1                                CGROUPS
   2                                -------
   4Written by Paul Menage <> based on
   7Original copyright statements from cpusets.txt:
   8Portions Copyright (C) 2004 BULL SA.
   9Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
  10Modified by Paul Jackson <>
  11Modified by Christoph Lameter <>
  161. Control Groups
  17  1.1 What are cgroups ?
  18  1.2 Why are cgroups needed ?
  19  1.3 How are cgroups implemented ?
  20  1.4 What does notify_on_release do ?
  21  1.5 What does clone_children do ?
  22  1.6 How do I use cgroups ?
  232. Usage Examples and Syntax
  24  2.1 Basic Usage
  25  2.2 Attaching processes
  26  2.3 Mounting hierarchies by name
  27  2.4 Notification API
  283. Kernel API
  29  3.1 Overview
  30  3.2 Synchronization
  31  3.3 Subsystem API
  324. Questions
  341. Control Groups
  371.1 What are cgroups ?
  40Control Groups provide a mechanism for aggregating/partitioning sets of
  41tasks, and all their future children, into hierarchical groups with
  42specialized behaviour.
  46A *cgroup* associates a set of tasks with a set of parameters for one
  47or more subsystems.
  49A *subsystem* is a module that makes use of the task grouping
  50facilities provided by cgroups to treat groups of tasks in
  51particular ways. A subsystem is typically a "resource controller" that
  52schedules a resource or applies per-cgroup limits, but it may be
  53anything that wants to act on a group of processes, e.g. a
  54virtualization subsystem.
  56A *hierarchy* is a set of cgroups arranged in a tree, such that
  57every task in the system is in exactly one of the cgroups in the
  58hierarchy, and a set of subsystems; each subsystem has system-specific
  59state attached to each cgroup in the hierarchy.  Each hierarchy has
  60an instance of the cgroup virtual filesystem associated with it.
  62At any one time there may be multiple active hierarchies of task
  63cgroups. Each hierarchy is a partition of all tasks in the system.
  65User level code may create and destroy cgroups by name in an
  66instance of the cgroup virtual file system, specify and query to
  67which cgroup a task is assigned, and list the task pids assigned to
  68a cgroup. Those creations and assignments only affect the hierarchy
  69associated with that instance of the cgroup file system.
  71On their own, the only use for cgroups is for simple job
  72tracking. The intention is that other subsystems hook into the generic
  73cgroup support to provide new attributes for cgroups, such as
  74accounting/limiting the resources which processes in a cgroup can
  75access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows
  76you to associate a set of CPUs and a set of memory nodes with the
  77tasks in each cgroup.
  791.2 Why are cgroups needed ?
  82There are multiple efforts to provide process aggregations in the
  83Linux kernel, mainly for resource tracking purposes. Such efforts
  84include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
  85namespaces. These all require the basic notion of a
  86grouping/partitioning of processes, with newly forked processes ending
  87in the same group (cgroup) as their parent process.
  89The kernel cgroup patch provides the minimum essential kernel
  90mechanisms required to efficiently implement such groups. It has
  91minimal impact on the system fast paths, and provides hooks for
  92specific subsystems such as cpusets to provide additional behaviour as
  95Multiple hierarchy support is provided to allow for situations where
  96the division of tasks into cgroups is distinctly different for
  97different subsystems - having parallel hierarchies allows each
  98hierarchy to be a natural division of tasks, without having to handle
  99complex combinations of tasks that would be present if several
 100unrelated subsystems needed to be forced into the same tree of
 103At one extreme, each resource controller or subsystem could be in a
 104separate hierarchy; at the other extreme, all subsystems
 105would be attached to the same hierarchy.
 107As an example of a scenario (originally proposed by
 108that can benefit from multiple hierarchies, consider a large
 109university server with various users - students, professors, system
 110tasks etc. The resource planning for this server could be along the
 111following lines:
 113       CPU :          "Top cpuset"
 114                       /       \
 115               CPUSet1         CPUSet2
 116                  |               |
 117               (Professors)    (Students)
 119               In addition (system tasks) are attached to topcpuset (so
 120               that they can run anywhere) with a limit of 20%
 122       Memory : Professors (50%), Students (30%), system (20%)
 124       Disk : Professors (50%), Students (30%), system (20%)
 126       Network : WWW browsing (20%), Network File System (60%), others (20%)
 127                               / \
 128               Professors (15%)  students (5%)
 130Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd go
 131into NFS network class.
