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NAME | DESCRIPTION | CGROUPS VERSION 1 | CGROUPS VERSION 2 | CGROUPS VERSION 2 THREAD MODE | ERRORS | NOTES | SEE ALSO | COLOPHON |
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cgroups(7) Miscellaneous Information Manual cgroups(7)
cgroups - Linux control groups
Control groups, usually referred to as cgroups, are a Linux
kernel feature which allow processes to be organized into
hierarchical groups whose usage of various types of resources can
then be limited and monitored. The kernel's cgroup interface is
provided through a pseudo-filesystem called cgroupfs. Grouping
is implemented in the core cgroup kernel code, while resource
tracking and limits are implemented in a set of per-resource-type
subsystems (memory, CPU, and so on).
Terminology
A cgroup is a collection of processes that are bound to a set of
limits or parameters defined via the cgroup filesystem.
A subsystem is a kernel component that modifies the behavior of
the processes in a cgroup. Various subsystems have been
implemented, making it possible to do things such as limiting the
amount of CPU time and memory available to a cgroup, accounting
for the CPU time used by a cgroup, and freezing and resuming
execution of the processes in a cgroup. Subsystems are sometimes
also known as resource controllers (or simply, controllers).
The cgroups for a controller are arranged in a hierarchy. This
hierarchy is defined by creating, removing, and renaming
subdirectories within the cgroup filesystem. At each level of
the hierarchy, attributes (e.g., limits) can be defined. The
limits, control, and accounting provided by cgroups generally
have effect throughout the subhierarchy underneath the cgroup
where the attributes are defined. Thus, for example, the limits
placed on a cgroup at a higher level in the hierarchy cannot be
exceeded by descendant cgroups.
Cgroups version 1 and version 2
The initial release of the cgroups implementation was in Linux
2.6.24. Over time, various cgroup controllers have been added to
allow the management of various types of resources. However, the
development of these controllers was largely uncoordinated, with
the result that many inconsistencies arose between controllers
and management of the cgroup hierarchies became rather complex.
A longer description of these problems can be found in the kernel
source file Documentation/admin-guide/cgroup-v2.rst (or
Documentation/cgroup-v2.txt in Linux 4.17 and earlier).
Because of the problems with the initial cgroups implementation
(cgroups version 1), starting in Linux 3.10, work began on a new,
orthogonal implementation to remedy these problems. Initially
marked experimental, and hidden behind the
-o __DEVEL__sane_behavior mount option, the new version (cgroups
version 2) was eventually made official with the release of Linux
4.5. Differences between the two versions are described in the
text below. The file cgroup.sane_behavior, present in cgroups
v1, is a relic of this mount option. The file always reports "0"
and is only retained for backward compatibility.
Although cgroups v2 is intended as a replacement for cgroups v1,
the older system continues to exist (and for compatibility
reasons is unlikely to be removed). Currently, cgroups v2
implements only a subset of the controllers available in cgroups
v1. The two systems are implemented so that both v1 controllers
and v2 controllers can be mounted on the same system. Thus, for
example, it is possible to use those controllers that are
supported under version 2, while also using version 1 controllers
where version 2 does not yet support those controllers. The only
restriction here is that a controller can't be simultaneously
employed in both a cgroups v1 hierarchy and in the cgroups v2
hierarchy.
Under cgroups v1, each controller may be mounted against a
separate cgroup filesystem that provides its own hierarchical
organization of the processes on the system. It is also possible
to comount multiple (or even all) cgroups v1 controllers against
the same cgroup filesystem, meaning that the comounted
controllers manage the same hierarchical organization of
processes.
For each mounted hierarchy, the directory tree mirrors the
control group hierarchy. Each control group is represented by a
directory, with each of its child control cgroups represented as
a child directory. For instance, /user/joe/1.session represents
control group 1.session, which is a child of cgroup joe, which is
a child of /user. Under each cgroup directory is a set of files
which can be read or written to, reflecting resource limits and a
few general cgroup properties.
Tasks (threads) versus processes
In cgroups v1, a distinction is drawn between processes and
tasks. In this view, a process can consist of multiple tasks
(more commonly called threads, from a user-space perspective, and
called such in the remainder of this man page). In cgroups v1,
it is possible to independently manipulate the cgroup memberships
of the threads in a process.
The cgroups v1 ability to split threads across different cgroups
caused problems in some cases. For example, it made no sense for
the memory controller, since all of the threads of a process
share a single address space. Because of these problems, the
ability to independently manipulate the cgroup memberships of the
threads in a process was removed in the initial cgroups v2
implementation, and subsequently restored in a more limited form
(see the discussion of "thread mode" below).
Mounting v1 controllers
The use of cgroups requires a kernel built with the CONFIG_CGROUP
option. In addition, each of the v1 controllers has an
associated configuration option that must be set in order to
employ that controller.
In order to use a v1 controller, it must be mounted against a
cgroup filesystem. The usual place for such mounts is under a
tmpfs(5) filesystem mounted at /sys/fs/cgroup. Thus, one might
mount the cpu controller as follows:
mount -t cgroup -o cpu none /sys/fs/cgroup/cpu
It is possible to comount multiple controllers against the same
hierarchy. For example, here the cpu and cpuacct controllers are
comounted against a single hierarchy:
mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct
Comounting controllers has the effect that a process is in the
same cgroup for all of the comounted controllers. Separately
mounting controllers allows a process to be in cgroup /foo1 for
one controller while being in /foo2/foo3 for another.
It is possible to comount all v1 controllers against the same
hierarchy:
mount -t cgroup -o all cgroup /sys/fs/cgroup
(One can achieve the same result by omitting -o all, since it is
the default if no controllers are explicitly specified.)
It is not possible to mount the same controller against multiple
cgroup hierarchies. For example, it is not possible to mount
both the cpu and cpuacct controllers against one hierarchy, and
to mount the cpu controller alone against another hierarchy. It
is possible to create multiple mount with exactly the same set of
comounted controllers. However, in this case all that results is
multiple mount points providing a view of the same hierarchy.
