# cgroups

## 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/admin-guide/cgroup-v2.rst_ (or
           _Documentation/cgroup-v2.txt_ in Linux 4.17 and earlier).
    
           Because of the problems with the initial cgroups implementation
           (cgroups version 1), starting in Linux 3.10, work began on a new,
           orthogonal implementation to remedy these problems.  Initially
           marked experimental, and hidden behind the
           _-o __DEVEL__sane_behavior_ mount option, the new version (cgroups
           version 2) was eventually made official with the release of Linux
           4.5.  Differences between the two versions are described in the
           text below.  The file _cgroup.sane_behavior_ , present in cgroups v1,
           is a relic of this mount option.  The file always reports "0" and
           is only retained for backward compatibility.
    
           Although cgroups v2 is intended as a replacement for cgroups v1,
           the older system continues to exist (and for compatibility reasons
           is unlikely to be removed).  Currently, cgroups v2 implements only
           a subset of the controllers available in cgroups v1.  The two
           systems are implemented so that both v1 controllers and v2
           controllers can be mounted on the same system.  Thus, for example,
           it is possible to use those controllers that are supported under
           version 2, while also using version 1 controllers where version 2
           does not yet support those controllers.  The only restriction here
           is that a controller can't be simultaneously employed in both a
           cgroups v1 hierarchy and in the cgroups v2 hierarchy.
    


## 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 mounts with exactly the same set of
           comounted controllers.  However, in this case all that results is
           multiple mount points providing a view of the same hierarchy.
    
           Note that on many systems, the v1 controllers are automatically
           mounted under _/sys/fs/cgroup_ ; in particular, systemd(1)
           automatically creates such mounts.
    
       **Unmounting v1 controllers**
           A mounted cgroup filesystem can be unmounted using the umount(8)
           command, as in the following example:
    
               umount /sys/fs/cgroup/pids
    
           _But note well_ : a cgroup filesystem is unmounted only if it is not
           busy, that is, it has no child cgroups.  If this is not the case,
           then the only effect of the umount(8) is to make the mount
           invisible.  Thus, to ensure that the mount is really removed, one
           must first remove all child cgroups, which in turn can be done
           only after all member processes have been moved from those cgroups
           to the root cgroup.
    
       **Cgroups version 1 controllers**
           Each of the cgroups version 1 controllers is governed by a kernel
           configuration option (listed below).  Additionally, the
           availability of the cgroups feature is governed by the
           **CONFIG_CGROUPS** kernel configuration option.
    
           _cpu_ (since Linux 2.6.24; **CONFIG_CGROUP_SCHED**)
                  Cgroups can be guaranteed a minimum number of "CPU shares"
                  when a system is busy.  This does not limit a cgroup's CPU
                  usage if the CPUs are not busy.  For further information,
                  see _Documentation/scheduler/sched-design-CFS.rst_ (or
                  _Documentation/scheduler/sched-design-CFS.txt_ in Linux 5.2
                  and earlier).
    
                  In Linux 3.2, this controller was extended to provide CPU
                  "bandwidth" control.  If the kernel is configured with
                  **CONFIG_CFS_BANDWIDTH** , then within each scheduling period
                  (defined via a file in the cgroup directory), it is
                  possible to define an upper limit on the CPU time allocated
                  to the processes in a cgroup.  This upper limit applies
                  even if there is no other competition for the CPU.  Further
                  information can be found in the kernel source file
                  _Documentation/scheduler/sched-bwc.rst_ (or
                  _Documentation/scheduler/sched-bwc.txt_ in Linux 5.2 and
                  earlier).
    
           _cpuacct_ (since Linux 2.6.24; **CONFIG_CGROUP_CPUACCT**)
                  This provides accounting for CPU usage by groups of
                  processes.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/cpuacct.rst_ (or
                  _Documentation/cgroup-v1/cpuacct.txt_ in Linux 5.2 and
                  earlier).
    
           _cpuset_ (since Linux 2.6.24; **CONFIG_CPUSETS**)
                  This cgroup can be used to bind the processes in a cgroup
                  to a specified set of CPUs and NUMA nodes.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/cpusets.rst_ (or
                  _Documentation/cgroup-v1/cpusets.txt_ in Linux 5.2 and
                  earlier).
    
           _memory_ (since Linux 2.6.25; **CONFIG_MEMCG**)
                  The memory controller supports reporting and limiting of
                  process memory, kernel memory, and swap used by cgroups.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/memory.rst_ (or
                  _Documentation/cgroup-v1/memory.txt_ in Linux 5.2 and
                  earlier).
    
           _devices_ (since Linux 2.6.26; **CONFIG_CGROUP_DEVICE**)
                  This supports controlling which processes may create
                  (mknod) devices as well as open them for reading or
                  writing.  The policies may be specified as allow-lists and
                  deny-lists.  Hierarchy is enforced, so new rules must not
                  violate existing rules for the target or ancestor cgroups.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/devices.rst_ (or
                  _Documentation/cgroup-v1/devices.txt_ in Linux 5.2 and
                  earlier).
    
           _freezer_ (since Linux 2.6.28; **CONFIG_CGROUP_FREEZER**)
                  The _freezer_ cgroup can suspend and restore (resume) all
                  processes in a cgroup.  Freezing a cgroup _/A_ also causes
                  its children, for example, processes in _/A/B_ , to be frozen.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst_
                  (or _Documentation/cgroup-v1/freezer-subsystem.txt_ in Linux
                  5.2 and earlier).
    
