cgroups

CGROUPS(7)                 Linux Programmer's Manual                CGROUPS(7)



NAME
       cgroups - Linux control groups

DESCRIPTION
       Control groups, usually referred to as cgroups, are a Linux kernel
       feature which allow processes to be organized into hierarchical groups
       whose usage of various types of resources can then be limited and
       monitored.  The kernel's cgroup interface is provided through a pseudo-
       filesystem called cgroupfs.  Grouping is implemented in the core cgroup
       kernel code, while resource tracking and limits are implemented in a
       set of per-resource-type subsystems (memory, CPU, and so on).

   Terminology
       A cgroup is a collection of processes that are bound to a set of limits
       or parameters defined via the cgroup 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.

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.

       For each mounted hierarchy, the directory tree mirrors the control
       group hierarchy.  Each control group is represented by a directory,
       with each of its child control cgroups represented as a child
       directory.  For instance, /user/joe/1.session represents control group
       1.session, which is a child of cgroup joe, which is a child of /user.
       Under each cgroup directory is a set of files which can be read or
       written to, reflecting resource limits and a few general cgroup
       properties.

   Tasks (threads) versus processes
       In cgroups v1, a distinction is drawn between processes and tasks.  In
       this view, a process can consist of multiple tasks (more commonly
       called threads, from a user-space perspective, and called such in the
       remainder of this man page).  In cgroups v1, it is possible to
       independently manipulate the cgroup memberships of the threads in a
       process.

       The cgroups v1 ability to split threads across different cgroups caused
       problems in some cases.  For example, it made no sense for the memory
       controller, since all of the threads of a process share a single
       address space.  Because of these problems, the ability to independently
       manipulate the cgroup memberships of the threads in a process was
       removed in the initial cgroups v2 implementation, and subsequently
       restored in a more limited form (see the discussion of "thread mode"
       below).

   Mounting v1 controllers
       The use of cgroups requires a kernel built with the CONFIG_CGROUP
       option.  In addition, each of the v1 controllers has an associated
       configuration option that must be set in order to employ that
       controller.

       In order to use a v1 controller, it must be mounted against a cgroup
       filesystem.  The usual place for such mounts is under a tmpfs(5)
       filesystem mounted at /sys/fs/cgroup.  Thus, one might mount the cpu
       controller as follows:

           mount -t cgroup -o cpu none /sys/fs/cgroup/cpu

       It is possible to comount multiple controllers against the same
       hierarchy.  For example, here the cpu and cpuacct controllers are
       comounted against a single hierarchy:

           mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct

       Comounting controllers has the effect that a process is in the same
       cgroup for all of the comounted controllers.  Separately mounting
       controllers allows a process to be in cgroup /foo1 for one controller
       while being in /foo2/foo3 for another.

       It is possible to comount all v1 controllers against the same
       hierarchy:

           mount -t cgroup -o all cgroup /sys/fs/cgroup

       (One can achieve the same result by omitting -o all, since it is the
       default if no controllers are explicitly specified.)

       It is not possible to mount the same controller against multiple cgroup
       hierarchies.  For example, it is not possible to mount both the cpu and
       cpuacct controllers against one hierarchy, and to mount the cpu
       controller alone against another hierarchy.  It is possible to create
       multiple mount 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)
       command, as in the following example:

           umount /sys/fs/cgroup/pids

       But note well: a cgroup filesystem is unmounted only if it is not busy,
       that is, it has no child cgroups.  If this is not the case, then the
       only effect of the umount(8) is to make the mount invisible.  Thus, to
       ensure that the mount point is really removed, one must first remove
       all child cgroups, which in turn can be done only after all member
       processes have been moved from those cgroups to the root cgroup.

   Cgroups version 1 controllers
       Each of the cgroups version 1 controllers is governed by a kernel
       configuration option (listed below).  Additionally, the availability of
       the cgroups feature is governed by the CONFIG_CGROUPS kernel
       configuration option.

