It includes both creation of new device files  using mknod   and access to the existing device files   Cgroup v2 device controller has no interface files and is implemented on top of cgroup BPF  To control access to device files  a user may create bpf programs of type BPF PROG TYPE CGROUP DEVICE and attach them to cgroups with BPF CGROUP DEVICE flag  On an attempt to access a device file  corresponding BPF programs will be executed  and depending on the return value the attempt will succeed or fail with  EPERM   A BPF PROG TYPE CGROUP DEVICE program takes a pointer to the bpf cgroup dev ctx structure  which describes the device access attempt  access type  mknod read write  and device  type  major and minor numbers   If the program returns 0  the attempt fails with  EPERM  otherwise it succeeds   An example of BPF PROG TYPE CGROUP DEVICE program may be found in tools testing selftests bpf progs dev cgroup c in the kernel source tree    RDMA       The  rdma  controller regulates the distribution and accounting of RDMA resources   RDMA Interface Files                         rdma max  A readwrite nested keyed file that exists for all the cgroups  except root that describes current configured resource limit  for a RDMA IB device    Lines are keyed by device name and are not ordered   Each line contains space separated resource name and its configured  limit that can be distributed    The following nested keys are defined                                                  hca handle Maximum number of HCA Handles    hca object  Maximum number of HCA Objects                                               An example for mlx4 and ocrdma device follows       mlx4 0 hca handle 2 hca object 2000    ocrdma1 hca handle 3 hca object max    rdma current  A read only file that describes current resource usage   It exists for all the cgroup except root    An example for mlx4 and ocrdma device follows       mlx4 0 hca handle 1 hca object 20    ocrdma1 hca handle 1 hca object 23  HugeTLB          The HugeTLB controller allows to limit the HugeTLB usage per control group and enforces the controller limit during page fault   HugeTLB Interface Files                            hugetlb  hugepagesize  current  Show current usage for  hugepagesize  hugetlb   It exists for all  the cgroup except root     hugetlb  hugepagesize  max  Set show the hard limit of  hugepagesize  hugetlb usage   The default value is  max    It exists for all the cgroup except root     hugetlb  hugepagesize  events  A read only flat keyed file which exists on non root cgroups      max   The number of allocation failure due to HugeTLB limit    hugetlb  hugepagesize  events local  Similar to hugetlb  hugepagesize  events but the fields in the file  are local to the cgroup i e  not hierarchical  The file modified event  generated on this file reflects only the local events     hugetlb  hugepagesize  numa stat  Similar to memory numa stat  it shows the numa information of the         hugetlb pages of  hugepagesize  in this cgroup   Only active in         use hugetlb pages are included   The per node values are in bytes   Misc       The Miscellaneous cgroup provides the resource limiting and tracking mechanism for the scalar resources which cannot be abstracted like the other cgroup resources  Controller is enabled by the CONFIG CGROUP MISC config option   A resource can be added to the controller via enum misc res type   in the include linux misc cgroup h file and the corresponding name via misc res name   in the kernel cgroup misc c file  Provider of the resource must set its capacity prior to using the resource by calling misc cg set capacity     Once a capacity is set then the resource usage can be updated using charge and uncharge APIs  All of the APIs to interact with misc controller are in include linux misc cgroup h   Misc Interface Files                       Miscellaneous controller provides 3 interface files  If two misc resources  res a and res b  are registered then     misc capacity         A read only flat keyed file shown only in the root cgroup   It shows         miscellaneous scalar resources available on the platform along with         their quantities         cat misc capacity    res a 50    res b 10    misc current         A read only flat keyed file shown in the all cgroups   It shows         the current usage of the resources in the cgroup and its children          cat misc current    res a 3    res b 0    misc peak         A read only flat keyed file shown in all cgroups   It shows the         historical maximum usage of the resources in the cgroup and its         children          cat misc peak    res a 10    res b 8    misc max         A read write flat keyed file shown in the non root cgroups  Allowed         maximum usage of the resources in the cgroup and its children          cat misc max    res a max    res b 4   Limit can be set by         echo res a 1   misc max   Limit can be set to max by         echo res a max   misc max          Limits can be set higher than the capacity value in the misc capacity         file     misc events  A read only flat keyed file which exists on non root cgroups  The  following entries are defined  Unless specified otherwise  a value  change in this file generates a file modified event  All fields in  this file are hierarchical      max   The number of times the cgroup s resource usage