cgroups.7: Rework text on threads and cgroups v2

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.TH CGROUPS 7 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
cgroups \- Linux control groups
.SH DESCRIPTION
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).
.\"
.SS Terminology
A
.I cgroup
is a collection of processes that are bound to a set of
limits or parameters defined via the cgroup filesystem.
.PP
A
.I 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
.IR "resource controllers"
(or simply, controllers).
.PP
The cgroups for a controller are arranged in a
.IR 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.
.\"
.SS 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
.IR Documentation/cgroup\-v2.txt .)
.PP
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
.I "\-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.
.PP
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.
.\"
.SH CGROUPS VERSION 1
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.
.PP
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,
.IR /user/joe/1.session
represents control group
.IR 1.session ,
which is a child of cgroup
.IR joe ,
which is a child of
.IR /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.
.PP
In addition, in cgroups v1,
cgroups can be mounted with no bound controller, in which case
they serve only to track processes.
(See the discussion of release notification below.)
An example of this is the
.I name=systemd
cgroup which is used by
.BR systemd (1)
to track services and user sessions.
.\"
.SS Tasks (threads) versus processes
In cgroups v1, a distinction is drawn between
.I processes
and
.IR 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.
Because splitting threads across different cgroups
caused problems in some cases,
.\" FIXME Add some text describing why this was a problem.
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).
.\"
.SS Mounting v1 controllers
The use of cgroups requires a kernel built with the
.BR 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.
.PP
In order to use a v1 controller,
it must be mounted against a cgroup filesystem.
The usual place for such mounts is under a
.BR tmpfs (5)
filesystem mounted at
.IR /sys/fs/cgroup .
Thus, one might mount the
.I cpu
controller as follows:
.PP
.in +4n
.EX
mount \-t cgroup \-o cpu none /sys/fs/cgroup/cpu
.EE
.in
.PP
It is possible to comount multiple controllers against the same hierarchy.
For example, here the
.IR cpu
and
.IR cpuacct
controllers are comounted against a single hierarchy:
.PP
.in +4n
.EX
mount \-t cgroup \-o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct
.EE
.in
.PP
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
.I /foo1
for one controller while being in
.I /foo2/foo3
for another.
.PP
It is possible to comount all v1 controllers against the same hierarchy:
.PP
.in +4n
.EX
mount \-t cgroup \-o all cgroup /sys/fs/cgroup
.EE
.in
.PP
(One can achieve the same result by omitting
.IR "\-o all" ,
since it is the default if no controllers are explicitly specified.)
.PP
It is not possible to mount the same controller
against multiple cgroup hierarchies.
For example, it is not possible to mount both the
.I cpu
and
.I cpuacct
controllers against one hierarchy, and to mount the
.I 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.
.PP
Note that on many systems, the v1 controllers are automatically mounted under
.IR /sys/fs/cgroup ;
in particular,
.BR systemd (1)
automatically creates such mount points.
.\"
.SS Unmounting v1 controllers
A mounted cgroup filesystem can be unmounted using the
.BR umount (8)
command, as in the following example:
.PP
.in +4n
.EX
umount /sys/fs/cgroup/pids
.EE
.in
.PP
.IR "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
.BR 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.
.\"
.SS 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
.BR CONFIG_CGROUPS
kernel configuration option.
.TP
.IR cpu " (since Linux 2.6.24; " \fBCONFIG_CGROUP_SCHED\fP )
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
.IR Documentation/scheduler/sched-design-CFS.txt .
.IP
In Linux 3.2,
this controller was extended to provide CPU "bandwidth" control.
If the kernel is configured with
.BR 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
.IR Documentation/scheduler/sched\-bwc.txt .
.TP
.IR cpuacct " (since Linux 2.6.24; " \fBCONFIG_CGROUP_CPUACCT\fP )
This provides accounting for CPU usage by groups of processes.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup\-v1/cpuacct.txt .