       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
              CONFIG_CFS_BANDWIDTH, then within each scheduling period
              (defined via a file in the cgroup directory), it is possible to
              define an upper limit on the CPU time allocated to the processes
              in a cgroup.  This upper limit applies even if there is no other
              competition for the CPU.  Further information can be found in
              the kernel source file Documentation/scheduler/sched-bwc.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 allow-lists and deny-lists.
              Hierarchy is enforced, so new rules must not violate existing
              rules for the target or ancestor cgroups.

              Further information can be found in the kernel source file
              Documentation/cgroup-v1/devices.txt.

       freezer (since Linux 2.6.28; CONFIG_CGROUP_FREEZER)
              The freezer cgroup can suspend and restore (resume) all
              processes in a cgroup.  Freezing a cgroup /A also causes its
              children, for example, processes in /A/B, to be frozen.

              Further information can be found in the kernel source file
              Documentation/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 throttling
              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-specific
              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
       guaranteed to be in order.  Nor is it guaranteed to be free of
       duplicates.  (For example, a PID may be recycled while reading from the
       list.)

       In cgroups v1, an individual thread can be moved to another cgroup by
       writing its thread ID (i.e., the kernel thread ID returned by clone(2)
       and gettid(2)) to the tasks file in a cgroup directory.  This file can
       be read to discover the set of threads that are members of the cgroup.

   Removing cgroups
       To remove a cgroup, it must first have no child cgroups and contain no
       (nonzombie) processes.  So long as that is the case, one can simply
       remove the corresponding directory pathname.  Note that files in a
       cgroup directory cannot and need not be removed.

   Cgroups v1 release notification
       Two files can be used to determine whether the kernel provides
       notifications when a cgroup becomes empty.  A cgroup is considered to
       be empty when it contains no child cgroups and no member processes.

       A special file in the root directory of each cgroup hierarchy,
       release_agent, can be used to register the pathname of a program that
       may be invoked when a cgroup in the hierarchy becomes empty.  The
       pathname of the newly empty cgroup (relative to the cgroup mount point)
       is provided as the sole command-line argument when the release_agent
       program is invoked.  The release_agent program might remove the cgroup
       directory, or perhaps repopulate it with a process.

       The default value of the release_agent file is empty, meaning that no
       release agent is invoked.

       The content of the release_agent file can also be specified via a mount
       option when the cgroup filesystem is mounted:

           mount -o release_agent=pathname ...

       Whether or not the release_agent program is invoked when a particular
       cgroup becomes empty is determined by the value in the
       notify_on_release file in the corresponding cgroup directory.  If this
       file contains the value 0, then the release_agent program is not
       invoked.  If it contains the value 1, the release_agent program is
       invoked.  The default value for this file in the root cgroup is 0.  At
       the time when a new cgroup is created, the value in this file is
       inherited from the corresponding file in the parent cgroup.

   Cgroup v1 named hierarchies
       In cgroups v1, it is possible to mount a cgroup hierarchy that has no
       attached controllers:

           mount -t cgroup -o none,name=somename none /some/mount/point

       Multiple instances of such hierarchies can be mounted; each hierarchy
       must have a unique name.  The only purpose of such hierarchies is to
       track processes.  (See the discussion of release notification below.)
       An example of this is the name=systemd cgroup hierarchy that is used by
       systemd(1) to track services and user sessions.

       Since Linux 5.0, the cgroup_no_v1 kernel boot option (described below)
       can be used to disable cgroup v1 named hierarchies, by specifying
       cgroup_no_v1=named.


CGROUPS VERSION 2
       In cgroups v2, all mounted controllers reside in a single unified
       hierarchy.  While (different) controllers may be simultaneously mounted
       under the v1 and v2 hierarchies, it is not possible to mount the same
       controller simultaneously under both the v1 and the v2 hierarchies.

       The new behaviors in cgroups v2 are summarized here, and in some cases
       elaborated in the following subsections.