           _net_cls_ (since Linux 2.6.29; **CONFIG_CGROUP_NET_CLASSID**)
                  This places a classid, specified for the cgroup, on network
                  packets created by a cgroup.  These classids can then be
                  used in firewall rules, as well as used to shape traffic
                  using tc(8).  This applies only to packets leaving the
                  cgroup, not to traffic arriving at the cgroup.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/net_cls.rst_ (or
                  _Documentation/cgroup-v1/net_cls.txt_ in Linux 5.2 and
                  earlier).
    
           _blkio_ (since Linux 2.6.33; **CONFIG_BLK_CGROUP**)
                  The _blkio_ cgroup controls and limits access to specified
                  block devices by applying IO control in the form of
                  throttling and upper limits against leaf nodes and
                  intermediate nodes in the storage hierarchy.
    
                  Two policies are available.  The first is a proportional-
                  weight time-based division of disk implemented with CFQ.
                  This is in effect for leaf nodes using CFQ.  The second is
                  a throttling policy which specifies upper I/O rate limits
                  on a device.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/blkio-controller.rst_
                  (or _Documentation/cgroup-v1/blkio-controller.txt_ in Linux
                  5.2 and earlier).
    
           _perf_event_ (since Linux 2.6.39; **CONFIG_CGROUP_PERF**)
                  This controller allows _perf_ monitoring of the set of
                  processes grouped in a cgroup.
    
                  Further information can be found in the kernel source files
    
           _net_prio_ (since Linux 3.3; **CONFIG_CGROUP_NET_PRIO**)
                  This allows priorities to be specified, per network
                  interface, for cgroups.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/net_prio.rst_ (or
                  _Documentation/cgroup-v1/net_prio.txt_ in Linux 5.2 and
                  earlier).
    
           _hugetlb_ (since Linux 3.5; **CONFIG_CGROUP_HUGETLB**)
                  This supports limiting the use of huge pages by cgroups.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/hugetlb.rst_ (or
                  _Documentation/cgroup-v1/hugetlb.txt_ in Linux 5.2 and
                  earlier).
    
           _pids_ (since Linux 4.3; **CONFIG_CGROUP_PIDS**)
                  This controller permits limiting the number of process that
                  may be created in a cgroup (and its descendants).
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/pids.rst_ (or
                  _Documentation/cgroup-v1/pids.txt_ in Linux 5.2 and earlier).
    
           _rdma_ (since Linux 4.11; **CONFIG_CGROUP_RDMA**)
                  The RDMA controller permits limiting the use of RDMA/IB-
                  specific resources per cgroup.
    
                  Further information can be found in the kernel source file
                  _Documentation/admin-guide/cgroup-v1/rdma.rst_ (or
                  _Documentation/cgroup-v1/rdma.txt_ in Linux 5.2 and earlier).
    
       **Creating cgroups and moving processes**
           A cgroup filesystem initially contains a single root cgroup, '/',
           which all processes belong to.  A new cgroup is created by
           creating a directory in the cgroup filesystem:
    
               mkdir /sys/fs/cgroup/cpu/cg1
    
           This creates a new empty cgroup.
    
           A process may be moved to this cgroup by writing its PID into the
           cgroup's _cgroup.procs_ file:
    
               echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs
    
           Only one PID at a time should be written to this file.
    
           Writing the value 0 to a _cgroup.procs_ file causes the writing
           process to be moved to the corresponding cgroup.
    
           When writing a PID into the _cgroup.procs_ , all threads in the
           process are moved into the new cgroup at once.
    
           Within a hierarchy, a process can be a member of exactly one
           cgroup.  Writing a process's PID to a _cgroup.procs_ file
           automatically removes it from the cgroup of which it was
           previously a member.
    
           The _cgroup.procs_ file can be read to obtain a list of the
           processes that are members of a cgroup.  The returned list of PIDs
           is not guaranteed to be in order.  Nor is it guaranteed to be free
           of duplicates.  (For example, a PID may be recycled while reading
           from the list.)
    
           In cgroups v1, an individual thread can be moved to another cgroup
           by writing its thread ID (i.e., the kernel thread ID returned by
           clone(2) and gettid(2)) to the _tasks_ file in a cgroup directory.
           This file can be read to discover the set of threads that are
           members of the cgroup.
    
       **Removing cgroups**
           To remove a cgroup, it must first have no child cgroups and
           contain no (nonzombie) processes.  So long as that is the case,
           one can simply remove the corresponding directory pathname.  Note
           that files in a cgroup directory cannot and need not be removed.
    
       **Cgroups v1 release notification**
           Two files can be used to determine whether the kernel provides
           notifications when a cgroup becomes empty.  A cgroup is considered
           to be empty when it contains no child cgroups and no member
           processes.
    
           A special file in the root directory of each cgroup hierarchy,
           _release_agent_ , can be used to register the pathname of a program
           that may be invoked when a cgroup in the hierarchy becomes empty.
           The pathname of the newly empty cgroup (relative to the cgroup
           mount point) is provided as the sole command-line argument when
           the _release_agent_ program is invoked.  The _release_agent_ program
           might remove the cgroup directory, or perhaps repopulate it with a
           process.
    
           The default value of the _release_agent_ file is empty, meaning that
           no release agent is invoked.
    
           The content of the _release_agent_ file can also be specified via a
           mount option when the cgroup filesystem is mounted:
    
               mount -o release_agent=pathname ...
    