was   about to go over the max boundary     misc events local         Similar to misc events but the fields in the file are local to the         cgroup i e  not hierarchical  The file modified event generated on         this file reflects only the local events   Migration and Ownership                          A miscellaneous scalar resource is charged to the cgroup in which it is used first  and stays charged to that cgroup until that resource is freed  Migrating a process to a different cgroup does not move the charge to the destination cgroup where the process has moved   Others         perf event             perf event controller  if not mounted on a legacy hierarchy  is automatically enabled on the v2 hierarchy so that perf events can always be filtered by cgroup v2 path   The controller can still be moved to a legacy hierarchy after v2 hierarchy is populated    Non normative information                            This section contains information that isn t considered to be a part of the stable kernel API and so is subject to change    CPU controller root cgroup process behaviour                                               When distributing CPU cycles in the root cgroup each thread in this cgroup is treated as if it was hosted in a separate child cgroup of the root cgroup  This child cgroup weight is dependent on its thread nice level   For details of this mapping see sched prio to weight array in kernel sched core c file  values from this array should be scaled appropriately so the neutral   nice 0   value is 100 instead of 1024     IO controller root cgroup process behaviour                                              Root cgroup processes are hosted in an implicit leaf child node  When distributing IO resources this implicit child node is taken into account as if it was a normal child cgroup of the root cgroup with a weight value of 200    Namespace            Basics         cgroup namespace provides a mechanism to virtualize the view of the   proc  PID cgroup  file and cgroup mounts   The CLONE NEWCGROUP clone flag can be used with clone 2  and unshare 2  to create a new cgroup namespace   The process running inside the cgroup namespace will have its   proc  PID cgroup  output restricted to cgroupns root   The cgroupns root is the cgroup of the process at the time of creation of the cgroup namespace   Without cgroup namespace  the   proc  PID cgroup  file shows the complete path of the cgroup of a process   In a container setup where a set of cgroups and namespaces are intended to isolate processes the   proc  PID cgroup  file may leak potential system level information to the isolated processes   For example        cat  proc self cgroup   0   batchjobs container id1  The path   batchjobs container id1  can be considered as system data and undesirable to expose to the isolated processes   cgroup namespace can be used to restrict visibility of this path   For example  before creating a cgroup namespace  one would see        ls  l  proc self ns cgroup   lrwxrwxrwx 1 root root 0 2014 07 15 10 37  proc self ns cgroup    cgroup  4026531835      cat  proc self cgroup   0   batchjobs container id1  After unsharing a new namespace  the view changes        ls  l  proc self ns cgroup   lrwxrwxrwx 1 root root 0 2014 07 15 10 35  proc self ns cgroup    cgroup  4026532183      cat  proc self cgroup   0     When some thread from a multi threaded process unshares its cgroup namespace  the new cgroupns gets applied to the entire process  all the threads    This is natural for the v2 hierarchy  however  for the legacy hierarchies  this may be unexpected   A cgroup namespace is alive as long as there are processes inside or mounts pinning it   When the last usage goes away  the cgroup namespace is destroyed   The cgroupns root and the actual cgroups remain    The Root and Views                     The  cgroupns root  for a cgroup namespace is the cgroup in which the process calling unshare 2  is running   For example  if a process in  batchjobs container id1 cgroup calls unshare  cgroup  batchjobs container id1 becomes the cgroupns root   For the init cgroup ns  this is the real root       cgroup   The cgroupns root cgroup does not change even if the namespace creator process later moves to a different cgroup          unshare  c   unshare cgroupns in some cgroup     cat  proc self cgroup   0        mkdir sub cgrp 1     echo 0   sub cgrp 1 cgroup procs     cat  proc self cgroup   0   sub cgrp 1  Each process gets its namespace specific view of   proc  PID cgroup   Processes running inside the cgroup namespace will be able to see cgroup paths  in  proc self cgroup  only inside their root cgroup  From within an unshared cgroupns        sleep 100000      1  7353     echo 7353   sub cgrp 1 cgroup procs     cat  proc 7353 cgroup   0   sub cgrp 1  From the initial cgroup namespace  the real cgroup path will be visible        cat  proc 7353 cgroup   0   batchjobs container id1 sub cgrp 1  From a sibling cgroup namespace  that is  a namespace rooted at a different cgroup   the cgroup path relative to its own cgroup namespace root will be shown   For instance  if PID 7353 s cgroup namespace root is at   batchjobs container id2   then it will see        cat  proc 7353 cgroup   0      container id2 sub cgrp 1  Note that the relative path always starts with     to indicate that its relative to the cgroup