       1. Cgroups v2 provides a unified hierarchy against which all
          controllers are mounted.

       2. "Internal" processes are not permitted.  With the 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
       be less useful than expected, and in many cases added complexity.
       Therefore, in cgroups v2, all available controllers are mounted against
       a single hierarchy.  The available controllers are automatically
       mounted, meaning that it is not necessary (or possible) to specify the
       controllers when mounting the cgroup v2 filesystem using a command such
       as the following:

           mount -t cgroup2 none /mnt/cgroup2

       A cgroup v2 controller is available only if it is not currently in use
       via a mount against a cgroup v1 hierarchy.  Or, to put things another
       way, it is not possible to employ the same controller against both a v1
       hierarchy and the unified v2 hierarchy.  This means that it may be
       necessary first to unmount a v1 controller (as described above) before
       that controller is available in v2.  Since systemd(1) makes heavy use
       of some v1 controllers by default, it can in some cases be simpler to
       boot the system with selected v1 controllers disabled.  To do this,
       specify the cgroup_no_v1=list option on the kernel boot command line;
       list is a comma-separated list of the names of the controllers to
       disable, or the word all to disable all v1 controllers.  (This
       situation is correctly handled by systemd(1), which falls back to
       operating without the specified controllers.)

       Note that on many modern systems, systemd(1) automatically mounts the
       cgroup2 filesystem at /sys/fs/cgroup/unified during the boot process.

   Cgroups v2 controllers
       The following controllers, documented in the kernel source file
       Documentation/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
              controllers.

   Cgroups v2 subtree control
       Each cgroup in the v2 hierarchy contains the following two files:

       cgroup.controllers
              This read-only file exposes a list of the controllers that are
              available in this cgroup.  The contents of this file match the
              contents of the cgroup.subtree_control file in the parent
              cgroup.

       cgroup.subtree_control
              This is a list of controllers that are active (enabled) in the
              cgroup.  The set of controllers in this file is a subset of the
              set in the cgroup.controllers of this cgroup.  The set of active
              controllers is modified by writing strings to this file
              containing space-delimited controller names, each preceded by
              '+' (to enable a controller) or '-' (to disable a controller),
              as in the following example:

                  echo '+pids -memory' > x/y/cgroup.subtree_control

              An attempt to enable a controller that is not present in
              cgroup.controllers leads to an ENOENT error when writing to the
              cgroup.subtree_control file.

       Because the list of controllers in cgroup.subtree_control is a subset
       of those cgroup.controllers, a controller that has been disabled in one
       cgroup in the hierarchy can never be re-enabled in the subtree below
       that cgroup.

       A cgroup's cgroup.subtree_control file determines the set of
       controllers that are exercised in the child cgroups.  When a controller
       (e.g., pids) is present in the cgroup.subtree_control file of a parent
       cgroup, then the corresponding controller-interface files (e.g.,
       pids.max) are automatically created in the children of that cgroup and
       can be used to exert resource control in the child cgroups.

   Cgroups v2 "no internal processes" rule
       Cgroups v2 enforces a so-called "no internal processes" rule.  Roughly
       speaking, this rule means that, with the exception of the root cgroup,
       processes may reside only in leaf nodes (cgroups that do not themselves
       contain child cgroups).  This avoids the need to decide how to
       partition resources between processes which are members of cgroup A and
       processes in child cgroups of A.

       For instance, if cgroup /cg1/cg2 exists, then a process may reside in
       /cg1/cg2, but not in /cg1.  This is to avoid an ambiguity in cgroups v1
       with respect to the delegation of resources between processes in /cg1
       and its child cgroups.  The recommended approach in cgroups v2 is to
       create a subdirectory called leaf for any nonleaf cgroup which should
       contain processes, but no child cgroups.  Thus, processes which
       previously would have gone into /cg1 would now go into /cg1/leaf.  This
       has the advantage of making explicit the relationship between processes
       in /cg1/leaf and /cg1's other children.

       The "no internal processes" rule is in fact more subtle than stated
       above.  More precisely, the rule is that a (nonroot) cgroup can't both
       (1) have member processes, and (2) distribute resources into child
       cgroups—that is, have a nonempty cgroup.subtree_control file.  Thus, it
       is possible for a cgroup to have both member processes and child
       cgroups, but before controllers can be enabled for that cgroup, the
       member processes must be moved out of the cgroup (e.g., perhaps into
       the child cgroups).

