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- CGROUPS
- -------
-
-Written by Paul Menage <menage@google.com> based on
-Documentation/cgroups/cpusets.txt
-
-Original copyright statements from cpusets.txt:
-Portions Copyright (C) 2004 BULL SA.
-Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
-Modified by Paul Jackson <pj@sgi.com>
-Modified by Christoph Lameter <clameter@sgi.com>
-
-CONTENTS:
-=========
-
-1. Control Groups
- 1.1 What are cgroups ?
- 1.2 Why are cgroups needed ?
- 1.3 How are cgroups implemented ?
- 1.4 What does notify_on_release do ?
- 1.5 What does clone_children do ?
- 1.6 How do I use cgroups ?
-2. Usage Examples and Syntax
- 2.1 Basic Usage
- 2.2 Attaching processes
- 2.3 Mounting hierarchies by name
- 2.4 Notification API
-3. Kernel API
- 3.1 Overview
- 3.2 Synchronization
- 3.3 Subsystem API
-4. Questions
-
-1. Control Groups
-=================
-
-1.1 What are cgroups ?
-----------------------
-
-Control Groups provide a mechanism for aggregating/partitioning sets of
-tasks, and all their future children, into hierarchical groups with
-specialized behaviour.
-
-Definitions:
-
-A *cgroup* associates a set of tasks with a set of parameters for one
-or more subsystems.
-
-A *subsystem* is a module that makes use of the task grouping
-facilities provided by cgroups to treat groups of tasks in
-particular ways. A subsystem is typically a "resource controller" that
-schedules a resource or applies per-cgroup limits, but it may be
-anything that wants to act on a group of processes, e.g. a
-virtualization subsystem.
-
-A *hierarchy* is a set of cgroups arranged in a tree, such that
-every task in the system is in exactly one of the cgroups in the
-hierarchy, and a set of subsystems; each subsystem has system-specific
-state attached to each cgroup in the hierarchy. Each hierarchy has
-an instance of the cgroup virtual filesystem associated with it.
-
-At any one time there may be multiple active hierarchies of task
-cgroups. Each hierarchy is a partition of all tasks in the system.
-
-User level code may create and destroy cgroups by name in an
-instance of the cgroup virtual file system, specify and query to
-which cgroup a task is assigned, and list the task pids assigned to
-a cgroup. Those creations and assignments only affect the hierarchy
-associated with that instance of the cgroup file system.
-
-On their own, the only use for cgroups is for simple job
-tracking. The intention is that other subsystems hook into the generic
-cgroup support to provide new attributes for cgroups, such as
-accounting/limiting the resources which processes in a cgroup can
-access. For example, cpusets (see Documentation/cgroups/cpusets.txt) allows
-you to associate a set of CPUs and a set of memory nodes with the
-tasks in each cgroup.
-
-1.2 Why are cgroups needed ?
-----------------------------
-
-There are multiple efforts to provide process aggregations in the
-Linux kernel, mainly for resource tracking purposes. Such efforts
-include cpusets, CKRM/ResGroups, UserBeanCounters, and virtual server
-namespaces. These all require the basic notion of a
-grouping/partitioning of processes, with newly forked processes ending
-in the same group (cgroup) as their parent process.
-
-The kernel cgroup patch provides the minimum essential kernel
-mechanisms required to efficiently implement such groups. It has
-minimal impact on the system fast paths, and provides hooks for
-specific subsystems such as cpusets to provide additional behaviour as
-desired.
-
-Multiple hierarchy support is provided to allow for situations where
-the division of tasks into cgroups is distinctly different for
-different subsystems - having parallel hierarchies allows each
-hierarchy to be a natural division of tasks, without having to handle
-complex combinations of tasks that would be present if several
-unrelated subsystems needed to be forced into the same tree of
-cgroups.
-
-At one extreme, each resource controller or subsystem could be in a
-separate hierarchy; at the other extreme, all subsystems
-would be attached to the same hierarchy.