namespace root of the caller    Migration and setns 2                          Processes inside a cgroup namespace can move into and out of the namespace root if they have proper access to external cgroups   For example  from inside a namespace with cgroupns root at  batchjobs container id1  and assuming that the global hierarchy is still accessible inside cgroupns        cat  proc 7353 cgroup   0   sub cgrp 1     echo 7353   batchjobs container id2 cgroup procs     cat  proc 7353 cgroup   0      container id2  Note that this kind of setup is not encouraged   A task inside cgroup namespace should only be exposed to its own cgroupns hierarchy   setns 2  to another cgroup namespace is allowed when    a  the process has CAP SYS ADMIN against its current user namespace  b  the process has CAP SYS ADMIN against the target cgroup     namespace s userns  No implicit cgroup changes happen with attaching to another cgroup namespace   It is expected that the someone moves the attaching process under the target cgroup namespace root    Interaction with Other Namespaces                                    Namespace specific cgroup hierarchy can be mounted by a process running inside a non init cgroup namespace        mount  t cgroup2 none  MOUNT POINT  This will mount the unified cgroup hierarchy with cgroupns root as the filesystem root   The process needs CAP SYS ADMIN against its user and mount namespaces   The virtualization of  proc self cgroup file combined with restricting the view of cgroup hierarchy by namespace private cgroupfs mount provides a properly isolated cgroup view inside the container    Information on Kernel Programming                                    This section contains kernel programming information in the areas where interacting with cgroup is necessary   cgroup core and controllers are not covered    Filesystem Support for Writeback                                   A filesystem can support cgroup writeback by updating address space operations  writepage s    to annotate bio s using the following two functions     wbc init bio  wbc   bio   Should be called for each bio carrying writeback data and  associates the bio with the inode s owner cgroup and the  corresponding request queue   This must be called after  a queue  device  has been associated with the bio and  before submission     wbc account cgroup owner  wbc   folio   bytes   Should be called for each data segment being written out   While this function doesn t care exactly when it s called  during the writeback session  it s the easiest and most  natural to call it as data segments are added to a bio   With writeback bio s annotated  cgroup support can be enabled per super block by setting SB I CGROUPWB in   s iflags   This allows for selective disabling of cgroup writeback support which is helpful when certain filesystem features  e g  journaled data mode  are incompatible   wbc init bio   binds the specified bio to its cgroup   Depending on the configuration  the bio may be executed at a lower priority and if the writeback session is holding shared resources  e g  a journal entry  may lead to priority inversion   There is no one easy solution for the problem   Filesystems can try to work around specific problem cases by skipping wbc init bio   and using bio associate blkg   directly    Deprecated v1 Core Features                                Multiple hierarchies including named ones are not supported     All v1 mount options are not supported     The  tasks  file is removed and  cgroup procs  is not sorted      cgroup clone children  is removed      proc cgroups is meaningless for v2   Use  cgroup controllers  or    cgroup stat  files at the root instead    Issues with v1 and Rationales for v2                                       Multiple Hierarchies                       cgroup v1 allowed an arbitrary number of hierarchies and each hierarchy could host any number of controllers   While this seemed to provide a high level of flexibility  it wasn t useful in practice   For example  as there is only one instance of each controller  utility type controllers such as freezer which can be useful in all hierarchies could only be used in one   The issue is exacerbated by the fact that controllers couldn t be moved to another hierarchy once hierarchies were populated   Another issue was that all controllers bound to a hierarchy were forced to have exactly the same view of the hierarchy   It wasn t possible to vary the granularity depending on the specific controller   In practice  these issues heavily limited which controllers could be put on the same hierarchy and most configurations resorted to putting each controller on its own hierarchy   Only closely related ones  such as the cpu and cpuacct controllers  made sense to be put on the same hierarchy   This often meant that userland ended up managing multiple similar hierarchies repeating the same steps on each hierarchy whenever a hierarchy management operation was necessary   Furthermore  support for multiple hierarchies came at a steep cost  It greatly complicated cgroup core implementation but more importantly the support for multiple hierarchies restricted how cgroup could be used in general and what controllers was able to do   There was no limit on how many hierarchies there might be  which meant that a thread s cgroup membership couldn t be described in finite