       With the Linux 4.14 addition of "thread mode" (described below), the
       "no internal processes" rule has been relaxed in some cases.

   Cgroups v2 cgroup.events file
       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
       general-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
       contains member processes.

       The 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
       inotify(7), transitions generate IN_MODIFY events, and when monitoring
       the file using poll(2), transitions cause the bits POLLPRI and POLLERR
       to be returned in the revents field.

       The cgroups v2 release-notification mechanism provided by the populated
       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, notification 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 underneath
              this cgroup.  A cgroup enters the dying state after being
              deleted.  It remains in that state for an undefined period
              (which will depend on system load) while resources are freed
              before the cgroup is destroyed.  Note that the presence of some
              cgroups in the dying state is normal, and is not indicative of
              any problem.

              A process can't be made a member of a dying cgroup, and a dying
              cgroup can't be brought back to life.

   Limiting the number of descendant cgroups
       Each cgroup in the v2 hierarchy contains the following files, which can
       be used to view and set limits on the number of descendant cgroups
       under that cgroup:

       cgroup.max.depth (since Linux 4.14)
              This file defines a limit on the depth of nesting of descendant
              cgroups.  A value of 0 in this file means that no descendant
              cgroups can be created.  An attempt to create a descendant whose
              nesting level exceeds the limit fails (mkdir(2) fails with the
              error EAGAIN).

              Writing the string "max" to this file means that no limit is
              imposed.  The default value in this file is "max".

       cgroup.max.descendants (since Linux 4.14)
              This file defines a limit on the number of live descendant
              cgroups that this cgroup may have.  An attempt to create more
              descendants than allowed by the limit fails (mkdir(2) fails with
              the error EAGAIN).

              Writing the string "max" to this file means that no limit is
              imposed.  The default value in this file is "max".

CGROUPS DELEGATION: DELEGATING A HIERARCHY TO A LESS PRIVILEGED USER
       In the context of cgroups, delegation means passing management of some
       subtree of the cgroup hierarchy to a nonprivileged user.  Cgroups v1
       provides support for delegation based on file permissions in the cgroup
       hierarchy but with less strict containment rules than v2 (as noted
       below).  Cgroups v2 supports delegation with containment by explicit
       design.  The focus of the discussion in this section is on delegation
       in cgroups v2, with some differences for cgroups v1 noted along the
       way.

       Some terminology is required in order to describe delegation.  A
       delegater is a privileged user (i.e., root) who owns a parent cgroup.
       A delegatee is a nonprivileged user who will be granted the permissions
       needed to manage some subhierarchy under that parent cgroup, known as
       the delegated subtree.

       To perform delegation, the delegater makes certain directories and
       files writable by the delegatee, typically by changing the ownership of
       the objects to be the user ID of the delegatee.  Assuming that we want
       to delegate the hierarchy rooted at (say) /dlgt_grp and that there are
       not yet any child cgroups under that cgroup, the ownership of the
       following is changed to the user ID of the delegatee:

       /dlgt_grp
              Changing the ownership of the root of the subtree means that any
              new cgroups created under the subtree (and the files they
              contain) will also be owned by the delegatee.

       /dlgt_grp/cgroup.procs
              Changing the ownership of this file means that the delegatee can
              move processes into the root of the delegated subtree.

       /dlgt_grp/cgroup.subtree_control (cgroups v2 only)
              Changing the ownership of this file means that the delegatee can
              enable controllers (that are present in
              /dlgt_grp/cgroup.controllers) in order to further redistribute
              resources at lower levels in the subtree.  (As an alternative to
              changing the ownership of this file, the delegater might instead
              add selected controllers to this file.)

