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NAME

       cgroups - Linux control groups

DESCRIPTION

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

   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 whitelists  and
              blacklists.   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.

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 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 generate POLLPRI events.

       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 v2 delegation: delegation to a less privileged user
       In  the  context  of  cgroups,  delegation means passing management of some subtree of the
       cgroup hierarchy to a nonprivileged process.  Cgroups v1 provides support  for  delegation
       that  was  accidental  and  not  fully secure.  Cgroups v2 supports delegation by explicit
       design.

       Some terminology is required in order to describe delegation.  A 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
              Changing the ownership of this file  means  that  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
              Changing the ownership of this file is necessary if a  threaded  subtree  is  being
              delegated  (see  the  description  of  "thread  mode",  below).   This  permits the
              delegatee to write thread IDs to the file.  (The ownership of this file can also be
              changed  when  delegating  a  domain subtree, but currently this serves no purpose,
              since, as described below, it is not possible  to  move  a  thread  between  domain
              cgroups by writing its thread ID to the cgroup.tasks file.)

       The  delegater  should  not change the ownership of any of the 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.

       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.   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  the  that hierarchy.  (These resource controller settings are
          intended to allow control to be exercised from the parent cgroup; a process inside  the
          child cgroup should not be allowed to modify them.)

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

       Employing the nsdelegate mount option prevents both of these 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 desirable to

   Cgroup v2 delegation containment rules
       Some delegation containment rules ensure that the delegatee  can  move  processes  between
       cgroups  within the delegated subtree, but can't move processes from outside the delegated
       subtree into the subtree or vice versa.  A nonprivileged process (i.e., the delegatee) can
       write  the PID of a "target" process into a cgroup.procs file only if all of the following
       are true:

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

       *  The writer has write permission on the cgroup.procs file in the common ancestor of  the
          source  and destination cgroups.  (In some cases, the common ancestor may be the source
          or destination cgroup itself.)

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

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

       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 4.15, the following controllers are threaded: cpu, perf_event, and pids.

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

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

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

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

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

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

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

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

       1. In an existing cgroup, z, that currently has the type domain, we (1) enable one or more
          threaded controllers and (2) make a process a member of z.  (These  two  steps  can  be
          done in either order.)  This has the following consequences:

          *  The type of z becomes domain threaded.

          *  All  of  the  descendant  cgroups  of  x  that were not already of type threaded are
             converted to type domain invalid.

       2. As before, we make the threaded subtree usable by writing the string "threaded" to each
          of the domain invalid cgroups under y, in order to convert them to the type threaded.

       One  of  the consequences of the above pathways to creating a threaded subtree is that the
       threaded root cgroup can be a parent only to threaded (and domain invalid)  cgroups.   The
       threaded  root  cgroup  can't be a parent of a domain cgroups, and a threaded cgroup can't
       have a sibling that is a domain cgroup.

   Using a threaded subtree
       Within a threaded subtree, threaded controllers can be enabled in each subgroup whose type
       has  been changed to threaded; upon doing so, the corresponding controller interface files
       appear in the children of that cgroup.

       A process can be moved into a threaded subtree by writing its PID to the cgroup.procs file
       in  one  of the cgroups inside the tree.  This has the effect of making all of the threads
       in the process members of the corresponding cgroup and makes the process a member  of  the
       threaded  subtree.   The  threads  of  the  process can then be spread across the threaded
       subtree by writing their thread  IDs  (see  gettid(2))  to  the  cgroup.threads  files  in
       different  cgroups  inside  the  subtree.  The threads of a process must all reside in the
       same threaded subtree.

       As with writing to  cgroup.procs,  some  containment  rules  apply  when  writing  to  the
       cgroup.threads file:

       *  The  writer  must  have  write permission on the cgroup.threads file in the destination
          cgroup.

       *  The writer must have write permission on the cgroup.procs file in the  common  ancestor
          of  the source and destination cgroups.  (In some cases, the common ancestor may be the
          source or destination cgroup itself.)

       *  The source and destination cgroups must be in the same threaded  subtree.   (Outside  a
          threaded  subtree,  an  attempt  to  move  a  thread  by  writing  its thread ID to the
          cgroup.threads file in a different domain cgroup fails with the error EOPNOTSUPP.)

       The cgroup.threads file is present in each cgroup (including domain cgroups)  and  can  be
       read  in  order  to discover the set of threads that is present in the cgroup.  The set of
       thread IDs obtained when reading this file is not guaranteed to  be  ordered  or  free  of
       duplicates.

       The  cgroup.procs  file  in  the  threaded  root  shows the PIDs of all processes that are
       members of the threaded subtree.  The cgroup.procs files  in  the  other  cgroups  in  the
       subtree are not readable.

       Domain  controllers  can't be enabled in a threaded subtree; no controller-interface files
       appear inside the cgroups underneath the threaded root.  From  the  point  of  view  of  a
       domain  controller,  threaded  subtrees  are  invisible:  a multithreaded process inside a
       threaded subtree appears to a domain controller as a process that resides in the  threaded
       root cgroup.

       Within  a  threaded subtree, the "no internal processes" rule does not apply: a cgroup can
       both contain member processes (or thread) and exercise controllers on child cgroups.

   Rules for writing to cgroup.type and creating threaded subtrees
       A number of rules apply when writing to the cgroup.type file:

       *  Only the string  "threaded"  may  be  written.   In  other  words,  the  only  explicit
          transition that is possible is to convert a domain cgroup to type threaded.

       *  The string "threaded" can be written only if the current value in cgroup.type is one of
          the following

          ·  domain, to start the creation of a threaded subtree via the first  of  the  pathways
             described above;

          ·  domain invalid,  to  convert  one of the cgroups in a threaded subtree into a usable
             (i.e., threaded) state;

          ·  threaded, which has no effect (a "no-op").

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

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

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

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

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

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

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

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

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

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

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

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

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

       *  The type of that cgroup becomes threaded.

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

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

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

   The cgroups v2 "cpu" controller and realtime processes
       As at Linux 4.15, the cgroups v2 cpu controller  does  not  support  control  of  realtime
       processes,  and  the  controller  can  be  enabled in the root cgroup only if all realtime
       threads are in the root cgroup.  (If there are realtime processes in nonroot cgroups, then
       a  write(2)  of  the string "+cpu" to the cgroup.subtree_control file fails with the error
       EINVAL.  However, on some systems, systemd(1) places certain realtime processes in nonroot
       cgroups  in the v2 hierarchy.  On such systems, these processes must first be moved to the
       root cgroup before the cpu controller can be enabled.

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 4.15 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.