Note that on many systems, the v1 controllers are automatically
mounted under /sys/fs/cgroup; in particular, systemd(1)
automatically creates such mounts.
Unmounting v1 controllers
A mounted cgroup filesystem can be unmounted using the umount(8)
command, as in the following example:
umount /sys/fs/cgroup/pids
But note well: a cgroup filesystem is unmounted only if it is not
busy, that is, it has no child cgroups. If this is not the case,
then the only effect of the umount(8) is to make the mount
invisible. Thus, to ensure that the mount is really removed, one
must first remove all child cgroups, which in turn can be done
only after all member processes have been moved from those
cgroups to the root cgroup.
Cgroups version 1 controllers
Each of the cgroups version 1 controllers is governed by a kernel
configuration option (listed below). Additionally, the
availability of the cgroups feature is governed by the
CONFIG_CGROUPS kernel configuration option.
cpu (since Linux 2.6.24; CONFIG_CGROUP_SCHED)
Cgroups can be guaranteed a minimum number of "CPU shares"
when a system is busy. This does not limit a cgroup's CPU
usage if the CPUs are not busy. For further information,
see Documentation/scheduler/sched-design-CFS.rst (or
Documentation/scheduler/sched-design-CFS.txt in Linux 5.2
and earlier).
In Linux 3.2, this controller was extended to provide CPU
"bandwidth" control. If the kernel is configured with
CONFIG_CFS_BANDWIDTH, then within each scheduling period
(defined via a file in the cgroup directory), it is
possible to define an upper limit on the CPU time
allocated to the processes in a cgroup. This upper limit
applies even if there is no other competition for the CPU.
Further information can be found in the kernel source file
Documentation/scheduler/sched-bwc.rst (or
Documentation/scheduler/sched-bwc.txt in Linux 5.2 and
earlier).
cpuacct (since Linux 2.6.24; CONFIG_CGROUP_CPUACCT)
This provides accounting for CPU usage by groups of
processes.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/cpuacct.rst (or
Documentation/cgroup-v1/cpuacct.txt in Linux 5.2 and
earlier).
cpuset (since Linux 2.6.24; CONFIG_CPUSETS)
This cgroup can be used to bind the processes in a cgroup
to a specified set of CPUs and NUMA nodes.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/cpusets.rst (or
Documentation/cgroup-v1/cpusets.txt in Linux 5.2 and
earlier).
memory (since Linux 2.6.25; CONFIG_MEMCG)
The memory controller supports reporting and limiting of
process memory, kernel memory, and swap used by cgroups.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/memory.rst (or
Documentation/cgroup-v1/memory.txt in Linux 5.2 and
earlier).
devices (since Linux 2.6.26; CONFIG_CGROUP_DEVICE)
This supports controlling which processes may create
(mknod) devices as well as open them for reading or
writing. The policies may be specified as allow-lists and
deny-lists. Hierarchy is enforced, so new rules must not
violate existing rules for the target or ancestor cgroups.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/devices.rst (or
Documentation/cgroup-v1/devices.txt in Linux 5.2 and
earlier).
freezer (since Linux 2.6.28; CONFIG_CGROUP_FREEZER)
The freezer cgroup can suspend and restore (resume) all
processes in a cgroup. Freezing a cgroup /A also causes
its children, for example, processes in /A/B, to be
frozen.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst
(or Documentation/cgroup-v1/freezer-subsystem.txt in Linux
5.2 and earlier).
net_cls (since Linux 2.6.29; CONFIG_CGROUP_NET_CLASSID)
This places a classid, specified for the cgroup, on
network packets created by a cgroup. These classids can
then be used in firewall rules, as well as used to shape
traffic using tc(8). This applies only to packets leaving
the cgroup, not to traffic arriving at the cgroup.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/net_cls.rst (or
Documentation/cgroup-v1/net_cls.txt in Linux 5.2 and
earlier).
blkio (since Linux 2.6.33; CONFIG_BLK_CGROUP)
The blkio cgroup controls and limits access to specified
block devices by applying IO control in the form of
throttling and upper limits against leaf nodes and
intermediate nodes in the storage hierarchy.
Two policies are available. The first is a proportional-
weight time-based division of disk implemented with CFQ.
This is in effect for leaf nodes using CFQ. The second is
a throttling policy which specifies upper I/O rate limits
on a device.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/blkio-controller.rst
(or Documentation/cgroup-v1/blkio-controller.txt in Linux
5.2 and earlier).
perf_event (since Linux 2.6.39; CONFIG_CGROUP_PERF)
This controller allows perf monitoring of the set of
processes grouped in a cgroup.
Further information can be found in the kernel source
files
net_prio (since Linux 3.3; CONFIG_CGROUP_NET_PRIO)
This allows priorities to be specified, per network
interface, for cgroups.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/net_prio.rst (or
Documentation/cgroup-v1/net_prio.txt in Linux 5.2 and
earlier).
hugetlb (since Linux 3.5; CONFIG_CGROUP_HUGETLB)
This supports limiting the use of huge pages by cgroups.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/hugetlb.rst (or
Documentation/cgroup-v1/hugetlb.txt in Linux 5.2 and
earlier).
pids (since Linux 4.3; CONFIG_CGROUP_PIDS)
This controller permits limiting the number of process
that may be created in a cgroup (and its descendants).
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/pids.rst (or
Documentation/cgroup-v1/pids.txt in Linux 5.2 and
earlier).
rdma (since Linux 4.11; CONFIG_CGROUP_RDMA)
The RDMA controller permits limiting the use of RDMA/IB-
specific resources per cgroup.
Further information can be found in the kernel source file
Documentation/admin-guide/cgroup-v1/rdma.rst (or
Documentation/cgroup-v1/rdma.txt in Linux 5.2 and
earlier).
Creating cgroups and moving processes
A cgroup filesystem initially contains a single root cgroup, '/',
which all processes belong to. A new cgroup is created by
creating a directory in the cgroup filesystem:
mkdir /sys/fs/cgroup/cpu/cg1
This creates a new empty cgroup.