.TP
.IR cpuset " (since Linux 2.6.24; " \fBCONFIG_CPUSETS\fP )
This cgroup can be used to bind the processes in a cgroup to
a specified set of CPUs and NUMA nodes.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup\-v1/cpusets.txt .
.TP
.IR memory " (since Linux 2.6.25; " \fBCONFIG_MEMCG\fP )
The memory controller supports reporting and limiting of process memory, kernel
memory, and swap used by cgroups.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup\-v1/memory.txt .
.TP
.IR devices " (since Linux 2.6.26; " \fBCONFIG_CGROUP_DEVICE\fP )
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.
Hierarchy is enforced, so new rules must not
violate existing rules for the target or ancestor cgroups.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/devices.txt .
.TP
.IR freezer " (since Linux 2.6.28; " \fBCONFIG_CGROUP_FREEZER\fP )
The
.IR freezer
cgroup can suspend and restore (resume) all processes in a cgroup.
Freezing a cgroup
.I /A
also causes its children, for example, processes in
.IR /A/B ,
to be frozen.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/freezer-subsystem.txt .
.TP
.IR net_cls " (since Linux 2.6.29; " \fBCONFIG_CGROUP_NET_CLASSID\fP )
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
.BR tc (8).
This applies only to packets
leaving the cgroup, not to traffic arriving at the cgroup.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/net_cls.txt .
.TP
.IR blkio " (since Linux 2.6.33; " \fBCONFIG_BLK_CGROUP\fP )
The
.I 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.
.IP
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.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/blkio-controller.txt .
.TP
.IR perf_event " (since Linux 2.6.39; " \fBCONFIG_CGROUP_PERF\fP )
This controller allows
.I perf
monitoring of the set of processes grouped in a cgroup.
.IP
Further information can be found in the kernel source file
.IR tools/perf/Documentation/perf-record.txt .
.TP
.IR net_prio " (since Linux 3.3; " \fBCONFIG_CGROUP_NET_PRIO\fP )
This allows priorities to be specified, per network interface, for cgroups.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/net_prio.txt .
.TP
.IR hugetlb " (since Linux 3.5; " \fBCONFIG_CGROUP_HUGETLB\fP )
This supports limiting the use of huge pages by cgroups.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/hugetlb.txt .
.TP
.IR pids " (since Linux 4.3; " \fBCONFIG_CGROUP_PIDS\fP )
This controller permits limiting the number of process that may be created
in a cgroup (and its descendants).
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/pids.txt .
.TP
.IR rdma " (since Linux 4.11; " \fBCONFIG_CGROUP_RDMA\fP )
The RDMA controller permits limiting the use of
RDMA/IB-specific resources per cgroup.
.IP
Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/rdma.txt .
.\"
.SS 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:
.PP
.in +4n
.EX
mkdir /sys/fs/cgroup/cpu/cg1
.EE
.in
.PP
This creates a new empty cgroup.
.PP
A process may be moved to this cgroup by writing its PID into the cgroup's
.I cgroup.procs
file:
.PP
.in +4n
.EX
echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs
.EE
.in
.PP
Only one PID at a time should be written to this file.
.PP
Writing the value 0 to a
.IR cgroup.procs
file causes the writing process to be moved to the corresponding cgroup.
.PP
When writing a PID into the
.IR cgroup.procs ,
all threads in the process are moved into the new cgroup at once.
.PP
Within a hierarchy, a process can be a member of exactly one cgroup.
Writing a process's PID to a
.IR cgroup.procs
file automatically removes it from the cgroup of
which it was previously a member.
.PP
The
.I 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.)
.PP
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
.BR clone (2)
and
.BR gettid (2))
to the
.IR tasks
file in a cgroup directory.
This file can be read to discover the set of threads
that are members of the cgroup.
.\"
.SS 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.
.\"
.SS 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.
.PP
A special file in the root directory of each cgroup hierarchy,
.IR 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
.IR release_agent
program is invoked.