           Whether or not the _release_agent_ program is invoked when a
           particular cgroup becomes empty is determined by the value in the
           _notify_on_release_ file in the corresponding cgroup directory.  If
           this file contains the value 0, then the _release_agent_ program is
           not invoked.  If it contains the value 1, the _release_agent_
           program is invoked.  The default value for this file in the root
           cgroup is 0.  At the time when a new cgroup is created, the value
           in this file is inherited from the corresponding file in the
           parent cgroup.
    
       **Cgroup v1 named hierarchies**
           In cgroups v1, it is possible to mount a cgroup hierarchy that has
           no attached controllers:
    
               mount -t cgroup -o none,name=somename none /some/mount/point
    
           Multiple instances of such hierarchies can be mounted; each
           hierarchy must have a unique name.  The only purpose of such
           hierarchies is to track processes.  (See the discussion of release
           notification below.)  An example of this is the _name=systemd_
           cgroup hierarchy that is used by systemd(1) to track services and
           user sessions.
    
           Since Linux 5.0, the _cgroup_no_v1_ kernel boot option (described
           below) can be used to disable cgroup v1 named hierarchies, by
           specifying _cgroup_no_v1=named_.
    


## 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.
    
           •  Cgroups v2 provides a unified hierarchy against which all
              controllers are mounted.
    
           •  "Internal" processes are not permitted.  With the exception of
              the root cgroup, processes may reside only in leaf nodes
              (cgroups that do not themselves contain child cgroups).  The
              details are somewhat more subtle than this, and are described
              below.
    
           •  Active cgroups must be specified via the files
              _cgroup.controllers_ and _cgroup.subtree_control_.
    
           •  The _tasks_ file has been removed.  In addition, the
              _cgroup.clone_children_ file that is employed by the _cpuset_
              controller has been removed.
    
           •  An improved mechanism for notification of empty cgroups is
              provided by the _cgroup.events_ file.
    
           For more changes, see the _Documentation/admin-guide/cgroup-v2.rst_
           file in the kernel source (or _Documentation/cgroup-v2.txt_ in Linux
           4.17 and earlier).
    
           Some of the new behaviors listed above saw subsequent modification
           with the addition in Linux 4.14 of "thread mode" (described
           below).
    
       **Cgroups v2 unified hierarchy**
           In cgroups v1, the ability to mount different controllers against
           different hierarchies was intended to allow great flexibility for
           application design.  In practice, though, the flexibility turned
           out to be less useful than expected, and in many cases added
           complexity.  Therefore, in cgroups v2, all available controllers
           are mounted against a single hierarchy.  The available controllers
           are automatically mounted, meaning that it is not necessary (or
           possible) to specify the controllers when mounting the cgroup v2
           filesystem using a command such as the following:
    
               mount -t cgroup2 none /mnt/cgroup2
    
           A cgroup v2 controller is available only if it is not currently in
           use via a mount against a cgroup v1 hierarchy.  Or, to put things
           another way, it is not possible to employ the same controller
           against both a v1 hierarchy and the unified v2 hierarchy.  This
           means that it may be necessary first to unmount a v1 controller
           (as described above) before that controller is available in v2.
           Since systemd(1) makes heavy use of some v1 controllers by
           default, it can in some cases be simpler to boot the system with
           selected v1 controllers disabled.  To do this, specify the
           _cgroup_no_v1=list_ option on the kernel boot command line; _list_ is
           a comma-separated list of the names of the controllers to disable,
           or the word _all_ to disable all v1 controllers.  (This situation is
           correctly handled by systemd(1), which falls back to operating
           without the specified controllers.)
    
           Note that on many modern systems, systemd(1) automatically mounts
           the _cgroup2_ filesystem at _/sys/fs/cgroup/unified_ during the boot
           process.
    
       **Cgroups v2 mount options**
           The following options (_mount -o_) can be specified when mounting
           the group v2 filesystem:
    
           _nsdelegate_ (since Linux 4.15)
                  Treat cgroup namespaces as delegation boundaries.  For
                  details, see below.
    
           _memory_localevents_ (since Linux 5.2)
                  The _memory.events_ should show statistics only for the
                  cgroup itself, and not for any descendant cgroups.  This
                  was the behavior before Linux 5.2.  Since Linux 5.2, the
                  default behavior is to include statistics for descendant
                  cgroups in _memory.events_ , and this mount option can be used
                  to revert to the legacy behavior.  This option is system
                  wide and can be set on mount or modified through remount
                  only from the initial mount namespace; it is silently
                  ignored in noninitial namespaces.
    
       **Cgroups v2 controllers**
           The following controllers, documented in the kernel source file
           _Documentation/admin-guide/cgroup-v2.rst_ (or
           _Documentation/cgroup-v2.txt_ in Linux 4.17 and earlier), are
           supported in cgroups version 2:
    
           _cpu_ (since Linux 4.15)
                  This is the successor to the version 1 _cpu_ and _cpuacct_
                  controllers.
    
           _cpuset_ (since Linux 5.0)
                  This is the successor of the version 1 _cpuset_ controller.
    
           _freezer_ (since Linux 5.2)
                  This is the successor of the version 1 _freezer_ controller.
    
           _hugetlb_ (since Linux 5.6)
                  This is the successor of the version 1 _hugetlb_ controller.
    
           _io_ (since Linux 4.5)
                  This is the successor of the version 1 _blkio_ controller.
    
           _memory_ (since Linux 4.5)
                  This is the successor of the version 1 _memory_ controller.
    
           _perf_event_ (since Linux 4.11)
                  This is the same as the version 1 _perf_event_ controller.
    
           _pids_ (since Linux 4.5)
                  This is the same as the version 1 _pids_ controller.
    