A process may be moved to this cgroup by writing its PID into the
cgroup's cgroup.procs file:
echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs
Only one PID at a time should be written to this file.
Writing the value 0 to a cgroup.procs file causes the writing
process to be moved to the corresponding cgroup.
When writing a PID into the cgroup.procs, all threads in the
process are moved into the new cgroup at once.
Within a hierarchy, a process can be a member of exactly one
cgroup. Writing a process's PID to a cgroup.procs file
automatically removes it from the cgroup of which it was
previously a member.
The cgroup.procs file can be read to obtain a list of the
processes that are members of a cgroup. The returned list of
PIDs is not guaranteed to be in order. Nor is it guaranteed to
be free of duplicates. (For example, a PID may be recycled while
reading from the list.)
In cgroups v1, an individual thread can be moved to another
cgroup by writing its thread ID (i.e., the kernel thread ID
returned by clone(2) and gettid(2)) to the tasks file in a cgroup
directory. This file can be read to discover the set of threads
that are members of the cgroup.
Removing cgroups
To remove a cgroup, it must first have no child cgroups and
contain no (nonzombie) processes. So long as that is the case,
one can simply remove the corresponding directory pathname. Note
that files in a cgroup directory cannot and need not be removed.
Cgroups v1 release notification
Two files can be used to determine whether the kernel provides
notifications when a cgroup becomes empty. A cgroup is
considered to be empty when it contains no child cgroups and no
member processes.
A special file in the root directory of each cgroup hierarchy,
release_agent, can be used to register the pathname of a program
that may be invoked when a cgroup in the hierarchy becomes empty.
The pathname of the newly empty cgroup (relative to the cgroup
mount point) is provided as the sole command-line argument when
the release_agent program is invoked. The release_agent program
might remove the cgroup directory, or perhaps repopulate it with
a process.
The default value of the release_agent file is empty, meaning
that no release agent is invoked.
The content of the release_agent file can also be specified via a
mount option when the cgroup filesystem is mounted:
mount -o release_agent=pathname ...
Whether or not the release_agent program is invoked when a
particular cgroup becomes empty is determined by the value in the
notify_on_release file in the corresponding cgroup directory. If
this file contains the value 0, then the release_agent program is
not invoked. If it contains the value 1, the release_agent
program is invoked. The default value for this file in the root
cgroup is 0. At the time when a new cgroup is created, the value
in this file is inherited from the corresponding file in the
parent cgroup.
Cgroup v1 named hierarchies
In cgroups v1, it is possible to mount a cgroup hierarchy that
has no attached controllers:
mount -t cgroup -o none,name=somename none /some/mount/point
Multiple instances of such hierarchies can be mounted; each
hierarchy must have a unique name. The only purpose of such
hierarchies is to track processes. (See the discussion of
release notification below.) An example of this is the
name=systemd cgroup hierarchy that is used by systemd(1) to track
services and user sessions.
Since Linux 5.0, the cgroup_no_v1 kernel boot option (described
below) can be used to disable cgroup v1 named hierarchies, by
specifying cgroup_no_v1=named.
In cgroups v2, all mounted controllers reside in a single unified
hierarchy. While (different) controllers may be simultaneously
mounted under the v1 and v2 hierarchies, it is not possible to
mount the same controller simultaneously under both the v1 and
the v2 hierarchies.
The new behaviors in cgroups v2 are summarized here, and in some
cases elaborated in the following subsections.
• Cgroups v2 provides a unified hierarchy against which all
controllers are mounted.
• "Internal" processes are not permitted. With the exception of
the root cgroup, processes may reside only in leaf nodes
(cgroups that do not themselves contain child cgroups). The
details are somewhat more subtle than this, and are described
below.
• Active cgroups must be specified via the files
cgroup.controllers and cgroup.subtree_control.
• The tasks file has been removed. In addition, the
cgroup.clone_children file that is employed by the cpuset
controller has been removed.
• An improved mechanism for notification of empty cgroups is
provided by the cgroup.events file.
For more changes, see the Documentation/admin-guide/cgroup-v2.rst
file in the kernel source (or Documentation/cgroup-v2.txt in
Linux 4.17 and earlier).
Some of the new behaviors listed above saw subsequent
modification with the addition in Linux 4.14 of "thread mode"
(described below).
Cgroups v2 unified hierarchy
In cgroups v1, the ability to mount different controllers against
different hierarchies was intended to allow great flexibility for
application design. In practice, though, the flexibility turned
out to be less useful than expected, and in many cases added
complexity. Therefore, in cgroups v2, all available controllers
are mounted against a single hierarchy. The available
controllers are automatically mounted, meaning that it is not
necessary (or possible) to specify the controllers when mounting
the cgroup v2 filesystem using a command such as the following:
mount -t cgroup2 none /mnt/cgroup2
A cgroup v2 controller is available only if it is not currently
in use via a mount against a cgroup v1 hierarchy. Or, to put
things another way, it is not possible to employ the same
controller against both a v1 hierarchy and the unified v2
hierarchy. This means that it may be necessary first to unmount
a v1 controller (as described above) before that controller is
available in v2. Since systemd(1) makes heavy use of some v1
controllers by default, it can in some cases be simpler to boot
the system with selected v1 controllers disabled. To do this,
specify the cgroup_no_v1=list option on the kernel boot command
line; list is a comma-separated list of the names of the
controllers to disable, or the word all to disable all v1
controllers. (This situation is correctly handled by systemd(1),
which falls back to operating without the specified controllers.)
Note that on many modern systems, systemd(1) automatically mounts
the cgroup2 filesystem at /sys/fs/cgroup/unified during the boot
process.
