Control Group v2
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================
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 Documentation/cgroup-v1/.
… CONTENTS
- Introduction
1-1. Terminology
1-2. What is cgroup? - 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 - Resource Distribution Models
3-1. Weights
3-2. Limits
3-3. Protections
3-4. Allocations - Interface Files
4-1. Format
4-2. Conventions
4-3. Core Interface Files - 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
5-4. PID
5-4-1. PID Interface Files
5-5. Device
5-6. RDMA
5-6-1. RDMA Interface Files
5-7. Misc
5-7-1. 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 - 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.
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.
[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, the
kernel rejects writes to all files other than "cgroup.procs" and
"cgroup.subtree_control" on a namespace root from inside the
namespace.
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::
~ 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.
Limits
------
A child can only consume upto 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.
Protections
-----------
A cgroup is protected to be allocated upto the configured amount of
the resource if 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 upto 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. Also,
informational files on the root cgroup which end up showing global
information available elsewhere shouldn't exist.
- 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.
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.
Controllers
===========
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.
WARNING: cgroup2 doesn't yet support control of realtime processes and
the cpu controller can only be enabled when all RT processes are in
the root cgroup. 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.
CPU Interface Files
~~~~~~~~~~~~~~~~~~~
All time durations are in microseconds.
cpu.stat
A read-only flat-keyed file which exists on non-root cgroups.
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 three when the controller is enabled:
- nr_periods
- nr_throttled
- throttled_usec
cpu.weight
A read-write single value file which exists on non-root
cgroups. The default is "100".
The weight in the range [1, 10000].
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 upto $MAX in each
$PERIOD duration. "max" for $MAX indicates no limit. If only
one number is written, $MAX is updated.
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.low
A read-write single value file which exists on non-root
cgroups. The default is “0”.
Best-effort memory protection. If the memory usages of a
cgroup and all its ancestors are below their low boundaries,
the cgroup's memory won't be reclaimed unless memory can be
reclaimed from unprotected cgroups.
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. This is the main mechanism to
control memory usage of a cgroup. 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.
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 final protection
mechanism. 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.
This is the ultimate protection mechanism. As long as the
high limit is used and monitored properly, this limit's
utility is limited to providing the final safety net.
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.
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.
Depending on context result could be invocation of OOM
killer and retrying allocation or failing allocation.
Failed allocation in its turn could be returned into
userspace as -ENOMEM or silently ignored in cases like
disk readahead. For now OOM in memory cgroup kills
tasks iff shortage has happened inside page fault.
oom_kill
The number of processes belonging to this cgroup
killed by any kind of OOM killer.
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!
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_stack
Amount of memory allocated to kernel stacks.
slab
Amount of memory used for storing in-kernel data
structures.
sock
Amount of memory used in network transmission buffers
shmem
Amount of cached filesystem data that is swap-backed,
such as tmpfs, shm segments, shared anonymous mmap()s
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
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
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.
pgfault
Total number of page faults incurred
pgmajfault
Number of major page faults incurred
workingset_refault
Number of refaults of previously evicted pages
workingset_activate
Number of refaulted pages that were immediately activated
workingset_nodereclaim
Number of times a shadow node has been reclaimed
pgrefill
Amount of scanned pages (in an active LRU list)
pgscan
Amount of scanned pages (in an inactive LRU list)
pgsteal
Amount of reclaimed pages
pgactivate
Amount of pages moved to the active LRU list
pgdeactivate
Amount of pages moved to the inactive LRU lis
pglazyfree
Amount of pages postponed to be freed under memory pressure
pglazyfreed
Amount of reclaimed lazyfree pages
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.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.
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 which exists on non-root
cgroups.
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
====== ===================
An example read output follows:
8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353
8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252
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
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
and btrfs. 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.
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 all cgroups.
The number of processes currently in the cgroup and its
descendants.
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.
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 the BPF_CGROUP_DEVICE type and attach them
to cgroups. 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_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_CGROUP_DEVICE program may be found in the kernel
source tree in the tools/testing/selftests/bpf/dev_cgroup.c file.
RDMA
The “rdma” controller regulates the distribution and accounting of
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
Misc
----
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. Can be
called anytime between bio allocation and submission.
wbc_account_io(@wbc, @page, @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
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