           _rdma_ (since Linux 4.11)
                  This is the same as the version 1 _rdma_ controller.
    
           There is no direct equivalent of the _net_cls_ and _net_prio_
           controllers from cgroups version 1.  Instead, support has been
           added to iptables(8) to allow eBPF filters that hook on cgroup v2
           pathnames to make decisions about network traffic on a per-cgroup
           basis.
    
           The v2 _devices_ controller provides no interface files; instead,
           device control is gated by attaching an eBPF (**BPF_CGROUP_DEVICE**)
           program to a v2 cgroup.
    
       **Cgroups v2 subtree control**
           Each cgroup in the v2 hierarchy contains the following two files:
    
           _cgroup.controllers_
                  This read-only file exposes a list of the controllers that
                  are _available_ in this cgroup.  The contents of this file
                  match the contents of the _cgroup.subtree_control_ file in
                  the parent cgroup.
    
           _cgroup.subtree_control_
                  This is a list of controllers that are _active_ (_enabled_) in
                  the cgroup.  The set of controllers in this file is a
                  subset of the set in the _cgroup.controllers_ of this cgroup.
                  The set of active controllers is modified by writing
                  strings to this file containing space-delimited controller
                  names, each preceded by '+' (to enable a controller) or '-'
                  (to disable a controller), as in the following example:
    
                      echo '+pids -memory' > x/y/cgroup.subtree_control
    
                  An attempt to enable a controller that is not present in
                  _cgroup.controllers_ leads to an **ENOENT** error when writing to
                  the _cgroup.subtree_control_ file.
    
           Because the list of controllers in _cgroup.subtree_control_ is a
           subset of those _cgroup.controllers_ , a controller that has been
           disabled in one cgroup in the hierarchy can never be re-enabled in
           the subtree below that cgroup.
    
           A cgroup's _cgroup.subtree_control_ file determines the set of
           controllers that are exercised in the _child_ cgroups.  When a
           controller (e.g., _pids_) is present in the _cgroup.subtree_control_
           file of a parent cgroup, then the corresponding controller-
           interface files (e.g., _pids.max_) are automatically created in the
           children of that cgroup and can be used to exert resource control
           in the child cgroups.
    
       **Cgroups v2 "no internal processes" rule**
           Cgroups v2 enforces a so-called "no internal processes" rule.
           Roughly speaking, this rule means that, with the exception of the
           root cgroup, processes may reside only in leaf nodes (cgroups that
           do not themselves contain child cgroups).  This avoids the need to
           decide how to partition resources between processes which are
           members of cgroup A and processes in child cgroups of A.
    
           For instance, if cgroup _/cg1/cg2_ exists, then a process may reside
           in _/cg1/cg2_ , but not in _/cg1_.  This is to avoid an ambiguity in
           cgroups v1 with respect to the delegation of resources between
           processes in _/cg1_ and its child cgroups.  The recommended approach
           in cgroups v2 is to create a subdirectory called _leaf_ for any
           nonleaf cgroup which should contain processes, but no child
           cgroups.  Thus, processes which previously would have gone into
           _/cg1_ would now go into _/cg1/leaf_.  This has the advantage of
           making explicit the relationship between processes in _/cg1/leaf_
           and _/cg1_ 's other children.
    
           The "no internal processes" rule is in fact more subtle than
           stated above.  More precisely, the rule is that a (nonroot) cgroup
           can't both (1) have member processes, and (2) distribute resources
           into child cgroups—that is, have a nonempty _cgroup.subtree_control_
           file.  Thus, it _is_ possible for a cgroup to have both member
           processes and child cgroups, but before controllers can be enabled
           for that cgroup, the member processes must be moved out of the
           cgroup (e.g., perhaps into the child cgroups).
    
           With the Linux 4.14 addition of "thread mode" (described below),
           the "no internal processes" rule has been relaxed in some cases.
    
       **Cgroups v2 cgroup.events file**
           Each nonroot cgroup in the v2 hierarchy contains a read-only file,
           _cgroup.events_ , whose contents are key-value pairs (delimited by
           newline characters, with the key and value separated by spaces)
           providing state information about the cgroup:
    
               $ **cat mygrp/cgroup.events**
               populated 1
               frozen 0
    
           The following keys may appear in this file:
    
           _populated_
                  The value of this key is either 1, if this cgroup or any of
                  its descendants has member processes, or otherwise 0.
    
           _frozen_ (since Linux 5.2)
                  The value of this key is 1 if this cgroup is currently
                  frozen, or 0 if it is not.
    
           The _cgroup.events_ file can be monitored, in order to receive
           notification when the value of one of its keys changes.  Such
           monitoring can be done using inotify(7), which notifies changes as
           **IN_MODIFY** events, or poll(2), which notifies changes by returning
           the **POLLPRI** and **POLLERR** bits in the _revents_ field.
    
       **Cgroup v2 release notification**
           Cgroups v2 provides a new mechanism for obtaining notification
           when a cgroup becomes empty.  The cgroups v1 _release_agent_ and
           _notify_on_release_ files are removed, and replaced by the _populated_
           key in the _cgroup.events_ file.  This key either has the value 0,
           meaning that the cgroup (and its descendants) contain no
           (nonzombie) member processes, or 1, meaning that the cgroup (or
           one of its descendants) contains member processes.
    
           The cgroups v2 release-notification mechanism offers the following
           advantages over the cgroups v1 _release_agent_ mechanism:
    
           •  It allows for cheaper notification, since a single process can
              monitor multiple _cgroup.events_ files (using the techniques
              described earlier).  By contrast, the cgroups v1 mechanism
              requires the expense of creating a process for each
              notification.
    