Cgroups v2 mount options
The following options (mount -o) can be specified when mounting
the group v2 filesystem:
nsdelegate (since Linux 4.15)
Treat cgroup namespaces as delegation boundaries. For
details, see below.
memory_localevents (since Linux 5.2)
The memory.events should show statistics only for the
cgroup itself, and not for any descendant cgroups. This
was the behavior before Linux 5.2. Starting in Linux 5.2,
the default behavior is to include statistics for
descendant cgroups in memory.events, and this mount option
can be used to revert to the legacy behavior. This option
is system wide and can be set on mount or modified through
remount only from the initial mount namespace; it is
silently ignored in noninitial namespaces.
Cgroups v2 controllers
The following controllers, documented in the kernel source file
Documentation/admin-guide/cgroup-v2.rst (or
Documentation/cgroup-v2.txt in Linux 4.17 and earlier), are
supported in cgroups version 2:
cpu (since Linux 4.15)
This is the successor to the version 1 cpu and cpuacct
controllers.
cpuset (since Linux 5.0)
This is the successor of the version 1 cpuset controller.
freezer (since Linux 5.2)
This is the successor of the version 1 freezer controller.
hugetlb (since Linux 5.6)
This is the successor of the version 1 hugetlb controller.
io (since Linux 4.5)
This is the successor of the version 1 blkio controller.
memory (since Linux 4.5)
This is the successor of the version 1 memory controller.
perf_event (since Linux 4.11)
This is the same as the version 1 perf_event controller.
pids (since Linux 4.5)
This is the same as the version 1 pids controller.
rdma (since Linux 4.11)
This is the same as the version 1 rdma controller.
There is no direct equivalent of the net_cls and net_prio
controllers from cgroups version 1. Instead, support has been
added to iptables(8) to allow eBPF filters that hook on cgroup v2
pathnames to make decisions about network traffic on a per-cgroup
basis.
The v2 devices controller provides no interface files; instead,
device control is gated by attaching an eBPF (BPF_CGROUP_DEVICE)
program to a v2 cgroup.
Cgroups v2 subtree control
Each cgroup in the v2 hierarchy contains the following two files:
cgroup.controllers
This read-only file exposes a list of the controllers that
are available in this cgroup. The contents of this file
match the contents of the cgroup.subtree_control file in
the parent cgroup.
cgroup.subtree_control
This is a list of controllers that are active (enabled) in
the cgroup. The set of controllers in this file is a
subset of the set in the cgroup.controllers of this
cgroup. The set of active controllers is modified by
writing strings to this file containing space-delimited
controller names, each preceded by '+' (to enable a
controller) or '-' (to disable a controller), as in the
following example:
echo '+pids -memory' > x/y/cgroup.subtree_control
An attempt to enable a controller that is not present in
cgroup.controllers leads to an ENOENT error when writing
to the cgroup.subtree_control file.
Because the list of controllers in cgroup.subtree_control is a
subset of those cgroup.controllers, a controller that has been
disabled in one cgroup in the hierarchy can never be re-enabled
in the subtree below that cgroup.
A cgroup's cgroup.subtree_control file determines the set of
controllers that are exercised in the child cgroups. When a
controller (e.g., pids) is present in the cgroup.subtree_control
file of a parent cgroup, then the corresponding controller-
interface files (e.g., pids.max) are automatically created in the
children of that cgroup and can be used to exert resource control
in the child cgroups.
Cgroups v2 "no internal processes" rule
Cgroups v2 enforces a so-called "no internal processes" rule.
Roughly speaking, this rule means that, with the exception of the
root cgroup, processes may reside only in leaf nodes (cgroups
that do not themselves contain child cgroups). This avoids the
need to decide how to partition resources between processes which
are members of cgroup A and processes in child cgroups of A.
For instance, if cgroup /cg1/cg2 exists, then a process may
reside in /cg1/cg2, but not in /cg1. This is to avoid an
ambiguity in cgroups v1 with respect to the delegation of
resources between processes in /cg1 and its child cgroups. The
recommended approach in cgroups v2 is to create a subdirectory
called leaf for any nonleaf cgroup which should contain
processes, but no child cgroups. Thus, processes which
previously would have gone into /cg1 would now go into /cg1/leaf.
This has the advantage of making explicit the relationship
between processes in /cg1/leaf and /cg1's other children.
The "no internal processes" rule is in fact more subtle than
stated above. More precisely, the rule is that a (nonroot)
cgroup can't both (1) have member processes, and (2) distribute
resources into child cgroups—that is, have a nonempty
cgroup.subtree_control file. Thus, it is possible for a cgroup
to have both member processes and child cgroups, but before
controllers can be enabled for that cgroup, the member processes
must be moved out of the cgroup (e.g., perhaps into the child
cgroups).
With the Linux 4.14 addition of "thread mode" (described below),
the "no internal processes" rule has been relaxed in some cases.
Cgroups v2 cgroup.events file
Each nonroot cgroup in the v2 hierarchy contains a read-only
file, cgroup.events, whose contents are key-value pairs
(delimited by newline characters, with the key and value
separated by spaces) providing state information about the
cgroup:
$ cat mygrp/cgroup.events
populated 1
frozen 0
The following keys may appear in this file:
populated
The value of this key is either 1, if this cgroup or any
of its descendants has member processes, or otherwise 0.
frozen (since Linux 5.2)
The value of this key is 1 if this cgroup is currently
frozen, or 0 if it is not.
The cgroup.events file can be monitored, in order to receive
notification when the value of one of its keys changes. Such
monitoring can be done using inotify(7), which notifies changes
as IN_MODIFY events, or poll(2), which notifies changes by
returning the POLLPRI and POLLERR bits in the revents field.
Cgroup v2 release notification
Cgroups v2 provides a new mechanism for obtaining notification
when a cgroup becomes empty. The cgroups v1 release_agent and
notify_on_release files are removed, and replaced by the
populated key in the cgroup.events file. This key either has the
value 0, meaning that the cgroup (and its descendants) contain no
(nonzombie) member processes, or 1, meaning that the cgroup (or
one of its descendants) contains member processes.