 133At the same time Firefox/Lynx will share an appropriate CPU/Memory class
 134depending on who launched it (prof/student).
 136With the ability to classify tasks differently for different resources
 137(by putting those resource subsystems in different hierarchies) then
 138the admin can easily set up a script which receives exec notifications
 139and depending on who is launching the browser he can
 141    # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks
 143With only a single hierarchy, he now would potentially have to create
 144a separate cgroup for every browser launched and associate it with
 145appropriate network and other resource class.  This may lead to
 146proliferation of such cgroups.
 148Also lets say that the administrator would like to give enhanced network
 149access temporarily to a student's browser (since it is night and the user
 150wants to do online gaming :))  OR give one of the students simulation
 151apps enhanced CPU power,
 153With ability to write pids directly to resource classes, it's just a
 154matter of :
 156       # echo pid > /sys/fs/cgroup/network/<new_class>/tasks
 157       (after some time)
 158       # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks
 160Without this ability, he would have to split the cgroup into
 161multiple separate ones and then associate the new cgroups with the
 162new resource classes.
 1661.3 How are cgroups implemented ?
 169Control Groups extends the kernel as follows:
 171 - Each task in the system has a reference-counted pointer to a
 172   css_set.
 174 - A css_set contains a set of reference-counted pointers to
 175   cgroup_subsys_state objects, one for each cgroup subsystem
 176   registered in the system. There is no direct link from a task to
 177   the cgroup of which it's a member in each hierarchy, but this
 178   can be determined by following pointers through the
 179   cgroup_subsys_state objects. This is because accessing the
 180   subsystem state is something that's expected to happen frequently
 181   and in performance-critical code, whereas operations that require a
 182   task's actual cgroup assignments (in particular, moving between
 183   cgroups) are less common. A linked list runs through the cg_list
 184   field of each task_struct using the css_set, anchored at
 185   css_set->tasks.
 187 - A cgroup hierarchy filesystem can be mounted  for browsing and
 188   manipulation from user space.
 190 - You can list all the tasks (by pid) attached to any cgroup.
 192The implementation of cgroups requires a few, simple hooks
 193into the rest of the kernel, none in performance critical paths:
 195 - in init/main.c, to initialize the root cgroups and initial
 196   css_set at system boot.
 198 - in fork and exit, to attach and detach a task from its css_set.
 200In addition a new file system, of type "cgroup" may be mounted, to
 201enable browsing and modifying the cgroups presently known to the
 202kernel.  When mounting a cgroup hierarchy, you may specify a
 203comma-separated list of subsystems to mount as the filesystem mount
 204options.  By default, mounting the cgroup filesystem attempts to
 205mount a hierarchy containing all registered subsystems.
 207If an active hierarchy with exactly the same set of subsystems already
 208exists, it will be reused for the new mount. If no existing hierarchy
 209matches, and any of the requested subsystems are in use in an existing
 210hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
 211is activated, associated with the requested subsystems.
 213It's not currently possible to bind a new subsystem to an active
 214cgroup hierarchy, or to unbind a subsystem from an active cgroup
 215hierarchy. This may be possible in future, but is fraught with nasty
 216error-recovery issues.
 218When a cgroup filesystem is unmounted, if there are any
 219child cgroups created below the top-level cgroup, that hierarchy
 220will remain active even though unmounted; if there are no
 221child cgroups then the hierarchy will be deactivated.
 223No new system calls are added for cgroups - all support for
 224querying and modifying cgroups is via this cgroup file system.
 226Each task under /proc has an added file named 'cgroup' displaying,
 227for each active hierarchy, the subsystem names and the cgroup name
 228as the path relative to the root of the cgroup file system.
 230Each cgroup is represented by a directory in the cgroup file system
 231containing the following files describing that cgroup:
 233 - tasks: list of tasks (by pid) attached to that cgroup.  This list
 234   is not guaranteed to be sorted.  Writing a thread id into this file
 235   moves the thread into this cgroup.