           •  Notification for different cgroup subhierarchies can be
              delegated to different processes.  By contrast, the cgroups v1
              mechanism allows only one release agent for an entire
              hierarchy.
    
       **Cgroups v2 cgroup.stat file**
           Each cgroup in the v2 hierarchy contains a read-only _cgroup.stat_
           file (first introduced in Linux 4.14) that consists of lines
           containing key-value pairs.  The following keys currently appear
           in this file:
    
           _nr_descendants_
                  This is the total number of visible (i.e., living)
                  descendant cgroups underneath this cgroup.
    
           _nr_dying_descendants_
                  This is the total number of dying descendant cgroups
                  underneath this cgroup.  A cgroup enters the dying state
                  after being deleted.  It remains in that state for an
                  undefined period (which will depend on system load) while
                  resources are freed before the cgroup is destroyed.  Note
                  that the presence of some cgroups in the dying state is
                  normal, and is not indicative of any problem.
    
                  A process can't be made a member of a dying cgroup, and a
                  dying cgroup can't be brought back to life.
    
       **Limiting the number of descendant cgroups**
           Each cgroup in the v2 hierarchy contains the following files,
           which can be used to view and set limits on the number of
           descendant cgroups under that cgroup:
    
           _cgroup.max.depth_ (since Linux 4.14)
                  This file defines a limit on the depth of nesting of
                  descendant cgroups.  A value of 0 in this file means that
                  no descendant cgroups can be created.  An attempt to create
                  a descendant whose nesting level exceeds the limit fails
                  (_mkdir_(2) fails with the error **EAGAIN**).
    
                  Writing the string _"max"_ to this file means that no limit
                  is imposed.  The default value in this file is _"max"_.
    
           _cgroup.max.descendants_ (since Linux 4.14)
                  This file defines a limit on the number of live descendant
                  cgroups that this cgroup may have.  An attempt to create
                  more descendants than allowed by the limit fails (_mkdir_(2)
                  fails with the error **EAGAIN**).
    
                  Writing the string _"max"_ to this file means that no limit
                  is imposed.  The default value in this file is _"max"_.
    
    **CGROUPS DELEGATION: DELEGATING A HIERARCHY TO A LESS PRIVILEGED USER**
           **%%%SH%%%**
           In the context of cgroups, delegation means passing management of
           some subtree of the cgroup hierarchy to a nonprivileged user.
           Cgroups v1 provides support for delegation based on file
           permissions in the cgroup hierarchy but with less strict
           containment rules than v2 (as noted below).  Cgroups v2 supports
           delegation with containment by explicit design.  The focus of the
           discussion in this section is on delegation in cgroups v2, with
           some differences for cgroups v1 noted along the way.
    
           Some terminology is required in order to describe delegation.  A
           _delegater_ is a privileged user (i.e., root) who owns a parent
           cgroup.  A _delegatee_ is a nonprivileged user who will be granted
           the permissions needed to manage some subhierarchy under that
           parent cgroup, known as the _delegated subtree_.
    
           To perform delegation, the delegater makes certain directories and
           files writable by the delegatee, typically by changing the
           ownership of the objects to be the user ID of the delegatee.
           Assuming that we want to delegate the hierarchy rooted at (say)
           _/dlgt_grp_ and that there are not yet any child cgroups under that
           cgroup, the ownership of the following is changed to the user ID
           of the delegatee:
    
           _/dlgt_grp_
                  Changing the ownership of the root of the subtree means
                  that any new cgroups created under the subtree (and the
                  files they contain) will also be owned by the delegatee.
    
           _/dlgt_grp/cgroup.procs_
                  Changing the ownership of this file means that the
                  delegatee can move processes into the root of the delegated
                  subtree.
    
           _/dlgt_grp/cgroup.subtree_control_ (cgroups v2 only)
                  Changing the ownership of this file means that the
                  delegatee can enable controllers (that are present in
                  _/dlgt_grp/cgroup.controllers_) in order to further
                  redistribute resources at lower levels in the subtree.  (As
                  an alternative to changing the ownership of this file, the
                  delegater might instead add selected controllers to this
                  file.)
    
           _/dlgt_grp/cgroup.threads_ (cgroups v2 only)
                  Changing the ownership of this file is necessary if a
                  threaded subtree is being delegated (see the description of
                  "thread mode", below).  This permits the delegatee to write
                  thread IDs to the file.  (The ownership of this file can
                  also be changed when delegating a domain subtree, but
                  currently this serves no purpose, since, as described
                  below, it is not possible to move a thread between domain
                  cgroups by writing its thread ID to the _cgroup.threads_
                  file.)
    
                  In cgroups v1, the corresponding file that should instead
                  be delegated is the _tasks_ file.
    
           The delegater should _not_ change the ownership of any of the
           controller interfaces files (e.g., _pids.max_ , _memory.high_) in
           _dlgt_grp_.  Those files are used from the next level above the
           delegated subtree in order to distribute resources into the
           subtree, and the delegatee should not have permission to change
           the resources that are distributed into the delegated subtree.
    
           See also the discussion of the _/sys/kernel/cgroup/delegate_ file in
           NOTES for information about further delegatable files in cgroups
           v2.
    
           After the aforementioned steps have been performed, the delegatee
           can create child cgroups within the delegated subtree (the cgroup
           subdirectories and the files they contain will be owned by the
           delegatee) and move processes between cgroups in the subtree.  If
           some controllers are present in _dlgt_grp/cgroup.subtree_control_ ,
           or the ownership of that file was passed to the delegatee, the
           delegatee can also control the further redistribution of the
           corresponding resources into the delegated subtree.
    