The cgroups v2 release-notification mechanism offers the
following advantages over the cgroups v1 release_agent mechanism:
• It allows for cheaper notification, since a single process can
monitor multiple cgroup.events files (using the techniques
described earlier). By contrast, the cgroups v1 mechanism
requires the expense of creating a process for each
notification.
• Notification for different cgroup subhierarchies can be
delegated to different processes. By contrast, the cgroups v1
mechanism allows only one release agent for an entire
hierarchy.
Cgroups v2 cgroup.stat file
Each cgroup in the v2 hierarchy contains a read-only cgroup.stat
file (first introduced in Linux 4.14) that consists of lines
containing key-value pairs. The following keys currently appear
in this file:
nr_descendants
This is the total number of visible (i.e., living)
descendant cgroups underneath this cgroup.
nr_dying_descendants
This is the total number of dying descendant cgroups
underneath this cgroup. A cgroup enters the dying state
after being deleted. It remains in that state for an
undefined period (which will depend on system load) while
resources are freed before the cgroup is destroyed. Note
that the presence of some cgroups in the dying state is
normal, and is not indicative of any problem.
A process can't be made a member of a dying cgroup, and a
dying cgroup can't be brought back to life.
Limiting the number of descendant cgroups
Each cgroup in the v2 hierarchy contains the following files,
which can be used to view and set limits on the number of
descendant cgroups under that cgroup:
cgroup.max.depth (since Linux 4.14)
This file defines a limit on the depth of nesting of
descendant cgroups. A value of 0 in this file means that
no descendant cgroups can be created. An attempt to
create a descendant whose nesting level exceeds the limit
fails (mkdir(2) fails with the error EAGAIN).
Writing the string "max" to this file means that no limit
is imposed. The default value in this file is "max".
cgroup.max.descendants (since Linux 4.14)
This file defines a limit on the number of live descendant
cgroups that this cgroup may have. An attempt to create
more descendants than allowed by the limit fails (mkdir(2)
fails with the error EAGAIN).
Writing the string "max" to this file means that no limit
is imposed. The default value in this file is "max".
CGROUPS DELEGATION: DELEGATING A HIERARCHY TO A LESS PRIVILEGED USER
%%%SH%%%
In the context of cgroups, delegation means passing management of
some subtree of the cgroup hierarchy to a nonprivileged user.
Cgroups v1 provides support for delegation based on file
permissions in the cgroup hierarchy but with less strict
containment rules than v2 (as noted below). Cgroups v2 supports
delegation with containment by explicit design. The focus of the
discussion in this section is on delegation in cgroups v2, with
some differences for cgroups v1 noted along the way.
Some terminology is required in order to describe delegation. A
delegater is a privileged user (i.e., root) who owns a parent
cgroup. A delegatee is a nonprivileged user who will be granted
the permissions needed to manage some subhierarchy under that
parent cgroup, known as the delegated subtree.
To perform delegation, the delegater makes certain directories
and files writable by the delegatee, typically by changing the
ownership of the objects to be the user ID of the delegatee.
Assuming that we want to delegate the hierarchy rooted at (say)
/dlgt_grp and that there are not yet any child cgroups under that
cgroup, the ownership of the following is changed to the user ID
of the delegatee:
/dlgt_grp
Changing the ownership of the root of the subtree means
that any new cgroups created under the subtree (and the
files they contain) will also be owned by the delegatee.
/dlgt_grp/cgroup.procs
Changing the ownership of this file means that the
delegatee can move processes into the root of the
delegated subtree.
/dlgt_grp/cgroup.subtree_control (cgroups v2 only)
Changing the ownership of this file means that the
delegatee can enable controllers (that are present in
/dlgt_grp/cgroup.controllers) in order to further
redistribute resources at lower levels in the subtree.
(As an alternative to changing the ownership of this file,
the delegater might instead add selected controllers to
this file.)
/dlgt_grp/cgroup.threads (cgroups v2 only)
Changing the ownership of this file is necessary if a
threaded subtree is being delegated (see the description
of "thread mode", below). This permits the delegatee to
write thread IDs to the file. (The ownership of this file
can also be changed when delegating a domain subtree, but
currently this serves no purpose, since, as described
below, it is not possible to move a thread between domain
cgroups by writing its thread ID to the cgroup.threads
file.)
In cgroups v1, the corresponding file that should instead
be delegated is the tasks file.
The delegater should not change the ownership of any of the
controller interfaces files (e.g., pids.max, memory.high) in
dlgt_grp. Those files are used from the next level above the
delegated subtree in order to distribute resources into the
subtree, and the delegatee should not have permission to change
the resources that are distributed into the delegated subtree.
See also the discussion of the /sys/kernel/cgroup/delegate file
in NOTES for information about further delegatable files in
cgroups v2.
After the aforementioned steps have been performed, the delegatee
can create child cgroups within the delegated subtree (the cgroup
subdirectories and the files they contain will be owned by the
delegatee) and move processes between cgroups in the subtree. If
some controllers are present in dlgt_grp/cgroup.subtree_control,
or the ownership of that file was passed to the delegatee, the
delegatee can also control the further redistribution of the
corresponding resources into the delegated subtree.
Cgroups v2 delegation: nsdelegate and cgroup namespaces
Starting with Linux 4.13, there is a second way to perform cgroup
delegation in the cgroups v2 hierarchy. This is done by mounting
or remounting the cgroup v2 filesystem with the nsdelegate mount
option. For example, if the cgroup v2 filesystem has already
been mounted, we can remount it with the nsdelegate option as
follows:
mount -t cgroup2 -o remount,nsdelegate \
none /sys/fs/cgroup/unified
The effect of this mount option is to cause cgroup namespaces to
automatically become delegation boundaries. More specifically,
the following restrictions apply for processes inside the cgroup
namespace:
• Writes to controller interface files in the root directory of
the namespace will fail with the error EPERM. Processes
inside the cgroup namespace can still write to delegatable
files in the root directory of the cgroup namespace such as
cgroup.procs and cgroup.subtree_control, and can create
subhierarchy underneath the root directory.