-
-As an example of a scenario (originally proposed by vatsa@in.ibm.com)
-that can benefit from multiple hierarchies, consider a large
-university server with various users - students, professors, system
-tasks etc. The resource planning for this server could be along the
-following lines:
-
- CPU : "Top cpuset"
- / \
- CPUSet1 CPUSet2
- | |
- (Professors) (Students)
-
- In addition (system tasks) are attached to topcpuset (so
- that they can run anywhere) with a limit of 20%
-
- Memory : Professors (50%), Students (30%), system (20%)
-
- Disk : Professors (50%), Students (30%), system (20%)
-
- Network : WWW browsing (20%), Network File System (60%), others (20%)
- / \
- Professors (15%) students (5%)
-
-Browsers like Firefox/Lynx go into the WWW network class, while (k)nfsd go
-into NFS network class.
-
-At the same time Firefox/Lynx will share an appropriate CPU/Memory class
-depending on who launched it (prof/student).
-
-With the ability to classify tasks differently for different resources
-(by putting those resource subsystems in different hierarchies) then
-the admin can easily set up a script which receives exec notifications
-and depending on who is launching the browser he can
-
- # echo browser_pid > /sys/fs/cgroup/<restype>/<userclass>/tasks
-
-With only a single hierarchy, he now would potentially have to create
-a separate cgroup for every browser launched and associate it with
-appropriate network and other resource class. This may lead to
-proliferation of such cgroups.
-
-Also lets say that the administrator would like to give enhanced network
-access temporarily to a student's browser (since it is night and the user
-wants to do online gaming :)) OR give one of the students simulation
-apps enhanced CPU power,
-
-With ability to write pids directly to resource classes, it's just a
-matter of :
-
- # echo pid > /sys/fs/cgroup/network/<new_class>/tasks
- (after some time)
- # echo pid > /sys/fs/cgroup/network/<orig_class>/tasks
-
-Without this ability, he would have to split the cgroup into
-multiple separate ones and then associate the new cgroups with the
-new resource classes.
-
-
-
-1.3 How are cgroups implemented ?
----------------------------------
-
-Control Groups extends the kernel as follows:
-
- - Each task in the system has a reference-counted pointer to a
- css_set.
-
- - A css_set contains a set of reference-counted pointers to
- cgroup_subsys_state objects, one for each cgroup subsystem
- registered in the system. There is no direct link from a task to
- the cgroup of which it's a member in each hierarchy, but this
- can be determined by following pointers through the
- cgroup_subsys_state objects. This is because accessing the
- subsystem state is something that's expected to happen frequently
- and in performance-critical code, whereas operations that require a
- task's actual cgroup assignments (in particular, moving between
- cgroups) are less common. A linked list runs through the cg_list
- field of each task_struct using the css_set, anchored at
- css_set->tasks.
-
- - A cgroup hierarchy filesystem can be mounted for browsing and
- manipulation from user space.
-
- - You can list all the tasks (by pid) attached to any cgroup.
-
-The implementation of cgroups requires a few, simple hooks
-into the rest of the kernel, none in performance critical paths:
-
- - in init/main.c, to initialize the root cgroups and initial
- css_set at system boot.
-
- - in fork and exit, to attach and detach a task from its css_set.
-
-In addition a new file system, of type "cgroup" may be mounted, to
-enable browsing and modifying the cgroups presently known to the
-kernel. When mounting a cgroup hierarchy, you may specify a
-comma-separated list of subsystems to mount as the filesystem mount
-options. By default, mounting the cgroup filesystem attempts to
-mount a hierarchy containing all registered subsystems.
-
-If an active hierarchy with exactly the same set of subsystems already
-exists, it will be reused for the new mount. If no existing hierarchy
-matches, and any of the requested subsystems are in use in an existing
-hierarchy, the mount will fail with -EBUSY. Otherwise, a new hierarchy
-is activated, associated with the requested subsystems.
-
-It's not currently possible to bind a new subsystem to an active
-cgroup hierarchy, or to unbind a subsystem from an active cgroup
-hierarchy. This may be possible in future, but is fraught with nasty
-error-recovery issues.