       **Cgroups v2 delegation: nsdelegate and cgroup namespaces**
           Starting with Linux 4.13, there is a second way to perform cgroup
           delegation in the cgroups v2 hierarchy.  This is done by mounting
           or remounting the cgroup v2 filesystem with the _nsdelegate_ mount
           option.  For example, if the cgroup v2 filesystem has already been
           mounted, we can remount it with the _nsdelegate_ option as follows:
    
               mount -t cgroup2 -o remount,nsdelegate \
                                none /sys/fs/cgroup/unified
    
           The effect of this mount option is to cause cgroup namespaces to
           automatically become delegation boundaries.  More specifically,
           the following restrictions apply for processes inside the cgroup
           namespace:
    
           •  Writes to controller interface files in the root directory of
              the namespace will fail with the error **EPERM**.  Processes inside
              the cgroup namespace can still write to delegatable files in
              the root directory of the cgroup namespace such as _cgroup.procs_
              and _cgroup.subtree_control_ , and can create subhierarchy
              underneath the root directory.
    
           •  Attempts to migrate processes across the namespace boundary are
              denied (with the error **ENOENT**).  Processes inside the cgroup
              namespace can still (subject to the containment rules described
              below) move processes between cgroups _within_ the subhierarchy
              under the namespace root.
    
           The ability to define cgroup namespaces as delegation boundaries
           makes cgroup namespaces more useful.  To understand why, suppose
           that we already have one cgroup hierarchy that has been delegated
           to a nonprivileged user, _cecilia_ , using the older delegation
           technique described above.  Suppose further that _cecilia_ wanted to
           further delegate a subhierarchy under the existing delegated
           hierarchy.  (For example, the delegated hierarchy might be
           associated with an unprivileged container run by _cecilia_.)  Even
           if a cgroup namespace was employed, because both hierarchies are
           owned by the unprivileged user _cecilia_ , the following illegitimate
           actions could be performed:
    
           •  A process in the inferior hierarchy could change the resource
              controller settings in the root directory of that hierarchy.
              (These resource controller settings are intended to allow
              control to be exercised from the _parent_ cgroup; a process
              inside the child cgroup should not be allowed to modify them.)
    
           •  A process inside the inferior hierarchy could move processes
              into and out of the inferior hierarchy if the cgroups in the
              superior hierarchy were somehow visible.
    
           Employing the _nsdelegate_ mount option prevents both of these
           possibilities.
    
           The _nsdelegate_ mount option only has an effect when performed in
           the initial mount namespace; in other mount namespaces, the option
           is silently ignored.
    
           _Note_ : On some systems, systemd(1) automatically mounts the cgroup
           v2 filesystem.  In order to experiment with the _nsdelegate_
           operation, it may be useful to boot the kernel with the following
           command-line options:
    
               cgroup_no_v1=all systemd.legacy_systemd_cgroup_controller
    
           These options cause the kernel to boot with the cgroups v1
           controllers disabled (meaning that the controllers are available
           in the v2 hierarchy), and tells systemd(1) not to mount and use
           the cgroup v2 hierarchy, so that the v2 hierarchy can be manually
           mounted with the desired options after boot-up.
    
       **Cgroup delegation containment rules**
           Some delegation _containment rules_ ensure that the delegatee can
           move processes between cgroups within the delegated subtree, but
           can't move processes from outside the delegated subtree into the
           subtree or vice versa.  A nonprivileged process (i.e., the
           delegatee) can write the PID of a "target" process into a
           _cgroup.procs_ file only if all of the following are true:
    
           •  The writer has write permission on the _cgroup.procs_ file in the
              destination cgroup.
    
           •  The writer has write permission on the _cgroup.procs_ file in the
              nearest common ancestor of the source and destination cgroups.
              Note that in some cases, the nearest common ancestor may be the
              source or destination cgroup itself.  This requirement is not
              enforced for cgroups v1 hierarchies, with the consequence that
              containment in v1 is less strict than in v2.  (For example, in
              cgroups v1 the user that owns two distinct delegated
              subhierarchies can move a process between the hierarchies.)
    
           •  If the cgroup v2 filesystem was mounted with the _nsdelegate_
              option, the writer must be able to see the source and
              destination cgroups from its cgroup namespace.
    
           •  In cgroups v1: the effective UID of the writer (i.e., the
              delegatee) matches the real user ID or the saved set-user-ID of
              the target process.  Before Linux 4.11, this requirement also
              applied in cgroups v2 (This was a historical requirement
              inherited from cgroups v1 that was later deemed unnecessary,
              since the other rules suffice for containment in cgroups v2.)
    
           _Note_ : one consequence of these delegation containment rules is
           that the unprivileged delegatee can't place the first process into
           the delegated subtree; instead, the delegater must place the first
           process (a process owned by the delegatee) into the delegated
           subtree.
    


## 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 (1.1) enable one or more threaded controllers and (1.2)
                make a process a member of _z_.  (These two steps can be done
                in either order.)  This has the following consequences:
    
                •  The type of _z_ becomes _domain threaded_.
    
                •  All of the descendant cgroups of _z_ that were not already
                   of type _threaded_ are converted to type _domain invalid_.
    
           (2)  As before, we make the threaded subtree usable by writing the
                string _"threaded"_ to each of the _domain invalid_ cgroups under
                _z_ , in order to convert them to the type _threaded_.
    