length   The key might contain any number of entries and was unlimited in length  which made it highly awkward to manipulate and led to addition of controllers which existed only to identify membership  which in turn exacerbated the original problem of proliferating number of hierarchies   Also  as a controller couldn t have any expectation regarding the topologies of hierarchies other controllers might be on  each controller had to assume that all other controllers were attached to completely orthogonal hierarchies   This made it impossible  or at least very cumbersome  for controllers to cooperate with each other   In most use cases  putting controllers on hierarchies which are completely orthogonal to each other isn t necessary   What usually is called for is the ability to have differing levels of granularity depending on the specific controller   In other words  hierarchy may be collapsed from leaf towards root when viewed from specific controllers   For example  a given configuration might not care about how memory is distributed beyond a certain level while still wanting to control how CPU cycles are distributed    Thread Granularity                     cgroup v1 allowed threads of a process to belong to different cgroups  This didn t make sense for some controllers and those controllers ended up implementing different ways to ignore such situations but much more importantly it blurred the line between API exposed to individual applications and system management interface   Generally  in process knowledge is available only to the process itself  thus  unlike service level organization of processes  categorizing threads of a process requires active participation from the application which owns the target process   cgroup v1 had an ambiguously defined delegation model which got abused in combination with thread granularity   cgroups were delegated to individual applications so that they can create and manage their own sub hierarchies and control resource distributions along them   This effectively raised cgroup to the status of a syscall like API exposed to lay programs   First of all  cgroup has a fundamentally inadequate interface to be exposed this way   For a process to access its own knobs  it has to extract the path on the target hierarchy from  proc self cgroup  construct the path by appending the name of the knob to the path  open and then read and or write to it   This is not only extremely clunky and unusual but also inherently racy   There is no conventional way to define transaction across the required steps and nothing can guarantee that the process would actually be operating on its own sub hierarchy   cgroup controllers implemented a number of knobs which would never be accepted as public APIs because they were just adding control knobs to system management pseudo filesystem   cgroup ended up with interface knobs which were not properly abstracted or refined and directly revealed kernel internal details   These knobs got exposed to individual applications through the ill defined delegation mechanism effectively abusing cgroup as a shortcut to implementing public APIs without going through the required scrutiny   This was painful for both userland and kernel   Userland ended up with misbehaving and poorly abstracted interfaces and kernel exposing and locked into constructs inadvertently    Competition Between Inner Nodes and Threads                                              cgroup v1 allowed threads to be in any cgroups which created an interesting problem where threads belonging to a parent cgroup and its children cgroups competed for resources   This was nasty as two different types of entities competed and there was no obvious way to settle it   Different controllers did different things   The cpu controller considered threads and cgroups as equivalents and mapped nice levels to cgroup weights   This worked for some cases but fell flat when children wanted to be allocated specific ratios of CPU cycles and the number of internal threads fluctuated   the ratios constantly changed as the number of competing entities fluctuated  There also were other issues   The mapping from nice level to weight wasn t obvious or universal  and there were various other knobs which simply weren t available for threads   The io controller implicitly created a hidden leaf node for each cgroup to host the threads   The hidden leaf had its own copies of all the knobs with   leaf    prefixed   While this allowed equivalent control over internal threads  it was with serious drawbacks   It always added an extra layer of nesting which wouldn t be necessary otherwise  made the interface messy and significantly complicated the implementation   The memory controller didn t have a way to control what happened between internal tasks and child cgroups and the behavior was not clearly defined   There were attempts to add ad hoc behaviors and knobs to tailor the behavior to specific workloads which would have led to problems extremely difficult to resolve in the long term   Multiple controllers struggled with internal tasks and came up with different ways to deal with it  unfortunately  all the approaches were severely flawed and  furthermore  the widely different behaviors made cgroup as a whole highly inconsistent   This clearly is a problem which needs to be addressed from cgroup core in a uniform