       /dlgt_grp/cgroup.threads (cgroups v2 only)
              Changing the ownership of this file is necessary if a threaded
              subtree is being delegated (see the description of "thread
              mode", below).  This permits the delegatee to write thread IDs
              to the file.  (The ownership of this file can also be changed
              when delegating a domain subtree, but currently this serves no
              purpose, since, as described below, it is not possible to move a
              thread between domain cgroups by writing its thread ID to the
              cgroup.threads file.)

              In cgroups v1, the corresponding file that should instead be
              delegated is the tasks file.

       The delegater should not change the ownership of any of the controller
       interfaces files (e.g., pids.max, memory.high) in dlgt_grp.  Those
       files are used from the next level above the delegated subtree in order
       to distribute resources into the subtree, and the delegatee should not
       have permission to change the resources that are distributed into the
       delegated subtree.

       See also the discussion of the /sys/kernel/cgroup/delegate file in
       NOTES for information about further delegatable files in cgroups v2.

       After the aforementioned steps have been performed, the delegatee can
       create child cgroups within the delegated subtree (the cgroup
       subdirectories and the files they contain will be owned by the
       delegatee) and move processes between cgroups in the subtree.  If some
       controllers are present in dlgt_grp/cgroup.subtree_control, or the
       ownership of that file was passed to the delegatee, the delegatee can
       also control the further redistribution of the corresponding resources
       into the delegated subtree.

   Cgroups v2 delegation: nsdelegate and cgroup namespaces
       Starting with Linux 4.13, there is a second way to perform cgroup
       delegation in the cgroups v2 hierarchy.  This is done by mounting or
       remounting the cgroup v2 filesystem with the nsdelegate mount option.
       For example, if the cgroup v2 filesystem has already been mounted, we
       can remount it with the nsdelegate option as follows:

           mount -t cgroup2 -o remount,nsdelegate \
                            none /sys/fs/cgroup/unified

       The effect of this mount option is to cause cgroup namespaces to
       automatically become delegation boundaries.  More specifically, the
       following restrictions apply for processes inside the cgroup namespace:

       *  Writes to controller interface files in the root directory of the
          namespace will fail with the error EPERM.  Processes inside the
          cgroup namespace can still write to delegatable files in the root
          directory of the cgroup namespace such as cgroup.procs and
          cgroup.subtree_control, and can create subhierarchy underneath the
          root directory.

       *  Attempts to migrate processes across the namespace boundary are
          denied (with the error ENOENT).  Processes inside the cgroup
          namespace can still (subject to the containment rules described
          below) move processes between cgroups within the subhierarchy under
          the namespace root.

       The ability to define cgroup namespaces as delegation boundaries makes
       cgroup namespaces more useful.  To understand why, suppose that we
       already have one cgroup hierarchy that has been delegated to a
       nonprivileged user, cecilia, using the older delegation technique
       described above.  Suppose further that cecilia wanted to further
       delegate a subhierarchy under the existing delegated hierarchy.  (For
       example, the delegated hierarchy might be associated with an
       unprivileged container run by cecilia.)  Even if a cgroup namespace was
       employed, because both hierarchies are owned by the unprivileged user
       cecilia, the following illegitimate actions could be performed:

       *  A process in the inferior hierarchy could change the resource
          controller settings in the root directory of that hierarchy.  (These
          resource controller settings are intended to allow control to be
          exercised from the parent cgroup; a process inside the child cgroup
          should not be allowed to modify them.)

       *  A process inside the inferior hierarchy could move processes into
          and out of the inferior hierarchy if the cgroups in the superior
          hierarchy were somehow visible.

       Employing the nsdelegate mount option prevents both of these
       possibilities.

       The nsdelegate mount option only has an effect when performed in the
       initial mount namespace; in other mount namespaces, the option is
       silently ignored.

       Note: On some systems, systemd(1) automatically mounts the cgroup v2
       filesystem.  In order to experiment with the nsdelegate operation, it
       may be useful to boot the kernel with the following command-line
       options:

           cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller

       These options cause the kernel to boot with the cgroups v1 controllers
       disabled (meaning that the controllers are available in the v2
       hierarchy), and tells systemd(1) not to mount and use the cgroup v2
       hierarchy, so that the v2 hierarchy can be manually mounted with the
       desired options after boot-up.