The
.IR release_agent
program might remove the cgroup directory,
or perhaps repopulate it with a process.
.PP
The default value of the
.IR release_agent
file is empty, meaning that no release agent is invoked.
.PP
Whether or not the
.IR release_agent
program is invoked when a particular cgroup becomes empty is determined
by the value in the
.IR notify_on_release
file in the corresponding cgroup directory.
If this file contains the value 0, then the
.IR release_agent
program is not invoked.
If it contains the value 1, the
.IR 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.
.\"
.SH 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.
.PP
The new behaviors in cgroups v2 are summarized here,
and in some cases elaborated in the following subsections.
.IP 1. 3
Cgroups v2 provides a unified hierarchy against
which all controllers are mounted.
.IP 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.
.IP 3.
Active cgroups must be specified via the files
.IR cgroup.controllers
and
.IR cgroup.subtree_control .
.IP 4.
The
.I tasks
file has been removed.
In addition, the
.I cgroup.clone_children
file that is employed by the
.I cpuset
controller has been removed.
.IP 5.
An improved mechanism for notification of empty cgroups is provided by the
.IR cgroup.events
file.
.PP
For more changes, see the
.I Documentation/cgroup-v2.txt
file in the kernel source.
.PP
Some of the new behaviors listed above saw subsequent modification with
the addition in Linux 4.14 of "thread mode" (described below).
.\"
.SS 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:
.PP
.in +4n
.EX
mount -t cgroup2 none /mnt/cgroup2
.EE
.in
.PP
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
.BR 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
.IR cgroup_no_v1=list
option on the kernel boot command line;
.I list
is a comma-separated list of the names of the controllers to disable,
or the word
.I all
to disable all v1 controllers.
(This situation is correctly handled by
.BR systemd (1),
which falls back to operating without the specified controllers.)
.PP
Note that on many modern systems,
.BR systemd (1)
automatically mounts the
.I cgroup2
filesystem at
.I /sys/fs/cgroup/unified
during the boot process.
.\"
.SS Cgroups v2 controllers
The following controllers, documented in the kernel source file
.IR Documentation/cgroup-v2.txt ,
are supported in cgroups version 2:
.TP
.IR io " (since Linux 4.5)"
This is the successor of the version 1
.I blkio
controller.
.TP
.IR memory " (since Linux 4.5)"
This is the successor of the version 1
.I memory
controller.
.TP
.IR pids " (since Linux 4.5)"
This is the same as the version 1
.I pids
controller.
.TP
.IR perf_event " (since Linux 4.11)"
This is the same as the version 1
.I perf_event
controller.
.TP
.IR rdma " (since Linux 4.11)"
This is the same as the version 1
.I rdma
controller.
.TP
.IR cpu " (since Linux 4.15)"
This is the successor to the version 1
.I cpu
and
.I cpuacct
controllers.
.\"
.SS Cgroups v2 subtree control
Each cgroup in the v2 hierarchy contains the following two files:
.TP
.IR cgroup.controllers
This is a list of the controllers that are
.I available
in this cgroup.
The contents of this file match the contents of the
.I cgroup.subtree_control
file in the parent cgroup.
.TP
.I cgroup.subtree_control
This is a list of controllers that are
.IR active
.RI ( enabled )
in the cgroup.
The set of controllers in this file is a subset of the set in the
.IR 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:
.IP
.in +4n
.EX
echo '+pids -memory' > x/y/cgroup.subtree_control
.EE
.in
.IP
An attempt to enable a controller
that is not present in
.I cgroup.controllers
leads to an
.B ENOENT
error when writing to the
.I cgroup.subtree_control
file.
.PP
Because the list of controllers in
.I cgroup.subtree_control
is a subset of those
.IR 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.
.PP
A cgroup's
.I cgroup.subtree_control
file determines the set of controllers that are exercised in the
.I child
cgroups.