way    Other Interface Issues                         cgroup v1 grew without oversight and developed a large number of idiosyncrasies and inconsistencies   One issue on the cgroup core side was how an empty cgroup was notified   a userland helper binary was forked and executed for each event   The event delivery wasn t recursive or delegatable   The limitations of the mechanism also led to in kernel event delivery filtering mechanism further complicating the interface   Controller interfaces were problematic too   An extreme example is controllers completely ignoring hierarchical organization and treating all cgroups as if they were all located directly under the root cgroup   Some controllers exposed a large amount of inconsistent implementation details to userland   There also was no consistency across controllers   When a new cgroup was created  some controllers defaulted to not imposing extra restrictions while others disallowed any resource usage until explicitly configured   Configuration knobs for the same type of control used widely differing naming schemes and formats   Statistics and information knobs were named arbitrarily and used different formats and units even in the same controller   cgroup v2 establishes common conventions where appropriate and updates controllers so that they expose minimal and consistent interfaces    Controller Issues and Remedies                                 Memory         The original lower boundary  the soft limit  is defined as a limit that is per default unset   As a result  the set of cgroups that global reclaim prefers is opt in  rather than opt out   The costs for optimizing these mostly negative lookups are so high that the implementation  despite its enormous size  does not even provide the basic desirable behavior   First off  the soft limit has no hierarchical meaning   All configured groups are organized in a global rbtree and treated like equal peers  regardless where they are located in the hierarchy   This makes subtree delegation impossible   Second  the soft limit reclaim pass is so aggressive that it not just introduces high allocation latencies into the system  but also impacts system performance due to overreclaim  to the point where the feature becomes self defeating   The memory low boundary on the other hand is a top down allocated reserve   A cgroup enjoys reclaim protection when it s within its effective low  which makes delegation of subtrees possible  It also enjoys having reclaim pressure proportional to its overage when above its effective low   The original high boundary  the hard limit  is defined as a strict limit that can not budge  even if the OOM killer has to be called  But this generally goes against the goal of making the most out of the available memory   The memory consumption of workloads varies during runtime  and that requires users to overcommit   But doing that with a strict upper limit requires either a fairly accurate prediction of the working set size or adding slack to the limit   Since working set size estimation is hard and error prone  and getting it wrong results in OOM kills  most users tend to err on the side of a looser limit and end up wasting precious resources   The memory high boundary on the other hand can be set much more conservatively   When hit  it throttles allocations by forcing them into direct reclaim to work off the excess  but it never invokes the OOM killer   As a result  a high boundary that is chosen too aggressively will not terminate the processes  but instead it will lead to gradual performance degradation   The user can monitor this and make corrections until the minimal memory footprint that still gives acceptable performance is found   In extreme cases  with many concurrent allocations and a complete breakdown of reclaim progress within the group  the high boundary can be exceeded   But even then it s mostly better to satisfy the allocation from the slack available in other groups or the rest of the system than killing the group   Otherwise  memory max is there to limit this type of spillover and ultimately contain buggy or even malicious applications   Setting the original memory limit in bytes below the current usage was subject to a race condition  where concurrent charges could cause the limit setting to fail  memory max on the other hand will first set the limit to prevent new charges  and then reclaim and OOM kill until the new limit is met   or the task writing to memory max is killed   The combined memory swap accounting and limiting is replaced by real control over swap space   The main argument for a combined memory swap facility in the original cgroup design was that global or parental pressure would always be able to swap all anonymous memory of a child group  regardless of the child s own  possibly untrusted  configuration   However  untrusted groups can sabotage swapping by other means   such as referencing its anonymous memory in a tight loop   and an admin can not assume full swappability when overcommitting untrusted jobs   For trusted jobs  on the other hand  a combined counter is not an intuitive userspace interface  and it flies in the face of the idea that cgroup controllers should account and limit specific physical resources   Swap space is a resource like all others in the system  and that s why unified hierarchy allows distributing it separately