   Cgroup delegation containment rules
       Some delegation containment rules ensure that the delegatee can move
       processes between cgroups within the delegated subtree, but can't move
       processes from outside the delegated subtree into the subtree or vice
       versa.  A nonprivileged process (i.e., the delegatee) can write the PID
       of a "target" process into a cgroup.procs file only if all of the
       following are true:

       *  The writer has write permission on the cgroup.procs file in the
          destination cgroup.

       *  The writer has write permission on the cgroup.procs file in the
          nearest common ancestor of the source and destination cgroups.  Note
          that in some cases, the nearest common ancestor may be the source or
          destination cgroup itself.  This requirement is not enforced for
          cgroups v1 hierarchies, with the consequence that containment in v1
          is less strict than in v2.  (For example, in cgroups v1 the user
          that owns two distinct delegated subhierarchies can move a process
          between the hierarchies.)

       *  If the cgroup v2 filesystem was mounted with the nsdelegate option,
          the writer must be able to see the source and destination cgroups
          from its cgroup namespace.

       *  In cgroups v1: the effective UID of the writer (i.e., the delegatee)
          matches the real user ID or the saved set-user-ID of the target
          process.  Before Linux 4.11, this requirement also applied in
          cgroups v2 (This was a historical requirement inherited from cgroups
          v1 that was later deemed unnecessary, since the other rules suffice
          for containment in cgroups v2.)

       Note: one consequence of these delegation containment rules is that the
       unprivileged delegatee can't place the first process into the delegated
       subtree; instead, the delegater must place the first process (a process
       owned by the delegatee) into the delegated subtree.

CGROUPS VERSION 2 THREAD MODE
       Among the restrictions imposed by cgroups v2 that were not present in
       cgroups v1 are the following:

       *  No thread-granularity control: all of the threads of a process must
          be in the same cgroup.

       *  No internal processes: a cgroup can't both have member processes and
          exercise controllers on child cgroups.

       Both of these restrictions were added because the lack of these
       restrictions had caused problems in cgroups v1.  In particular, the
       cgroups v1 ability to allow thread-level granularity for cgroup
       membership made no sense for some controllers.  (A notable example was
       the memory controller: since threads share an address space, it made no
       sense to split threads across different memory cgroups.)

       Notwithstanding the initial design decision in cgroups v2, there were
       use cases for certain controllers, notably the cpu controller, for
       which thread-level granularity of control was meaningful and useful.
       To accommodate such use cases, Linux 4.14 added thread mode for cgroups
       v2.

       Thread mode allows the following:

       *  The creation of threaded subtrees in which the threads of a process
          may be spread across cgroups inside the tree.  (A threaded subtree
          may contain multiple multithreaded processes.)

       *  The concept of threaded controllers, which can distribute resources
          across the cgroups in a threaded subtree.

       *  A relaxation of the "no internal processes rule", so that, within a
          threaded subtree, a cgroup can both contain member threads and
          exercise resource control over child cgroups.

       With the addition of thread mode, each nonroot cgroup now contains a
       new file, cgroup.type, that exposes, and in some circumstances can be
       used to change, the "type" of a cgroup.  This file contains one of the
       following type values:

       domain This is a normal v2 cgroup that provides process-granularity
              control.  If a process is a member of this cgroup, then all
              threads of the process are (by definition) in the same cgroup.
              This is the default cgroup type, and provides the same behavior
              that was provided for cgroups in the initial cgroups v2
              implementation.

       threaded
              This cgroup is a member of a threaded subtree.  Threads can be
              added to this cgroup, and controllers can be enabled for the
              cgroup.

       domain threaded
              This is a domain cgroup that serves as the root of a threaded
              subtree.  This cgroup type is also known as "threaded root".

       domain invalid
              This is a cgroup inside a threaded subtree that is in an
              "invalid" state.  Processes can't be added to the cgroup, and
              controllers can't be enabled for the cgroup.  The only thing
              that can be done with this cgroup (other than deleting it) is to
              convert it to a threaded cgroup by writing the string "threaded"
              to the cgroup.type file.