• Attempts to migrate processes across the namespace boundary
are denied (with the error ENOENT). Processes inside the
cgroup namespace can still (subject to the containment rules
described below) move processes between cgroups within the
subhierarchy under the namespace root.
The ability to define cgroup namespaces as delegation boundaries
makes cgroup namespaces more useful. To understand why, suppose
that we already have one cgroup hierarchy that has been delegated
to a nonprivileged user, cecilia, using the older delegation
technique described above. Suppose further that cecilia wanted
to further delegate a subhierarchy under the existing delegated
hierarchy. (For example, the delegated hierarchy might be
associated with an unprivileged container run by cecilia.) Even
if a cgroup namespace was employed, because both hierarchies are
owned by the unprivileged user cecilia, the following
illegitimate actions could be performed:
• A process in the inferior hierarchy could change the resource
controller settings in the root directory of that hierarchy.
(These resource controller settings are intended to allow
control to be exercised from the parent cgroup; a process
inside the child cgroup should not be allowed to modify them.)
• A process inside the inferior hierarchy could move processes
into and out of the inferior hierarchy if the cgroups in the
superior hierarchy were somehow visible.
Employing the nsdelegate mount option prevents both of these
possibilities.
The nsdelegate mount option only has an effect when performed in
the initial mount namespace; in other mount namespaces, the
option is silently ignored.
Note: On some systems, systemd(1) automatically mounts the cgroup
v2 filesystem. In order to experiment with the nsdelegate
operation, it may be useful to boot the kernel with the following
command-line options:
cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller
These options cause the kernel to boot with the cgroups v1
controllers disabled (meaning that the controllers are available
in the v2 hierarchy), and tells systemd(1) not to mount and use
the cgroup v2 hierarchy, so that the v2 hierarchy can be manually
mounted with the desired options after boot-up.
Cgroup delegation containment rules
Some delegation containment rules ensure that the delegatee can
move processes between cgroups within the delegated subtree, but
can't move processes from outside the delegated subtree into the
subtree or vice versa. A nonprivileged process (i.e., the
delegatee) can write the PID of a "target" process into a
cgroup.procs file only if all of the following are true:
• The writer has write permission on the cgroup.procs file in
the destination cgroup.
• The writer has write permission on the cgroup.procs file in
the nearest common ancestor of the source and destination
cgroups. Note that in some cases, the nearest common ancestor
may be the source or destination cgroup itself. This
requirement is not enforced for cgroups v1 hierarchies, with
the consequence that containment in v1 is less strict than in
v2. (For example, in cgroups v1 the user that owns two
distinct delegated subhierarchies can move a process between
the hierarchies.)
• If the cgroup v2 filesystem was mounted with the nsdelegate
option, the writer must be able to see the source and
destination cgroups from its cgroup namespace.
• In cgroups v1: the effective UID of the writer (i.e., the
delegatee) matches the real user ID or the saved set-user-ID
of the target process. Before Linux 4.11, this requirement
also applied in cgroups v2 (This was a historical requirement
inherited from cgroups v1 that was later deemed unnecessary,
since the other rules suffice for containment in cgroups v2.)
Note: one consequence of these delegation containment rules is
that the unprivileged delegatee can't place the first process
into the delegated subtree; instead, the delegater must place the
first process (a process owned by the delegatee) into the
delegated subtree.
Among the restrictions imposed by cgroups v2 that were not
present in cgroups v1 are the following:
• No thread-granularity control: all of the threads of a process
must be in the same cgroup.
• No internal processes: a cgroup can't both have member
processes and exercise controllers on child cgroups.
Both of these restrictions were added because the lack of these
restrictions had caused problems in cgroups v1. In particular,
the cgroups v1 ability to allow thread-level granularity for
cgroup membership made no sense for some controllers. (A notable
example was the memory controller: since threads share an address
space, it made no sense to split threads across different memory
cgroups.)
Notwithstanding the initial design decision in cgroups v2, there
were use cases for certain controllers, notably the cpu
controller, for which thread-level granularity of control was
meaningful and useful. To accommodate such use cases, Linux 4.14
added thread mode for cgroups v2.
Thread mode allows the following:
• The creation of threaded subtrees in which the threads of a
process may be spread across cgroups inside the tree. (A
threaded subtree may contain multiple multithreaded
processes.)
• The concept of threaded controllers, which can distribute
resources across the cgroups in a threaded subtree.
• A relaxation of the "no internal processes rule", so that,
within a threaded subtree, a cgroup can both contain member
threads and exercise resource control over child cgroups.
With the addition of thread mode, each nonroot cgroup now
contains a new file, cgroup.type, that exposes, and in some
circumstances can be used to change, the "type" of a cgroup.
This file contains one of the following type values:
domain This is a normal v2 cgroup that provides process-
granularity control. If a process is a member of this
cgroup, then all threads of the process are (by
definition) in the same cgroup. This is the default
cgroup type, and provides the same behavior that was
provided for cgroups in the initial cgroups v2
implementation.
threaded
This cgroup is a member of a threaded subtree. Threads
can be added to this cgroup, and controllers can be
enabled for the cgroup.
domain threaded
This is a domain cgroup that serves as the root of a
threaded subtree. This cgroup type is also known as
"threaded root".
domain invalid
This is a cgroup inside a threaded subtree that is in an
"invalid" state. Processes can't be added to the cgroup,
and controllers can't be enabled for the cgroup. The only
thing that can be done with this cgroup (other than
deleting it) is to convert it to a threaded cgroup by
writing the string "threaded" to the cgroup.type file.
The rationale for the existence of this "interim" type
during the creation of a threaded subtree (rather than the
kernel simply immediately converting all cgroups under the
threaded root to the type threaded) is to allow for
possible future extensions to the thread mode model
Threaded versus domain controllers
With the addition of threads mode, cgroups v2 now distinguishes
two types of resource controllers:
• Threaded controllers: these controllers support thread-
granularity for resource control and can be enabled inside
threaded subtrees, with the result that the corresponding
controller-interface files appear inside the cgroups in the
threaded subtree. As at Linux 4.19, the following controllers
are threaded: cpu, perf_event, and pids.