           One of the consequences of the above pathways to creating a
           threaded subtree is that the threaded root cgroup can be a parent
           only to _threaded_ (and _domain invalid_) cgroups.  The threaded root
           cgroup can't be a parent of a _domain_ cgroups, and a _threaded_
           cgroup can't have a sibling that is a _domain_ cgroup.
    
       **Using a threaded subtree**
           Within a threaded subtree, threaded controllers can be enabled in
           each subgroup whose type has been changed to _threaded_ ; upon doing
           so, the corresponding controller interface files appear in the
           children of that cgroup.
    
           A process can be moved into a threaded subtree by writing its PID
           to the _cgroup.procs_ file in one of the cgroups inside the tree.
           This has the effect of making all of the threads in the process
           members of the corresponding cgroup and makes the process a member
           of the threaded subtree.  The threads of the process can then be
           spread across the threaded subtree by writing their thread IDs
           (see gettid(2)) to the _cgroup.threads_ files in different cgroups
           inside the subtree.  The threads of a process must all reside in
           the same threaded subtree.
    
           As with writing to _cgroup.procs_ , some containment rules apply when
           writing to the _cgroup.threads_ file:
    
           •  The writer must have write permission on the cgroup.threads
              file in the destination cgroup.
    
           •  The writer must have write permission on the _cgroup.procs_ file
              in the common ancestor of the source and destination cgroups.
              (In some cases, the common ancestor may be the source or
              destination cgroup itself.)
    
           •  The source and destination cgroups must be in the same threaded
              subtree.  (Outside a threaded subtree, an attempt to move a
              thread by writing its thread ID to the _cgroup.threads_ file in a
              different _domain_ cgroup fails with the error **EOPNOTSUPP**.)
    
           The _cgroup.threads_ file is present in each cgroup (including
           _domain_ cgroups) and can be read in order to discover the set of
           threads that is present in the cgroup.  The set of thread IDs
           obtained when reading this file is not guaranteed to be ordered or
           free of duplicates.
    
           The _cgroup.procs_ file in the threaded root shows the PIDs of all
           processes that are members of the threaded subtree.  The
           _cgroup.procs_ files in the other cgroups in the subtree are not
           readable.
    
           Domain controllers can't be enabled in a threaded subtree; no
           controller-interface files appear inside the cgroups underneath
           the threaded root.  From the point of view of a domain controller,
           threaded subtrees are invisible: a multithreaded process inside a
           threaded subtree appears to a domain controller as a process that
           resides in the threaded root cgroup.
    
           Within a threaded subtree, the "no internal processes" rule does
           not apply: a cgroup can both contain member processes (or thread)
           and exercise controllers on child cgroups.
    
       **Rules for writing to cgroup.type and creating threaded subtrees**
           A number of rules apply when writing to the _cgroup.type_ file:
    
           •  Only the string _"threaded"_ may be written.  In other words, the
              only explicit transition that is possible is to convert a
              _domain_ cgroup to type _threaded_.
    
           •  The effect of writing _"threaded"_ depends on the current value
              in _cgroup.type_ , as follows:
    
              •  _domain_ or _domain threaded_ : start the creation of a threaded
                 subtree (whose root is the parent of this cgroup) via the
                 first of the pathways described above;
    
              •  _domain invalid_ : convert this cgroup (which is inside a
                 threaded subtree) to a usable (i.e., _threaded_) state;
    
              •  _threaded_ : no effect (a "no-op").
    
           •  We can't write to a _cgroup.type_ file if the parent's type is
              _domain invalid_.  In other words, the cgroups of a threaded
              subtree must be converted to the _threaded_ state in a top-down
              manner.
    
           There are also some constraints that must be satisfied in order to
           create a threaded subtree rooted at the cgroup _x_ :
    
           •  There can be no member processes in the descendant cgroups of
              _x_.  (The cgroup _x_ can itself have member processes.)
    
           •  No domain controllers may be enabled in _x_ 's
              _cgroup.subtree_control_ file.
    
           If any of the above constraints is violated, then an attempt to
           write _"threaded"_ to a _cgroup.type_ file fails with the error
           **ENOTSUP**.
    
       **The "domain threaded" cgroup type**
           According to the pathways described above, the type of a cgroup
           can change to _domain threaded_ in either of the following cases:
    
           •  The string _"threaded"_ is written to a child cgroup.
    
           •  A threaded controller is enabled inside the cgroup and a
              process is made a member of the cgroup.
    
           A _domain threaded_ cgroup, _x_ , can revert to the type _domain_ if the
           above conditions no longer hold true—that is, if all _threaded_
           child cgroups of _x_ are removed and either _x_ no longer has threaded
           controllers enabled or no longer has member processes.
    
           When a _domain threaded_ cgroup _x_ reverts to the type _domain_ :
    
           •  All _domain invalid_ descendants of _x_ that are not in lower-level
              threaded subtrees revert to the type _domain_.
    
           •  The root cgroups in any lower-level threaded subtrees revert to
              the type _domain threaded_.
    
       **Exceptions for the root cgroup**
           The root cgroup of the v2 hierarchy is treated exceptionally: it
           can be the parent of both _domain_ and _threaded_ cgroups.  If the
           string _"threaded"_ is written to the _cgroup.type_ file of one of the
           children of the root cgroup, then
    
           •  The type of that cgroup becomes _threaded_.
    
           •  The type of any descendants of that cgroup that are not part of
              lower-level threaded subtrees changes to _domain invalid_.
    