 236 - cgroup.procs: list of tgids in the cgroup.  This list is not
 237   guaranteed to be sorted or free of duplicate tgids, and userspace
 238   should sort/uniquify the list if this property is required.
 239   Writing a thread group id into this file moves all threads in that
 240   group into this cgroup.
 241 - notify_on_release flag: run the release agent on exit?
 242 - release_agent: the path to use for release notifications (this file
 243   exists in the top cgroup only)
 245Other subsystems such as cpusets may add additional files in each
 246cgroup dir.
 248New cgroups are created using the mkdir system call or shell
 249command.  The properties of a cgroup, such as its flags, are
 250modified by writing to the appropriate file in that cgroups
 251directory, as listed above.
 253The named hierarchical structure of nested cgroups allows partitioning
 254a large system into nested, dynamically changeable, "soft-partitions".
 256The attachment of each task, automatically inherited at fork by any
 257children of that task, to a cgroup allows organizing the work load
 258on a system into related sets of tasks.  A task may be re-attached to
 259any other cgroup, if allowed by the permissions on the necessary
 260cgroup file system directories.
 262When a task is moved from one cgroup to another, it gets a new
 263css_set pointer - if there's an already existing css_set with the
 264desired collection of cgroups then that group is reused, else a new
 265css_set is allocated. The appropriate existing css_set is located by
 266looking into a hash table.
 268To allow access from a cgroup to the css_sets (and hence tasks)
 269that comprise it, a set of cg_cgroup_link objects form a lattice;
 270each cg_cgroup_link is linked into a list of cg_cgroup_links for
 271a single cgroup on its cgrp_link_list field, and a list of
 272cg_cgroup_links for a single css_set on its cg_link_list.
 274Thus the set of tasks in a cgroup can be listed by iterating over
 275each css_set that references the cgroup, and sub-iterating over
 276each css_set's task set.
 278The use of a Linux virtual file system (vfs) to represent the
 279cgroup hierarchy provides for a familiar permission and name space
 280for cgroups, with a minimum of additional kernel code.
 2821.4 What does notify_on_release do ?
 285If the notify_on_release flag is enabled (1) in a cgroup, then
 286whenever the last task in the cgroup leaves (exits or attaches to
 287some other cgroup) and the last child cgroup of that cgroup
 288is removed, then the kernel runs the command specified by the contents
 289of the "release_agent" file in that hierarchy's root directory,
 290supplying the pathname (relative to the mount point of the cgroup
 291file system) of the abandoned cgroup.  This enables automatic
 292removal of abandoned cgroups.  The default value of
 293notify_on_release in the root cgroup at system boot is disabled
 294(0).  The default value of other cgroups at creation is the current
 295value of their parents notify_on_release setting. The default value of
 296a cgroup hierarchy's release_agent path is empty.
 2981.5 What does clone_children do ?
 301If the clone_children flag is enabled (1) in a cgroup, then all
 302cgroups created beneath will call the post_clone callbacks for each
 303subsystem of the newly created cgroup. Usually when this callback is
 304implemented for a subsystem, it copies the values of the parent
 305subsystem, this is the case for the cpuset.
 3071.6 How do I use cgroups ?
 310To start a new job that is to be contained within a cgroup, using
 311the "cpuset" cgroup subsystem, the steps are something like:
 313 1) mount -t tmpfs cgroup_root /sys/fs/cgroup
 314 2) mkdir /sys/fs/cgroup/cpuset
 315 3) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
 316 4) Create the new cgroup by doing mkdir's and write's (or echo's) in
 317    the /sys/fs/cgroup virtual file system.
 318 5) Start a task that will be the "founding father" of the new job.
 319 6) Attach that task to the new cgroup by writing its pid to the
 320    /sys/fs/cgroup/cpuset/tasks file for that cgroup.
 321 7) fork, exec or clone the job tasks from this founding father task.
