CGROUPS(7) (2020-08-13) CGROUPS(7)
NAME
cgroups - Linux control groups
DESCRIPTION
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 filesys-
tem.
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 limit-
ing 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. Sub-
systems 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 renam-
ing 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 incon-
sistencies arose between controllers and management of the
cgroup hierarchies became rather complex. A longer descrip-
tion 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).
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Because of the problems with the initial cgroups implementa-
tion (cgroups version 1), starting in Linux 3.10, work began
on a new, orthogonal implementation to remedy these prob-
lems. 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 compatibil-
ity 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 con-
trollers 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.
CGROUPS VERSION 1
Under cgroups v1, each controller may be mounted against a
separate cgroup filesystem that provides its own hierarchi-
cal organization of the processes on the system. It is also
possible to comount multiple (or even all) cgroups v1 con-
trollers against the same cgroup filesystem, meaning that
the comounted controllers manage the same hierarchical orga-
nization 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 rep-
resented 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 perspec-
tive, 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.
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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 con-
trollers 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 con-
trollers 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. Sepa-
rately 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 speci-
fied.)
It is not possible to mount the same controller against mul-
tiple 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
points 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.
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Note that on many systems, the v1 controllers are automati-
cally mounted under /sys/fs/cgroup; in particular,
systemd(1) automatically creates such mount points.
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
point 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.
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 fur-
ther 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 ker-
nel source file Documentation/scheduler/sched-bwc.rst
(or Documentation/scheduler/sched-bwc.txt in Linux 5.2
and earlier).
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
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(or Documentation/cgroup-v1/cpuacct.txt in Linux 5.2
and earlier).
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).
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).
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).
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
.}f (or Documentation/cgroup-v1/freezer-subsystem.txt
in Linux 5.2 and earlier).
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
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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).
CONFIG_BLK_CGROUP)
The blkio cgroup controls and limits access to speci-
fied 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 imple-
mented 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
.}f (or Documentation/cgroup-v1/blkio-controller.txt in
Linux 5.2 and earlier).
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
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
.}f (or Documentation/cgroup-v1/net_prio.txt in Linux
5.2 and earlier).
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).
CONFIG_CGROUP_PIDS)
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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 ear-
lier).
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 ear-
lier).
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 writ-
ing 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 guaran-
teed 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
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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 pro-
vides 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 hierar-
chy, 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 (rel-
ative 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, mean-
ing 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
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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.
CGROUPS VERSION 2
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.
1. Cgroups v2 provides a unified hierarchy against which all
controllers are mounted.
2. "Internal" processes are not permitted. With the excep-
tion 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.
3. Active cgroups must be specified via the files
cgroup.controllers and cgroup.subtree_control.
4. The tasks file has been removed. In addition, the
cgroup.clone_children file that is employed by the cpuset
controller has been removed.
5. 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 modi-
fication 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,
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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 filesys-
tem using a command such as the following:
mount -t cgroup2 none /mnt/cgroup2
A cgroup v2 controller is available only if it is not cur-
rently 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 uni-
fied 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 con-
trollers 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 con-
trollers to disable, or the word all to disable all v1 con-
trollers. (This situation is correctly handled by
systemd(1), which falls back to operating without the speci-
fied controllers.)
Note that on many modern systems, systemd(1) automatically
mounts the cgroup2 filesystem at /sys/fs/cgroup/unified dur-
ing the boot process.
Cgroups v2 mount options
The following options (mount -o) can be specified when
mounting the group v2 filesystem:
Treat cgroup namespaces as delegation boundaries. For
details, see below.
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 statis-
tics for descendant cgroups in memory.events, and this
mount option can be used to revert to the legacy behav-
ior. 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
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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:
This is the successor to the version 1 cpu and cpuacct
controllers.
This is the successor of the version 1 cpuset con-
troller.
This is the successor of the version 1 freezer con-
troller.
This is the successor of the version 1 hugetlb con-
troller.
This is the successor of the version 1 blkio con-
troller.
This is the successor of the version 1 memory con-
troller.
This is the same as the version 1 perf_event con-
troller.
This is the same as the version 1 pids controller.
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:
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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 aq+pids -memoryaq > 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 subdi-
rectory called leaf for any nonleaf cgroup which should con-
tain processes, but no child cgroups. Thus, processes which
previously would have gone into /cg1 would now go into
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/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) dis-
tribute 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:
The value of this key is either 1, if this cgroup or
any of its descendants has member processes, or other-
wise 0.
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 notifica-
tion 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.
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This key either has the value 0, meaning that the cgroup
(and its descendants) contain no (nonzombie) member pro-
cesses, or 1, meaning that the cgroup (or one of its descen-
dants) contains member processes.
The cgroups v2 release-notification mechanism offers the
following advantages over the cgroups v1 release_agent mech-
anism:
* It allows for cheaper notification, since a single pro-
cess 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 con-
sists 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 num-
ber of descendant cgroups under that cgroup:
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
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CGROUPS(7) (2020-08-13) CGROUPS(7)
to create a descendant whose nesting level exceeds the
limit fails (mkdir(2) fails with the error EAGAIN).
Writing the string
to this file means that no limit is imposed. The
default value in this file is
This file defines a limit on the number of live descen-
dant 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
to this file means that no limit is imposed. The
default value in this file is
CGROUPS DELEGATION: DELEGATING A HIERARCHY TO
In the context of cgroups, delegation means passing manage-
ment of some subtree of the cgroup hierarchy to a nonprivi-
leged 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 delega-
tion. 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 directo-
ries 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 hier-
archy 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:
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 delega-
tee.
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CGROUPS(7) (2020-08-13) CGROUPS(7)
Changing the ownership of this file means that the del-
egatee can move processes into the root of the dele-
gated subtree.
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