When a controller (e.g.,
.IR pids )
is present in the
.I cgroup.subtree_control
file of a parent cgroup,
then the corresponding controller-interface files (e.g.,
.IR pids.max )
are automatically created in the children of that cgroup
and can be used to exert resource control in the child cgroups.
.\"
.SS 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.
.PP
For instance, if cgroup
.I /cg1/cg2
exists, then a process may reside in
.IR /cg1/cg2 ,
but not in
.IR /cg1 .
This is to avoid an ambiguity in cgroups v1
with respect to the delegation of resources between processes in
.I /cg1
and its child cgroups.
The recommended approach in cgroups v2 is to create a subdirectory called
.I leaf
for any nonleaf cgroup which should contain processes, but no child cgroups.
Thus, processes which previously would have gone into
.I /cg1
would now go into
.IR /cg1/leaf .
This has the advantage of making explicit
the relationship between processes in
.I /cg1/leaf
and
.IR /cg1 's
other children.
.PP
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\(emthat is, have a nonempty
.I cgroup.subtree_control
file.
Thus, it
.I 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).
.PP
With the Linux 4.14 addition of "thread mode" (described below),
the "no internal processes" rule has been relaxed in some cases.
.\"
.SS 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
.IR release_agent
and
.IR notify_on_release
files are removed, and replaced by a new, more general-purpose file,
.IR 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,
.IR populated ,
which has either the value 0,
meaning that the cgroup (and its descendants)
contain no (nonzombie) processes,
or 1, meaning that the cgroup contains member processes.
.PP
The
.IR cgroup.events
file can be monitored, in order to receive notification when a cgroup
transitions between the populated and unpopulated states (or vice versa).
When monitoring this file using
.BR inotify (7),
transitions generate
.BR IN_MODIFY
events, and when monitoring the file using
.BR poll (2),
transitions generate
.B POLLPRI
events.
.PP
The cgroups v2 release-notification mechanism provided by the
.I populated
field of the
.I cgroup.events
file offers at least two advantages over the cgroups v1
.IR release_agent
mechanism.
First, it allows for cheaper notification,
since a single process can monitor multiple
.IR cgroup.events
files.
By contrast, the cgroups v1 mechanism requires the creation
of a process for each notification.
Second, notification can be delegated to a process that lives inside
a container associated with the newly empty cgroup.
.\"
.SS Cgroups v2 cgroup.stat file
.\" commit ec39225cca42c05ac36853d11d28f877fde5c42e
Each cgroup in the v2 hierarchy contains a read-only
.IR 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:
.TP
.I nr_descendants
This is the total number of visible (i.e., living) descendant cgroups
underneath this cgroup.
.TP
.I 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.
.IP
A process can't be made a member of a dying cgroup,
and a dying cgroup can't be brought back to life.
.\"
.SS 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:
.TP
.IR cgroup.max.depth " (since Linux 4.14)"
.\" commit 1a926e0bbab83bae8207d05a533173425e0496d1
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
.RI ( mkdir (2)
fails with the error
.BR EAGAIN ).
.IP
Writing the string
.IR """max"""
to this file means that no limit is imposed.
The default value in this file is
.IR """max""" .
.TP
.IR cgroup.max.descendants " (since Linux 4.14)"
.\" commit 1a926e0bbab83bae8207d05a533173425e0496d1
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
.RI ( mkdir (2)
fails with the error
.BR EAGAIN ).
.IP
Writing the string
.IR """max"""
to this file means that no limit is imposed.
The default value in this file is
.IR """max""" .
.\"
.SS 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.
.PP
Some terminology is required in order to describe delegation.
A
.I delegater
is a privileged user (i.e., root) who owns a parent cgroup.
A
.I delegatee
is a nonprivileged user who will be granted the permissions needed
to manage some subhierarchy under that parent cgroup,
known as the
.IR "delegated subtree" .
.PP
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)
.I /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:
.TP
.IR /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.