 323For example, the following sequence of commands will setup a cgroup
 324named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
 325and then start a subshell 'sh' in that cgroup:
 327  mount -t tmpfs cgroup_root /sys/fs/cgroup
 328  mkdir /sys/fs/cgroup/cpuset
 329  mount -t cgroup cpuset -ocpuset /sys/fs/cgroup/cpuset
 330  cd /sys/fs/cgroup/cpuset
 331  mkdir Charlie
 332  cd Charlie
 333  /bin/echo 2-3 > cpuset.cpus
 334  /bin/echo 1 > cpuset.mems
 335  /bin/echo $$ > tasks
 336  sh
 337  # The subshell 'sh' is now running in cgroup Charlie
 338  # The next line should display '/Charlie'
 339  cat /proc/self/cgroup
 3412. Usage Examples and Syntax
 3442.1 Basic Usage
 347Creating, modifying, using the cgroups can be done through the cgroup
 348virtual filesystem.
 350To mount a cgroup hierarchy with all available subsystems, type:
 351# mount -t cgroup xxx /sys/fs/cgroup
 353The "xxx" is not interpreted by the cgroup code, but will appear in
 354/proc/mounts so may be any useful identifying string that you like.
 356Note: Some subsystems do not work without some user input first.  For instance,
 357if cpusets are enabled the user will have to populate the cpus and mems files
 358for each new cgroup created before that group can be used.
 360As explained in section `1.2 Why are cgroups needed?' you should create
 361different hierarchies of cgroups for each single resource or group of
 362resources you want to control. Therefore, you should mount a tmpfs on
 363/sys/fs/cgroup and create directories for each cgroup resource or resource
 366# mount -t tmpfs cgroup_root /sys/fs/cgroup
 367# mkdir /sys/fs/cgroup/rg1
 369To mount a cgroup hierarchy with just the cpuset and memory
 370subsystems, type:
 371# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
 373To change the set of subsystems bound to a mounted hierarchy, just
 374remount with different options:
 375# mount -o remount,cpuset,blkio hier1 /sys/fs/cgroup/rg1
 377Now memory is removed from the hierarchy and blkio is added.
 379Note this will add blkio to the hierarchy but won't remove memory or
 380cpuset, because the new options are appended to the old ones:
 381# mount -o remount,blkio /sys/fs/cgroup/rg1
 383To Specify a hierarchy's release_agent:
 384# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
 385  xxx /sys/fs/cgroup/rg1
 387Note that specifying 'release_agent' more than once will return failure.
 389Note that changing the set of subsystems is currently only supported
 390when the hierarchy consists of a single (root) cgroup. Supporting
 391the ability to arbitrarily bind/unbind subsystems from an existing
 392cgroup hierarchy is intended to be implemented in the future.
 394Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
 395tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
 396is the cgroup that holds the whole system.
 398If you want to change the value of release_agent:
 399# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
 401It can also be changed via remount.
 403If you want to create a new cgroup under /sys/fs/cgroup/rg1:
 404# cd /sys/fs/cgroup/rg1
 405# mkdir my_cgroup
 407Now you want to do something with this cgroup.
 408# cd my_cgroup
 410In this directory you can find several files:
 411# ls
 412cgroup.procs notify_on_release tasks
 413(plus whatever files added by the attached subsystems)
 415Now attach your shell to this cgroup:
 416# /bin/echo $$ > tasks
 418You can also create cgroups inside your cgroup by using mkdir in this
 420# mkdir my_sub_cs
 422To remove a cgroup, just use rmdir:
 423# rmdir my_sub_cs
 425This will fail if the cgroup is in use (has cgroups inside, or
 426has processes attached, or is held alive by other subsystem-specific
 4292.2 Attaching processes
 432# /bin/echo PID > tasks
 434Note that it is PID, not PIDs. You can only attach ONE task at a time.
 435If you have several tasks to attach, you have to do it one after another:
 437# /bin/echo PID1 > tasks
 438# /bin/echo PID2 > tasks
 439        ...
 440# /bin/echo PIDn > tasks
 442You can attach the current shell task by echoing 0:
 444# echo 0 > tasks
 446You can use the cgroup.procs file instead of the tasks file to move all
 447threads in a threadgroup at once. Echoing the pid of any task in a
 448threadgroup to cgroup.procs causes all tasks in that threadgroup to be
 449be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
 450in the writing task's threadgroup.