              The rationale for the existence of this "interim" type during
              the creation of a threaded subtree (rather than the kernel
              simply immediately converting all cgroups under the threaded
              root to the type threaded) is to allow for possible future
              extensions to the thread mode model

   Threaded versus domain controllers
       With the addition of threads mode, cgroups v2 now distinguishes two
       types of resource controllers:

       *  Threaded controllers: these controllers support thread-granularity
          for resource control and can be enabled inside threaded subtrees,
          with the result that the corresponding controller-interface files
          appear inside the cgroups in the threaded subtree.  As at Linux
          4.19, the following controllers are threaded: cpu, perf_event, and
          pids.

       *  Domain controllers: these controllers support only process
          granularity for resource control.  From the perspective of a domain
          controller, all threads of a process are always in the same cgroup.
          Domain controllers can't be enabled inside a threaded subtree.

   Creating a threaded subtree
       There are two pathways that lead to the creation of a threaded subtree.
       The first pathway proceeds as follows:

       1. We write the string "threaded" to the cgroup.type file of a cgroup
          y/z that currently has the type domain.  This has the following
          effects:

          *  The type of the cgroup y/z becomes threaded.

          *  The type of the parent cgroup, y, becomes domain threaded.  The
             parent cgroup is the root of a threaded subtree (also known as
             the "threaded root").

          *  All other cgroups under y that were not already of type threaded
             (because they were inside already existing threaded subtrees
             under the new threaded root) are converted to type domain
             invalid.  Any subsequently created cgroups under y will also have
             the type domain invalid.

       2. We write the string "threaded" to each of the domain invalid cgroups
          under y, in order to convert them to the type threaded.  As a
          consequence of this step, all threads under the threaded root now
          have the type threaded and the threaded subtree is now fully usable.
          The requirement to write "threaded" to each of these cgroups is
          somewhat cumbersome, but allows for possible future extensions to
          the thread-mode model.

       The second way of creating a threaded subtree is as follows:

       1. In an existing cgroup, z, that currently has the type domain, we (1)
          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 effect of writing "threaded" depends on the current value in
          cgroup.type, as follows:

          ·  domain or domain threaded: start the creation of a threaded
             subtree (whose root is the parent of this cgroup) via the first
             of the pathways described above;

          ·  domain invalid: convert this cgroup (which is inside a threaded
             subtree) to a usable (i.e., threaded) state;

          ·  threaded: no effect (a "no-op").

       *  We can't write to a cgroup.type file if the parent's type is domain
          invalid.  In other words, the cgroups of a threaded subtree must be
          converted to the threaded state in a top-down manner.

       There are also some constraints that must be satisfied in order to
       create a threaded subtree rooted at the cgroup x:

       *  There can be no member processes in the descendant cgroups of x.
          (The cgroup x can itself have member processes.)

       *  No domain controllers may be enabled in x's cgroup.subtree_control
          file.

       If any of the above constraints is violated, then an attempt to write
       "threaded" to a cgroup.type file fails with the error ENOTSUP.

   The "domain threaded" cgroup type
       According to the pathways described above, the type of a cgroup can
       change to domain threaded in either of the following cases:

       *  The string "threaded" is written to a child cgroup.

       *  A threaded controller is enabled inside the cgroup and a process is
          made a member of the cgroup.

       A domain threaded cgroup, x, can revert to the type domain if the above
       conditions no longer hold true—that is, if all threaded child cgroups
       of x are removed and either x no longer has threaded controllers
       enabled or no longer has member processes.

       When a domain threaded cgroup x reverts to the type domain:

       *  All domain invalid descendants of x that are not in lower-level
          threaded subtrees revert to the type domain.

       *  The root cgroups in any lower-level threaded subtrees revert to the
          type domain threaded.