.TP
.IR /dlgt_grp/cgroup.procs
Changing the ownership of this file means that the delegatee
can move processes into the root of the delegated subtree.
.TP
.IR /dlgt_grp/cgroup.subtree_control
Changing the ownership of this file means that that the delegatee
can enable controllers (that are present in
.IR /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.)
.PP
The delegater should
.I not
change the ownership of any of the controller interfaces files (e.g.,
.IR pids.max ,
.IR memory.high )
in
.IR 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.
.PP
See also the discussion of the
.IR /sys/kernel/cgroup/delegate
file in NOTES.
.PP
After the aforementioned steps have been performed,
the delegatee can create child cgroups within the delegated subtree
and move processes between cgroups in the subtree.
If some controllers are present in
.IR 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.
.\"
.SS Cgroups v2 delegation: nsdelegate and cgroup namespaces
.\"
.\" To test this, it can be useful to boot the kernel with the options:
.\"
.\"    cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller
.\"
.\" The effect of the latter option is to prevent systemd from employing
.\" its "hybrid" cgroup mode, where it tries to make use of cgroups v2.
.\"
Starting with Linux 4.13,
.\" commit 5136f6365ce3eace5a926e10f16ed2a233db5ba9
there is a second way to perform cgroup delegation.
This is done by mounting the cgroup v2 filesystem with the
.I nsdelegate
mount option:
.PP
.in +4n
.EX
$ mount -t cgroup2 -o nsdelegate none /sys/fs/cgroup/unified
.EE
.in
.PP
The effect of this option is to cause cgroup namespaces
to automatically become delegation boundaries.
More specifically,
the following restrictions apply for processes inside the cgroup namespace:
.IP * 3
Writes to controller interface files in the root directory
will fail with the error
.BR EPERM .
Processes inside the cgroup namespace can still write to delegatable
files such as
.IR cgroup.procs
and
.IR cgroup.subtree_control ,
and can create subhierarchy underneath the root directory of
the cgroup namespace.
.IP *
Attempts to migrate processes across the namespace boundary are denied
(with the error
.BR ENOENT ).
Processes inside the cgroup namespace can still
(subject to the containment rules described below)
move processes between cgroups
.I within
the subhierarchy under the namespace root.
.PP
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,
.IR cecilia ,
using the older delegation technique described above.
Suppose further that
.I 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
.IR cecilia .)
Even if a cgroup namespace was employed,
because both hierarchies are owned by the unprivileged user
.IR cecilia ,
the following illegitimate actions could be performed:
.IP * 3
A process in the inferior hierarchy could change the
resource controller settings in the root directory of the that hierarchy.
(These resource controller settings are intended to allow control to
be exercised from the
.I parent
cgroup;
a process inside the child cgroup should not be allowed to modify them.)
.IP *
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.
.PP
Employing the
.I nsdelegate
mount option prevents both of these possibilities.
.PP
The
.I nsdelegate
mount option only has an effect when performed in
the initial mount namespace;
in other mount namespaces, the option is silently ignored.
.\"
.SS Cgroup v2 delegation containment rules
Some delegation
.IR "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
.IR cgroup.procs
file only if all of the following are true:
.IP * 3
The writer has write permission on the
.I cgroup.procs
file in the destination cgroup.
.IP *
The writer has write permission on the
.I 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.)
.IP *
If the cgroup v2 filesystem was mounted with the
.I nsdelegate
option, the writer must be able to see the source and destination cgroup
from its cgroup namespace.
.IP *
Before Linux 4.11:
.\" commit 576dd464505fc53d501bb94569db76f220104d28
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.
(This was a historical requirement inherited from cgroups v1
that was later deemed unnecessary,
since the other rules suffice for containment in cgroups v2.)
.PP
.IR 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.
.\"
.SH CGROUPS VERSION 2 THREAD MODE
Among the restrictions imposed by cgroups v2 that were not present
in cgroups v1 are the following:
.IP * 3
.IR "No thread-granularity control" :
all of the threads of a process must be in the same cgroup.
