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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) Linux Programmer's Manual CGROUPS(7)
cgroups - Linux control groups
Control cgroups, 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/cgroup-v2.txt.)
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.
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 hier‐
archy. 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 hierar‐
chy:
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 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.
Note that on many systems, the v1 controllers are automatically
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) com‐
mand, 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 con‐
figuration 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.txt.
In Linux 3.2, this controller was extended to provide CPU
"bandwidth" control. If the kernel is configured with CON‐
FIG_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.txt.
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/cgroup-v1/cpuacct.txt.
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/cgroup-v1/cpusets.txt.
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/cgroup-v1/memory.txt.
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 whitelists and blacklists. Hier‐
archy 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/cgroup-v1/devices.txt.
freezer (since Linux 2.6.28; CONFIG_CGROUP_FREEZER)
The freezer cgroup can suspend and restore (resume) all pro‐
cesses 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/cgroup-v1/freezer-subsystem.txt.
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/cgroup-v1/net_cls.txt.
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 throt‐
tling policy which specifies upper I/O rate limits on a
device.
Further information can be found in the kernel source file
Documentation/cgroup-v1/blkio-controller.txt.
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 file
tools/perf/Documentation/perf-record.txt.
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/cgroup-v1/net_prio.txt.
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/cgroup-v1/hugetlb.txt.
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/cgroup-v1/pids.txt.
rdma (since Linux 4.11; CONFIG_CGROUP_RDMA)
The RDMA controller permits limiting the use of RDMA/IB-spe‐
cific resources per cgroup.
Further information can be found in the kernel source file
Documentation/cgroup-v1/rdma.txt.
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 guar‐
anteed to be in order. Nor is it guaranteed to be free of dupli‐
cates. (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 sim‐
ply 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 noti‐
fications 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.
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 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.
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/cgroup-v2.txt file in the
kernel source.
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 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 dis‐
abled. To do this, specify the cgroup_no_v1=list option on the ker‐
nel 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 con‐
trollers. (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 controllers
The following controllers, documented in the kernel source file Docu‐
mentation/cgroup-v2.txt, are supported in cgroups version 2:
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.
pids (since Linux 4.5)
This is the same as the version 1 pids controller.
perf_event (since Linux 4.11)
This is the same as the version 1 perf_event controller.
rdma (since Linux 4.11)
This is the same as the version 1 rdma controller.
cpu (since Linux 4.15)
This is the successor to the version 1 cpu and cpuacct con‐
trollers.
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 con‐
trollers that are exercised in the child cgroups. When a controller
(e.g., pids) is present in the cgroup.subtree_control file of a par‐
ent 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 relation‐
ship 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
With cgroups v2, a new mechanism is provided to obtain notification
about when a cgroup becomes empty. The cgroups v1 release_agent and
notify_on_release files are removed, and replaced by a new, more gen‐
eral-purpose file, cgroup.events. This read-only file contains key-
value pairs (delimited by newline characters, with the key and value
separated by spaces) that identify events or state for a cgroup.
Currently, only one key appears in this file, populated, which has
either the value 0, meaning that the cgroup (and its descendants)
contain no (nonzombie) processes, or 1, meaning that the cgroup con‐
tains member processes.
The cgroup.events file can be monitored, in order to receive notifi‐
cation when a cgroup transitions between the populated and unpopu‐
lated states (or vice versa). When monitoring this file using
inotify(7), transitions generate IN_MODIFY events, and when monitor‐
ing the file using poll(2), transitions generate POLLPRI events.
The cgroups v2 release-notification mechanism provided by the popu‐
lated field of the cgroup.events file offers at least two advantages
over the cgroups v1 release_agent mechanism. First, it allows for
cheaper notification, since a single process can monitor multiple
cgroup.events files. By contrast, the cgroups v1 mechanism requires
the creation of a process for each notification. Second, notifica‐
tion can be delegated to a process that lives inside a container
associated with the newly empty cgroup.
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 under‐
neath 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 descen‐
dant cgroups. A value of 0 in this file means that no descen‐
dant cgroups can be created. An attempt to create a descen‐
dant 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 v2 delegation: delegation to a less privileged user
In the context of cgroups, delegation means passing management of
some subtree of the cgroup hierarchy to a nonprivileged process.
Cgroups v1 provides support for delegation that was accidental and
not fully secure. Cgroups v2 supports delegation by explicit design.
Some terminology is required in order to describe delegation. A del‐
egater 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
Changing the ownership of this file means that that the dele‐
gatee 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
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.tasks file.)
The delegater should not change the ownership of any of the con‐
troller 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 distrib‐
uted into the delegated subtree.
See also the discussion of the /sys/kernel/cgroup/delegate file in
NOTES.
After the aforementioned steps have been performed, the delegatee can
create child cgroups within the delegated subtree (the cgroup subdi‐
rectories and the files they contain will be owned by the delegatee)
and move processes between cgroups in the subtree. If some con‐
trollers are present in dlgt_grp/cgroup.subtree_control, or the own‐
ership 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. 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 names‐
pace:
* 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 del‐
egate a subhierarchy under the existing delegated hierarchy. (For
example, the delegated hierarchy might be associated with an unprivi‐
leged 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 con‐
troller settings in the root directory of the 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 possi‐
bilities.
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 desirable to
Cgroup v2 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
common ancestor of the source and destination cgroups. (In some
cases, the common ancestor may be the source or destination cgroup
itself.)
* 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.
* Before Linux 4.11: the effective UID of the writer (i.e., the del‐
egatee) matches the real user ID or the saved set-user-ID of the
target process. (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 sub‐
tree.
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
4.15, 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) enable one or more threaded controllers and (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 x 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 y,
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 string "threaded" can be written only if the current value in
cgroup.type is one of the following
· domain, to start the creation of a threaded subtree via the
first of the pathways described above;
· domain invalid, to convert one of the cgroups in a threaded
subtree into a usable (i.e., threaded) state;
· threaded, which has 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 processes
As at Linux 4.15, the cgroups v2 cpu controller does not support
control of realtime processes, and the controller can be enabled in
the root cgroup only if all realtime threads are in the root cgroup.
(If there are realtime processes in nonroot cgroups, then a write(2)
of the string "+cpu" to the cgroup.subtree_control file fails with
the error EINVAL. However, on some systems, systemd(1) places
certain realtime processes in nonroot cgroups in the v2 hierarchy.
On such systems, these processes 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).
/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 con‐
troller 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:
a) the controller is not mounted on a cgroups v1 hierar‐
chy;
b) the controller is bound to the cgroups v2 single uni‐
fied hierarchy; or
c) 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_dis‐
able 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 dif‐
fers 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 hierar‐
chy ID in /proc/cgroups. For the cgroups version 2 hierar‐
chy, 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 hier‐
archy. 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 applica‐
tions 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 sup‐
ports and has enabled. Features are listed one per line:
$ cat /sys/kernel/cgroup/features
nsdelegate
The entries that can appear in this file are:
nsdelegate (since Linux 4.15)
The kernel supports the nsdelegate 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)
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2018-02-02 CGROUPS(7)
Pages that refer to this page: getrlimit(2), ioprio_set(2), poll(2), proc(5), sysfs(5), systemd.exec(5), cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7)
Copyright and license for this manual page
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