   Exceptions for the root cgroup
       The root cgroup of the v2 hierarchy is treated exceptionally: it can be
       the parent of both domain and threaded cgroups.  If the string
       "threaded" is written to the cgroup.type file of one of the children of
       the root cgroup, then

       *  The type of that cgroup becomes threaded.

       *  The type of any descendants of that cgroup that are not part of
          lower-level threaded subtrees changes to domain invalid.

       Note that in this case, there is no cgroup whose type becomes domain
       threaded.  (Notionally, the root cgroup can be considered as the
       threaded root for the cgroup whose type was changed to threaded.)

       The aim of this exceptional treatment for the root cgroup is to allow a
       threaded cgroup that employs the cpu controller to be placed as high as
       possible in the hierarchy, so as to minimize the (small) cost of
       traversing the cgroup hierarchy.

   The cgroups v2 "cpu" controller and realtime threads
       As at Linux 4.19, the cgroups v2 cpu controller does not support
       control of realtime threads (specifically threads scheduled under any
       of the policies SCHED_FIFO, SCHED_RR, described SCHED_DEADLINE; see
       sched(7)).  Therefore, the cpu controller can be enabled in the root
       cgroup only if all realtime threads are in the root cgroup.  (If there
       are realtime threads in nonroot cgroups, then a write(2) of the string
       "+cpu" to the cgroup.subtree_control file fails with the error EINVAL.)

       On some systems, systemd(1) places certain realtime threads in nonroot
       cgroups in the v2 hierarchy.  On such systems, these threads must first
       be moved to the root cgroup before the cpu controller can be enabled.

ERRORS
       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).

NOTES
       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
                 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:

                   a) the controller is not mounted on a cgroups v1 hierarchy;

                   b) the controller is bound to the cgroups v2 single unified
                      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_disable
                 kernel command-line boot parameter).

       /proc/[pid]/cgroup (since Linux 2.6.24)
              This file describes control groups to which the process with the
              corresponding PID belongs.  The displayed information differs
              for cgroups version 1 and version 2 hierarchies.

              For each cgroup hierarchy of which the process is a member,
              there is one entry containing three colon-separated fields:

                  hierarchy-ID:controller-list:cgroup-path

              For example:

                  5:cpuacct,cpu,cpuset:/daemons

              The colon-separated fields are, from left to right:

              1. For cgroups version 1 hierarchies, this field contains a
                 unique hierarchy ID number that can be matched to a hierarchy
                 ID in /proc/cgroups.  For the cgroups version 2 hierarchy,
                 this field contains the value 0.

              2. For cgroups version 1 hierarchies, this field contains a
                 comma-separated list of the controllers bound to the
                 hierarchy.  For the cgroups version 2 hierarchy, this field
                 is empty.

              3. This field contains the pathname of the control group in the
                 hierarchy to which the process belongs.  This pathname is
                 relative to the mount point of the hierarchy.

   /sys/kernel/cgroup files
       /sys/kernel/cgroup/delegate (since Linux 4.15)
              This file exports a list of the cgroups v2 files (one per line)
              that are delegatable (i.e., whose ownership should be changed to
              the user ID of the delegatee).  In the future, the set of
              delegatable files may change or grow, and this file provides a
              way for the kernel to inform user-space applications of which
              files must be delegated.  As at Linux 4.15, one sees the
              following when inspecting this file:

                  $ cat /sys/kernel/cgroup/delegate
                  cgroup.procs
                  cgroup.subtree_control
                  cgroup.threads

       /sys/kernel/cgroup/features (since Linux 4.15)
              Over time, the set of cgroups v2 features that are provided by
              the kernel may change or grow, or some features may not be
              enabled by default.  This file provides a way for user-space
              applications to discover what features the running kernel
              supports and has enabled.  Features are listed one per line:

                  $ cat /sys/kernel/cgroup/features
                  nsdelegate

              The entries that can appear in this file are:

              nsdelegate (since Linux 4.15)
                     The kernel supports the nsdelegate mount option.

SEE ALSO
       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)

COLOPHON
       This page is part of release 5.04 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                             2019-11-19                        CGROUPS(7)