.IP *
.IR "No internal processes" :
a cgroup can't both have member processes and
exercise controllers on child cgroups.
.PP
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
.I memory
controller: since threads share an address space,
it made no sense to split threads across different
.I memory
cgroups.)
.PP
Notwithstanding the initial design decision in cgroups v2,
there were use cases for certain controllers, notably the
.IR cpu
controller,
for which thread-level granularity of control was meaningful and useful.
To accommodate such use cases, Linux 4.14 added
.I "thread mode"
for cgroups v2.
.PP
Thread mode allows the following:
.IP * 3
The creation of
.IR "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.)
.IP *
The concept of
.IR "threaded controllers",
which can distribute resources across the cgroups in a threaded subtree.
.IP *
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.
.PP
With the addition of thread mode,
each nonroot cgroup now contains a new file,
.IR 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:
.TP
.I "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.
.TP
.I "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.
.TP
.I "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".
.TP
.I "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
.IR threaded
cgroup by writing the string
.IR """threaded"""
to the
.I cgroup.type
file.
.\"
.SS Threaded versus domain controllers
With the addition of threads mode,
cgroups v2 now distinguishes two types of resource controllers:
.IP * 3
.I 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:
.IR cpu ,
.IR perf_event ,
and
.IR pids .
.IP *
.I 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.
.\"
.SS Creating a threaded subtree
There are two pathways that lead to the creation of a threaded subtree.
The first pathway proceeds as follows:
.IP 1. 3
We write the string
.IR """threaded"""
to the
.I cgroup.type
file of a cgroup
.IR y/z
that currently has the type
.IR domain .
This has the following effects:
.RS
.IP * 3
The type of the cgroup
.IR y/z
becomes
.IR threaded .
.IP *
The type of the parent cgroup,
.IR y ,
becomes
.IR "domain threaded" .
The parent cgroup is the root of a threaded subtree
(also known as the "threaded root").
.IP *
All other cgroups under
.IR y
that were not already of type
.IR threaded
(because they were inside already existing threaded subtrees
under the new threaded root)
are converted to type
.IR "domain invalid" .
Any subsequently created cgroups under
.I y
will also have the type
.IR "domain invalid" .
.RE
.IP 2.
We write the string
.IR """threaded"""
to each of the
.IR "domain invalid"
cgroups under
.IR y ,
in order to convert them to the type
.IR threaded .
As a consequence of this step, all threads under the threaded root
now have the type
.IR threaded
and the threaded subtree is now fully usable.
The requirement to write
.IR """threaded"""
to each of these cgroups is somewhat cumbersome,
but allows for possible future extensions to the thread-mode model.
.PP
The second way of creating a threaded subtree is as follows:
.IP 1. 3
In an existing cgroup,
.IR z ,
that currently has the type
.IR domain ,
we (1) enable one or more threaded controllers and
(2) make a process a member of
.IR z .
(These two steps can be done in either order.)
This has the following consequences:
.RS
.IP * 3
The type of
.I z
becomes
.IR "domain threaded" .
.IP *
All of the descendant cgroups of
.I x
that are were not already of type
.IR threaded
are converted to type
.IR "domain invalid" .
.RE
.IP 2.
As before, we make the threaded subtree usable by writing the string
.IR """threaded"""
to each of the
.IR "domain invalid"
cgroups under
.IR y ,
in order to convert them to the type
.IR threaded .
.PP
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
.I threaded
(and
.IR "domain invalid" )
cgroups.
The threaded root cgroup can't be a parent of a
.I domain
cgroups, and a
.I threaded
cgroup
can't have a sibling that is a
.I domain
cgroup.
.\"
.SS Using a threaded subtree
Within a threaded subtree, threaded controllers can be enabled
in each subgroup whose type has been changed to
.IR threaded ;
upon doing so, the corresponding controller interface files
appear in the children of that cgroup.