 452Note: Since every task is always a member of exactly one cgroup in each
 453mounted hierarchy, to remove a task from its current cgroup you must
 454move it into a new cgroup (possibly the root cgroup) by writing to the
 455new cgroup's tasks file.
 457Note: Due to some restrictions enforced by some cgroup subsystems, moving
 458a process to another cgroup can fail.
 4602.3 Mounting hierarchies by name
 463Passing the name=<x> option when mounting a cgroups hierarchy
 464associates the given name with the hierarchy.  This can be used when
 465mounting a pre-existing hierarchy, in order to refer to it by name
 466rather than by its set of active subsystems.  Each hierarchy is either
 467nameless, or has a unique name.
 469The name should match [\w.-]+
 471When passing a name=<x> option for a new hierarchy, you need to
 472specify subsystems manually; the legacy behaviour of mounting all
 473subsystems when none are explicitly specified is not supported when
 474you give a subsystem a name.
 476The name of the subsystem appears as part of the hierarchy description
 477in /proc/mounts and /proc/<pid>/cgroups.
 4792.4 Notification API
 482There is mechanism which allows to get notifications about changing
 483status of a cgroup.
 485To register new notification handler you need:
 486 - create a file descriptor for event notification using eventfd(2);
 487 - open a control file to be monitored (e.g. memory.usage_in_bytes);
 488 - write "<event_fd> <control_fd> <args>" to cgroup.event_control.
 489   Interpretation of args is defined by control file implementation;
 491eventfd will be woken up by control file implementation or when the
 492cgroup is removed.
 494To unregister notification handler just close eventfd.
 496NOTE: Support of notifications should be implemented for the control
 497file. See documentation for the subsystem.
 4993. Kernel API
 5023.1 Overview
 505Each kernel subsystem that wants to hook into the generic cgroup
 506system needs to create a cgroup_subsys object. This contains
 507various methods, which are callbacks from the cgroup system, along
 508with a subsystem id which will be assigned by the cgroup system.
 510Other fields in the cgroup_subsys object include:
 512- subsys_id: a unique array index for the subsystem, indicating which
 513  entry in cgroup->subsys[] this subsystem should be managing.
 515- name: should be initialized to a unique subsystem name. Should be
 516  no longer than MAX_CGROUP_TYPE_NAMELEN.
 518- early_init: indicate if the subsystem needs early initialization
 519  at system boot.
 521Each cgroup object created by the system has an array of pointers,
 522indexed by subsystem id; this pointer is entirely managed by the
 523subsystem; the generic cgroup code will never touch this pointer.
 5253.2 Synchronization
 528There is a global mutex, cgroup_mutex, used by the cgroup
 529system. This should be taken by anything that wants to modify a
 530cgroup. It may also be taken to prevent cgroups from being
 531modified, but more specific locks may be more appropriate in that
 534See kernel/cgroup.c for more details.
 536Subsystems can take/release the cgroup_mutex via the functions
 539Accessing a task's cgroup pointer may be done in the following ways:
 540- while holding cgroup_mutex
 541- while holding the task's alloc_lock (via task_lock())
 542- inside an rcu_read_lock() section via rcu_dereference()
 5443.3 Subsystem API
 547Each subsystem should:
 549- add an entry in linux/cgroup_subsys.h
 550- define a cgroup_subsys object called <name>_subsys
 552If a subsystem can be compiled as a module, it should also have in its
 553module initcall a call to cgroup_load_subsys(), and in its exitcall a
 554call to cgroup_unload_subsys(). It should also set its_subsys.module =
 555THIS_MODULE in its .c file.
 557Each subsystem may export the following methods. The only mandatory
 558methods are create/destroy. Any others that are null are presumed to
 559be successful no-ops.
 561struct cgroup_subsys_state *create(struct cgroup *cgrp)
 562(cgroup_mutex held by caller)
 564Called to create a subsystem state object for a cgroup. The
 565subsystem should allocate its subsystem state object for the passed
 566cgroup, returning a pointer to the new object on success or a
 567negative error code. On success, the subsystem pointer should point to
 568a structure of type cgroup_subsys_state (typically embedded in a
 569larger subsystem-specific object), which will be initialized by the
 570cgroup system. Note that this will be called at initialization to
 571create the root subsystem state for this subsystem; this case can be
 572identified by the passed cgroup object having a NULL parent (since
 573it's the root of the hierarchy) and may be an appropriate place for
 574initialization code.