• Domain controllers: these controllers support only process
granularity for resource control. From the perspective of a
domain controller, all threads of a process are always in the
same cgroup. Domain controllers can't be enabled inside a
threaded subtree.
Creating a threaded subtree
There are two pathways that lead to the creation of a threaded
subtree. The first pathway proceeds as follows:
(1) We write the string "threaded" to the cgroup.type file of a
cgroup y/z that currently has the type domain. This has the
following effects:
• The type of the cgroup y/z becomes threaded.
• The type of the parent cgroup, y, becomes domain
threaded. The parent cgroup is the root of a threaded
subtree (also known as the "threaded root").
• All other cgroups under y that were not already of type
threaded (because they were inside already existing
threaded subtrees under the new threaded root) are
converted to type domain invalid. Any subsequently
created cgroups under y will also have the type domain
invalid.
(2) We write the string "threaded" to each of the domain invalid
cgroups under y, in order to convert them to the type
threaded. As a consequence of this step, all threads under
the threaded root now have the type threaded and the
threaded subtree is now fully usable. The requirement to
write "threaded" to each of these cgroups is somewhat
cumbersome, but allows for possible future extensions to the
thread-mode model.
The second way of creating a threaded subtree is as follows:
(1) In an existing cgroup, z, that currently has the type
domain, we (1.1) enable one or more threaded controllers and
(1.2) make a process a member of z. (These two steps can be
done in either order.) This has the following consequences:
• The type of z becomes domain threaded.
• All of the descendant cgroups of z that were not already
of type threaded are converted to type domain invalid.
(2) As before, we make the threaded subtree usable by writing
the string "threaded" to each of the domain invalid cgroups
under z, in order to convert them to the type threaded.
One of the consequences of the above pathways to creating a
threaded subtree is that the threaded root cgroup can be a parent
only to threaded (and domain invalid) cgroups. The threaded root
cgroup can't be a parent of a domain cgroups, and a threaded
cgroup can't have a sibling that is a domain cgroup.
Using a threaded subtree
Within a threaded subtree, threaded controllers can be enabled in
each subgroup whose type has been changed to threaded; upon doing
so, the corresponding controller interface files appear in the
children of that cgroup.
A process can be moved into a threaded subtree by writing its PID
to the cgroup.procs file in one of the cgroups inside the tree.
This has the effect of making all of the threads in the process
members of the corresponding cgroup and makes the process a
member of the threaded subtree. The threads of the process can
then be spread across the threaded subtree by writing their
thread IDs (see gettid(2)) to the cgroup.threads files in
different cgroups inside the subtree. The threads of a process
must all reside in the same threaded subtree.
As with writing to cgroup.procs, some containment rules apply
when writing to the cgroup.threads file:
• The writer must have write permission on the cgroup.threads
file in the destination cgroup.
• The writer must have write permission on the cgroup.procs file
in the common ancestor of the source and destination cgroups.
(In some cases, the common ancestor may be the source or
destination cgroup itself.)
• The source and destination cgroups must be in the same
threaded subtree. (Outside a threaded subtree, an attempt to
move a thread by writing its thread ID to the cgroup.threads
file in a different domain cgroup fails with the error
EOPNOTSUPP.)
The cgroup.threads file is present in each cgroup (including
domain cgroups) and can be read in order to discover the set of
threads that is present in the cgroup. The set of thread IDs
obtained when reading this file is not guaranteed to be ordered
or free of duplicates.
The cgroup.procs file in the threaded root shows the PIDs of all
processes that are members of the threaded subtree. The
cgroup.procs files in the other cgroups in the subtree are not
readable.
Domain controllers can't be enabled in a threaded subtree; no
controller-interface files appear inside the cgroups underneath
the threaded root. From the point of view of a domain
controller, threaded subtrees are invisible: a multithreaded
process inside a threaded subtree appears to a domain controller
as a process that resides in the threaded root cgroup.
Within a threaded subtree, the "no internal processes" rule does
not apply: a cgroup can both contain member processes (or thread)
and exercise controllers on child cgroups.
Rules for writing to cgroup.type and creating threaded subtrees
A number of rules apply when writing to the cgroup.type file:
• Only the string "threaded" may be written. In other words,
the only explicit transition that is possible is to convert a
domain cgroup to type threaded.
• The effect of writing "threaded" depends on the current value
in cgroup.type, as follows:
• domain or domain threaded: start the creation of a threaded
subtree (whose root is the parent of this cgroup) via the
first of the pathways described above;
• domain invalid: convert this cgroup (which is inside a
threaded subtree) to a usable (i.e., threaded) state;
• threaded: no effect (a "no-op").
• We can't write to a cgroup.type file if the parent's type is
domain invalid. In other words, the cgroups of a threaded
subtree must be converted to the threaded state in a top-down
manner.
There are also some constraints that must be satisfied in order
to create a threaded subtree rooted at the cgroup x:
• There can be no member processes in the descendant cgroups of
x. (The cgroup x can itself have member processes.)
• No domain controllers may be enabled in x's
cgroup.subtree_control file.
If any of the above constraints is violated, then an attempt to
write "threaded" to a cgroup.type file fails with the error
ENOTSUP.
The "domain threaded" cgroup type
According to the pathways described above, the type of a cgroup
can change to domain threaded in either of the following cases:
• The string "threaded" is written to a child cgroup.
• A threaded controller is enabled inside the cgroup and a
process is made a member of the cgroup.
A domain threaded cgroup, x, can revert to the type domain if the
above conditions no longer hold true—that is, if all threaded
child cgroups of x are removed and either x no longer has
threaded controllers enabled or no longer has member processes.
When a domain threaded cgroup x reverts to the type domain:
• All domain invalid descendants of x that are not in lower-
level threaded subtrees revert to the type domain.