.PP
A process can be moved into a threaded subtree by writing its PID to the
.I 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
.BR 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.
.PP
The
cgroup.threads
file is present in each cgroup (including
.I 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.
.PP
The
.I cgroup.procs
file in the threaded root shows the PIDs of all processes
that are members of the threaded subtree.
The
.I cgroup.procs
files in the other cgroups in the subtree are not readable.
.PP
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.
.PP
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.
.\"
.SS Rules for writing to cgroup.type and creating threaded subtrees
A number of rules apply when writing to the
.I cgroup.type
file:
.IP * 3
Only the string
.IR """threaded"""
may be written.
In other words, the only explicit transition that is possible is to convert a
.I domain
cgroup to type
.IR threaded .
.IP *
The string
.IR """threaded"""
can be written only if the current value in
.IR cgroup.type
is one of the following
.RS
.IP \(bu 3
.IR domain ,
to start the creation of a threaded subtree via
the first of the pathways described above;
.IP \(bu
.IR "domain\ invalid" ,
to convert one of the cgroups in a threaded subtree into a usable (i.e.,
.IR threaded )
state;
.IP \(bu
.IR threaded ,
which has no effect (a "no-op").
.RE
.IP *
We can't write to a
.I cgroup.type
file if the parent's type is
.IR "domain invalid" .
In other words, the cgroups of a threaded subtree must be converted to the
.I threaded
state in a top-down manner.
.PP
There are also some constraints that must be satisfied
in order to create a threaded subtree rooted at the cgroup
.IR x :
.IP * 3
There can be no member processes in the descendant cgroups of
.IR x .
(The cgroup
.I x
can itself have member processes.)
.IP *
No domain controllers may be enabled in
.IR x 's
.IR cgroup.subtree_control
file.
.PP
If any of the above constraints is violated, then an attempt to write
.IR """threaded"""
to a
.IR cgroup.type
file fails with the error
.BR ENOTSUP .
.\"
.SS The """domain threaded""" cgroup type
According to the pathways described above,
the type of a cgroup can change to
.IR "domain threaded"
in either of the following cases:
.IP * 3
The string
.IR """threaded"""
is written to a child cgroup.
.IP *
A threaded controller is enabled inside the cgroup and
a process is made a member of the cgroup.
.PP
A
.IR "domain threaded"
cgroup,
.IR x ,
can revert to the type
.IR domain
if the above conditions no longer hold true\(emthat is, if all
.I threaded
child cgroups of
.I x
are removed and either
.I x
no longer has threaded controllers enabled or
no longer has member processes.
.PP
When a
.IR "domain threaded"
cgroup
.IR x
reverts to the type
.IR domain :
.IP * 3
All
.IR "domain invalid"
descendants of
.I x
that are not in lower-level threaded subtrees revert to the type
.IR domain .
.IP *
The root cgroups in any lower-level threaded subtrees revert to the type
.IR "domain threaded" .
.\"
.SS Exceptions for the root cgroup
The root cgroup of the v2 hierarchy is treated exceptionally:
it can be the parent of both
.I domain
and
.I threaded
cgroups.
If the string
.I """threaded"""
is written to the
.I cgroup.type
file of one of the children of the root cgroup, then
.IP * 3
The type of that cgroup becomes
.IR threaded .
.IP *
The type of any descendants of that cgroup that
are not part of lower-level threaded subtrees changes to
.IR "domain invalid" .
.PP
Note that in this case, there is no cgroup whose type becomes
.IR "domain threaded" .
(Notionally, the root cgroup can be considered as the threaded root
for the cgroup whose type was changed to
.IR threaded .)
.PP
The aim of this exceptional treatment for the root cgroup is to
allow a threaded cgroup that employs the
.I cpu
controller to be placed as high as possible in the hierarchy,
so as to minimize the (small) cost of traversing the cgroup hierarchy.