 576void destroy(struct cgroup *cgrp)
 577(cgroup_mutex held by caller)
 579The cgroup system is about to destroy the passed cgroup; the subsystem
 580should do any necessary cleanup and free its subsystem state
 581object. By the time this method is called, the cgroup has already been
 582unlinked from the file system and from the child list of its parent;
 583cgroup->parent is still valid. (Note - can also be called for a
 584newly-created cgroup if an error occurs after this subsystem's
 585create() method has been called for the new cgroup).
 587int pre_destroy(struct cgroup *cgrp);
 589Called before checking the reference count on each subsystem. This may
 590be useful for subsystems which have some extra references even if
 591there are not tasks in the cgroup. If pre_destroy() returns error code,
 592rmdir() will fail with it. From this behavior, pre_destroy() can be
 593called multiple times against a cgroup.
 595int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 596(cgroup_mutex held by caller)
 598Called prior to moving one or more tasks into a cgroup; if the
 599subsystem returns an error, this will abort the attach operation.
 600@tset contains the tasks to be attached and is guaranteed to have at
 601least one task in it.
 603If there are multiple tasks in the taskset, then:
 604  - it's guaranteed that all are from the same thread group
 605  - @tset contains all tasks from the thread group whether or not
 606    they're switching cgroups
 607  - the first task is the leader
 609Each @tset entry also contains the task's old cgroup and tasks which
 610aren't switching cgroup can be skipped easily using the
 611cgroup_taskset_for_each() iterator. Note that this isn't called on a
 612fork. If this method returns 0 (success) then this should remain valid
 613while the caller holds cgroup_mutex and it is ensured that either
 614attach() or cancel_attach() will be called in future.
 616void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 617(cgroup_mutex held by caller)
 619Called when a task attach operation has failed after can_attach() has succeeded.
 620A subsystem whose can_attach() has some side-effects should provide this
 621function, so that the subsystem can implement a rollback. If not, not necessary.
 622This will be called only about subsystems whose can_attach() operation have
 623succeeded. The parameters are identical to can_attach().
 625void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 626(cgroup_mutex held by caller)
 628Called after the task has been attached to the cgroup, to allow any
 629post-attachment activity that requires memory allocations or blocking.
 630The parameters are identical to can_attach().
 632void fork(struct task_struct *task)
 634Called when a task is forked into a cgroup.
 636void exit(struct task_struct *task)
 638Called during task exit.
 640int populate(struct cgroup *cgrp)
 641(cgroup_mutex held by caller)
 643Called after creation of a cgroup to allow a subsystem to populate
 644the cgroup directory with file entries.  The subsystem should make
 645calls to cgroup_add_file() with objects of type cftype (see
 646include/linux/cgroup.h for details).  Note that although this
 647method can return an error code, the error code is currently not
 648always handled well.
 650void post_clone(struct cgroup *cgrp)
 651(cgroup_mutex held by caller)
 653Called during cgroup_create() to do any parameter
 654initialization which might be required before a task could attach.  For
 655example in cpusets, no task may attach before 'cpus' and 'mems' are set
 658void bind(struct cgroup *root)
 659(cgroup_mutex and ss->hierarchy_mutex held by caller)
 661Called when a cgroup subsystem is rebound to a different hierarchy
 662and root cgroup. Currently this will only involve movement between
 663the default hierarchy (which never has sub-cgroups) and a hierarchy
 664that is being created/destroyed (and hence has no sub-cgroups).
 6664. Questions
 669Q: what's up with this '/bin/echo' ?
 670A: bash's builtin 'echo' command does not check calls to write() against
 671   errors. If you use it in the cgroup file system, you won't be
 672   able to tell whether a command succeeded or failed.
 674Q: When I attach processes, only the first of the line gets really attached !
 675A: We can only return one error code per call to write(). So you should also
 676   put only ONE pid.
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