           Note that in this case, there is no cgroup whose type becomes
           _domain threaded_.  (Notionally, the root cgroup can be considered
           as the threaded root for the cgroup whose type was changed to
           _threaded_.)
    
           The aim of this exceptional treatment for the root cgroup is to
           allow a threaded cgroup that employs the _cpu_ controller to be
           placed as high as possible in the hierarchy, so as to minimize the
           (small) cost of traversing the cgroup hierarchy.
    
       **The cgroups v2 "cpu" controller and realtime threads**
           As at Linux 4.19, the cgroups v2 _cpu_ controller does not support
           control of realtime threads (specifically threads scheduled under
           any of the policies **SCHED_FIFO** , **SCHED_RR** , described
           **SCHED_DEADLINE** ; see sched(7)).  Therefore, the _cpu_ controller can
           be enabled in the root cgroup only if all realtime threads are in
           the root cgroup.  (If there are realtime threads in nonroot
           cgroups, then a write(2) of the string _"+cpu"_ to the
           _cgroup.subtree_control_ file fails with the error **EINVAL**.)
    
           On some systems, systemd(1) places certain realtime threads in
           nonroot cgroups in the v2 hierarchy.  On such systems, these
           threads must first be moved to the root cgroup before the _cpu_
           controller can be enabled.
    


## 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).
    
           The clone3(2) **CLONE_INTO_CGROUP** flag can be used to create a child
           process that begins its life in a different version 2 cgroup from
           the parent process.
    
       **/proc files**
           _/proc/cgroups_ (since Linux 2.6.24)
                  This file contains information about the controllers that
                  are compiled into the kernel.  An example of the contents
                  of this file (reformatted for readability) is the
                  following:
    
                      #subsys_name    hierarchy      num_cgroups    enabled
                      cpuset          4              1              1
                      cpu             8              1              1
                      cpuacct         8              1              1
                      blkio           6              1              1
                      memory          3              1              1
                      devices         10             84             1
                      freezer         7              1              1
                      net_cls         9              1              1
                      perf_event      5              1              1
                      net_prio        9              1              1
                      hugetlb         0              1              0
                      pids            2              1              1
    
                  The fields in this file are, from left to right:
    
                  [1]  The name of the controller.
    
                  [2]  The unique ID of the cgroup hierarchy on which this
                       controller is mounted.  If multiple cgroups v1
                       controllers are bound to the same hierarchy, then each
                       will show the same hierarchy ID in this field.  The
                       value in this field will be 0 if:
    
                       •  the controller is not mounted on a cgroups v1
                          hierarchy;
    
                       •  the controller is bound to the cgroups v2 single
                          unified hierarchy; or
    
                       •  the controller is disabled (see below).
    
                  [3]  The number of control groups in this hierarchy using
                       this controller.
    
                  [4]  This field contains the value 1 if this controller is
                       enabled, or 0 if it has been disabled (via the
                       _cgroup_disable_ kernel command-line boot parameter).
    
           _/proc/_ pid _/cgroup_ (since Linux 2.6.24)
                  This file describes control groups to which the process
                  with the corresponding PID belongs.  The displayed
                  information differs for cgroups version 1 and version 2
                  hierarchies.
    
                  For each cgroup hierarchy of which the process is a member,
                  there is one entry containing three colon-separated fields:
    
                      hierarchy-ID:controller-list:cgroup-path
    
                  For example:
    
                      5:cpuacct,cpu,cpuset:/daemons
    
                  The colon-separated fields are, from left to right:
    
                  [1]  For cgroups version 1 hierarchies, this field contains
                       a unique hierarchy ID number that can be matched to a
                       hierarchy ID in _/proc/cgroups_.  For the cgroups
                       version 2 hierarchy, this field contains the value 0.
    
                  [2]  For cgroups version 1 hierarchies, this field contains
                       a comma-separated list of the controllers bound to the
                       hierarchy.  For the cgroups version 2 hierarchy, this
                       field is empty.
    
                  [3]  This field contains the pathname of the control group
                       in the hierarchy to which the process belongs.  This
                       pathname is relative to the mount point of the
                       hierarchy.
    
       **/sys/kernel/cgroup files**
           _/sys/kernel/cgroup/delegate_ (since Linux 4.15)
                  This file exports a list of the cgroups v2 files (one per
                  line) that are delegatable (i.e., whose ownership should be
                  changed to the user ID of the delegatee).  In the future,
                  the set of delegatable files may change or grow, and this
                  file provides a way for the kernel to inform user-space
                  applications of which files must be delegated.  As at Linux
                  4.15, one sees the following when inspecting this file:
    
                      $ **cat /sys/kernel/cgroup/delegate**
                      cgroup.procs
                      cgroup.subtree_control
                      cgroup.threads
    
           _/sys/kernel/cgroup/features_ (since Linux 4.15)
                  Over time, the set of cgroups v2 features that are provided
                  by the kernel may change or grow, or some features may not
                  be enabled by default.  This file provides a way for user-
                  space applications to discover what features the running
                  kernel supports and has enabled.  Features are listed one
                  per line:
    
                      $ **cat /sys/kernel/cgroup/features**
                      nsdelegate
                      memory_localevents
    
                  The entries that can appear in this file are:
    
                  _memory_localevents_ (since Linux 5.2)
                         The kernel supports the _memory_localevents_ mount
                         option.
    
                  _nsdelegate_ (since Linux 4.15)
                         The kernel supports the _nsdelegate_ mount option.
    
                  _memory_recursiveprot_ (since Linux 5.7)
                         The kernel supports the _memory_recursiveprot_ mount
                         option.
    


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