• The root cgroups in any lower-level threaded subtrees revert
to the type domain threaded.
Exceptions for the root cgroup
The root cgroup of the v2 hierarchy is treated exceptionally: it
can be the parent of both domain and threaded cgroups. If the
string "threaded" is written to the cgroup.type file of one of
the children of the root cgroup, then
• The type of that cgroup becomes threaded.
• The type of any descendants of that cgroup that are not part
of lower-level threaded subtrees changes to domain invalid.
Note that in this case, there is no cgroup whose type becomes
domain threaded. (Notionally, the root cgroup can be considered
as the threaded root for the cgroup whose type was changed to
threaded.)
The aim of this exceptional treatment for the root cgroup is to
allow a threaded cgroup that employs the cpu controller to be
placed as high as possible in the hierarchy, so as to minimize
the (small) cost of traversing the cgroup hierarchy.
The cgroups v2 "cpu" controller and realtime threads
As at Linux 4.19, the cgroups v2 cpu controller does not support
control of realtime threads (specifically threads scheduled under
any of the policies SCHED_FIFO, SCHED_RR, described
SCHED_DEADLINE; see sched(7)). Therefore, the cpu controller can
be enabled in the root cgroup only if all realtime threads are in
the root cgroup. (If there are realtime threads in nonroot
cgroups, then a write(2) of the string "+cpu" to the
cgroup.subtree_control file fails with the error EINVAL.)
On some systems, systemd(1) places certain realtime threads in
nonroot cgroups in the v2 hierarchy. On such systems, these
threads must first be moved to the root cgroup before the cpu
controller can be enabled.
The following errors can occur for mount(2):
EBUSY An attempt to mount a cgroup version 1 filesystem
specified neither the name= option (to mount a named
hierarchy) nor a controller name (or all).
A child process created via fork(2) inherits its parent's cgroup
memberships. A process's cgroup memberships are preserved across
execve(2).
The clone3(2) CLONE_INTO_CGROUP flag can be used to create a
child process that begins its life in a different version 2
cgroup from the parent process.
/proc files
/proc/cgroups (since Linux 2.6.24)
This file contains information about the controllers that
are compiled into the kernel. An example of the contents
of this file (reformatted for readability) is the
following:
#subsys_name hierarchy num_cgroups enabled
cpuset 4 1 1
cpu 8 1 1
cpuacct 8 1 1
blkio 6 1 1
memory 3 1 1
devices 10 84 1
freezer 7 1 1
net_cls 9 1 1
perf_event 5 1 1
net_prio 9 1 1
hugetlb 0 1 0
pids 2 1 1
The fields in this file are, from left to right:
[1] The name of the controller.
[2] The unique ID of the cgroup hierarchy on which this
controller is mounted. If multiple cgroups v1
controllers are bound to the same hierarchy, then
each will show the same hierarchy ID in this field.
The value in this field will be 0 if:
• the controller is not mounted on a cgroups v1
hierarchy;
• the controller is bound to the cgroups v2 single
unified hierarchy; or
• the controller is disabled (see below).
[3] The number of control groups in this hierarchy using
this controller.
[4] This field contains the value 1 if this controller is
enabled, or 0 if it has been disabled (via the
cgroup_disable kernel command-line boot parameter).
/proc/pid/cgroup (since Linux 2.6.24)
This file describes control groups to which the process
with the corresponding PID belongs. The displayed
information differs for cgroups version 1 and version 2
hierarchies.
For each cgroup hierarchy of which the process is a
member, there is one entry containing three colon-
separated fields:
hierarchy-ID:controller-list:cgroup-path
For example:
5:cpuacct,cpu,cpuset:/daemons
The colon-separated fields are, from left to right:
[1] For cgroups version 1 hierarchies, this field
contains a unique hierarchy ID number that can be
matched to a hierarchy ID in /proc/cgroups. For the
cgroups version 2 hierarchy, this field contains the
value 0.
[2] For cgroups version 1 hierarchies, this field
contains a comma-separated list of the controllers
bound to the hierarchy. For the cgroups version 2
hierarchy, this field is empty.
[3] This field contains the pathname of the control group
in the hierarchy to which the process belongs. This
pathname is relative to the mount point of the
hierarchy.
/sys/kernel/cgroup files
/sys/kernel/cgroup/delegate (since Linux 4.15)
This file exports a list of the cgroups v2 files (one per
line) that are delegatable (i.e., whose ownership should
be changed to the user ID of the delegatee). In the
future, the set of delegatable files may change or grow,
and this file provides a way for the kernel to inform
user-space applications of which files must be delegated.
As at Linux 4.15, one sees the following when inspecting
this file:
$ cat /sys/kernel/cgroup/delegate
cgroup.procs
cgroup.subtree_control
cgroup.threads
/sys/kernel/cgroup/features (since Linux 4.15)
Over time, the set of cgroups v2 features that are
provided by the kernel may change or grow, or some
features may not be enabled by default. This file
provides a way for user-space applications to discover
what features the running kernel supports and has enabled.
Features are listed one per line:
$ cat /sys/kernel/cgroup/features
nsdelegate
memory_localevents
The entries that can appear in this file are:
memory_localevents (since Linux 5.2)
The kernel supports the memory_localevents mount
option.
nsdelegate (since Linux 4.15)
The kernel supports the nsdelegate mount option.
memory_recursiveprot (since Linux 5.7)
The kernel supports the memory_recursiveprot mount
option.
prlimit(1), systemd(1), systemd-cgls(1), systemd-cgtop(1),
clone(2), ioprio_set(2), perf_event_open(2), setrlimit(2),
cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7),
user_namespaces(7)
The kernel source file Documentation/admin-guide/cgroup-v2.rst.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-06-15 cgroups(7)
Pages that refer to this page: clone(2), getrlimit(2), ioprio_set(2), poll(2), proc_cgroups(5), proc_pid_cgroup(5), sysfs(5), systemd.exec(5), bpf-helpers(7), cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7)
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