.\"
.SS The cgroups v2 """cpu""" controller and realtime processes
As at Linux 4.15, the cgroups v2
.I 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
.BR write (2)
of the string
.IR """+cpu"""
to the
.I cgroup.subtree_control
file fails with the error
.BR EINVAL .
However, on some systems,
.BR 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
.I cpu
controller can be enabled.
.\"
.SH ERRORS
The following errors can occur for
.BR mount (2):
.TP
.B EBUSY
An attempt to mount a cgroup version 1 filesystem specified neither the
.I name=
option (to mount a named hierarchy) nor a controller name (or
.IR all ).
.SH NOTES
A child process created via
.BR fork (2)
inherits its parent's cgroup memberships.
A process's cgroup memberships are preserved across
.BR execve (2).
.\"
.SS /proc files
.TP
.IR /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:
.IP
.in +4n
.EX
#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
.EE
.in
.IP
The fields in this file are, from left to right:
.RS
.IP 1. 3
The name of the controller.
.IP 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:
.RS 5
.IP a) 3
the controller is not mounted on a cgroups v1 hierarchy;
.IP b)
the controller is bound to the cgroups v2 single unified hierarchy; or
.IP c)
the controller is disabled (see below).
.RE
.IP 3.
The number of control groups in this hierarchy using this controller.
.IP 4.
This field contains the value 1 if this controller is enabled,
or 0 if it has been disabled (via the
.IR cgroup_disable
kernel command-line boot parameter).
.RE
.TP
.IR /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.
.IP
For each cgroup hierarchy of which the process is a member,
there is one entry containing three colon-separated fields:
.IP
.in +4n
.EX
hierarchy-ID:controller-list:cgroup-path
.EE
.in
.IP
For example:
.IP
.in +4n
.EX
5:cpuacct,cpu,cpuset:/daemons
.EE
.in
.IP
The colon-separated fields are, from left to right:
.RS
.IP 1. 3
For cgroups version 1 hierarchies,
this field contains a unique hierarchy ID number
that can be matched to a hierarchy ID in
.IR /proc/cgroups .
For the cgroups version 2 hierarchy, this field contains the value 0.
.IP 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.
.IP 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.
.RE
.\"
.SS /sys/kernel/cgroup files
.TP
.IR /sys/kernel/cgroup/delegate " (since Linux 4.15)"
.\" commit 01ee6cfb1483fe57c9cbd8e73817dfbf9bacffd3
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:
.IP
.EX
.in +4n
$ \fBcat /sys/kernel/cgroup/delegate\fP
cgroup.procs
cgroup.subtree_control
.in
.EE
.TP
.IR /sys/kernel/cgroup/features " (since Linux 4.15)"
.\" commit 5f2e673405b742be64e7c3604ed4ed3ac14f35ce
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:
.IP
.in +4n
.EX
.EE
$ \fBcat /sys/kernel/cgroup/features\fP
nsdelegate
.in
.IP
The entries that can appear in this file are:
.RS
.TP
.IR nsdelegate " (since Linux 4.15)"
The kernel supports the
.I nsdelegate
mount option.
.RE
.SH ERRORS
The following errors can occur for
.BR mount (2):
.TP
.B EBUSY
An attempt to mount a cgroup version 1 filesystem specified neither the
.I name=
option (to mount a named hierarchy) nor a controller name (or
.IR all ).
.SH NOTES
A child process created via
.BR fork (2)
inherits its parent's cgroup memberships.
A process's cgroup memberships are preserved across
.BR execve (2).
.SH SEE ALSO
.BR prlimit (1),
.BR systemd (1),
.BR systemd-cgls (1),
.BR systemd-cgtop (1),
.BR clone (2),
.BR ioprio_set (2),
.BR perf_event_open (2),
.BR setrlimit (2),
.BR cgroup_namespaces (7),
.BR cpuset (7),
.BR namespaces (7),
.BR sched (7),
.BR user_namespaces (7)