-
-When a cgroup filesystem is unmounted, if there are any
-child cgroups created below the top-level cgroup, that hierarchy
-will remain active even though unmounted; if there are no
-child cgroups then the hierarchy will be deactivated.
-
-No new system calls are added for cgroups - all support for
-querying and modifying cgroups is via this cgroup file system.
-
-Each task under /proc has an added file named 'cgroup' displaying,
-for each active hierarchy, the subsystem names and the cgroup name
-as the path relative to the root of the cgroup file system.
-
-Each cgroup is represented by a directory in the cgroup file system
-containing the following files describing that cgroup:
-
- - tasks: list of tasks (by pid) attached to that cgroup. This list
- is not guaranteed to be sorted. Writing a thread id into this file
- moves the thread into this cgroup.
- - cgroup.procs: list of tgids in the cgroup. This list is not
- guaranteed to be sorted or free of duplicate tgids, and userspace
- should sort/uniquify the list if this property is required.
- Writing a thread group id into this file moves all threads in that
- group into this cgroup.
- - notify_on_release flag: run the release agent on exit?
- - release_agent: the path to use for release notifications (this file
- exists in the top cgroup only)
-
-Other subsystems such as cpusets may add additional files in each
-cgroup dir.
-
-New cgroups are created using the mkdir system call or shell
-command. The properties of a cgroup, such as its flags, are
-modified by writing to the appropriate file in that cgroups
-directory, as listed above.
-
-The named hierarchical structure of nested cgroups allows partitioning
-a large system into nested, dynamically changeable, "soft-partitions".
-
-The attachment of each task, automatically inherited at fork by any
-children of that task, to a cgroup allows organizing the work load
-on a system into related sets of tasks. A task may be re-attached to
-any other cgroup, if allowed by the permissions on the necessary
-cgroup file system directories.
-
-When a task is moved from one cgroup to another, it gets a new
-css_set pointer - if there's an already existing css_set with the
-desired collection of cgroups then that group is reused, else a new
-css_set is allocated. The appropriate existing css_set is located by
-looking into a hash table.
-
-To allow access from a cgroup to the css_sets (and hence tasks)
-that comprise it, a set of cg_cgroup_link objects form a lattice;
-each cg_cgroup_link is linked into a list of cg_cgroup_links for
-a single cgroup on its cgrp_link_list field, and a list of
-cg_cgroup_links for a single css_set on its cg_link_list.
-
-Thus the set of tasks in a cgroup can be listed by iterating over
-each css_set that references the cgroup, and sub-iterating over
-each css_set's task set.
-
-The use of a Linux virtual file system (vfs) to represent the
-cgroup hierarchy provides for a familiar permission and name space
-for cgroups, with a minimum of additional kernel code.
-
-1.4 What does notify_on_release do ?
-------------------------------------
-
-If the notify_on_release flag is enabled (1) in a cgroup, then
-whenever the last task in the cgroup leaves (exits or attaches to
-some other cgroup) and the last child cgroup of that cgroup
-is removed, then the kernel runs the command specified by the contents
-of the "release_agent" file in that hierarchy's root directory,
-supplying the pathname (relative to the mount point of the cgroup
-file system) of the abandoned cgroup. This enables automatic
-removal of abandoned cgroups. The default value of
-notify_on_release in the root cgroup at system boot is disabled
-(0). The default value of other cgroups at creation is the current
-value of their parents notify_on_release setting. The default value of
-a cgroup hierarchy's release_agent path is empty.
-
-1.5 What does clone_children do ?
----------------------------------
-
-If the clone_children flag is enabled (1) in a cgroup, then all
-cgroups created beneath will call the post_clone callbacks for each
-subsystem of the newly created cgroup. Usually when this callback is
-implemented for a subsystem, it copies the values of the parent
-subsystem, this is the case for the cpuset.
-
-1.6 How do I use cgroups ?
---------------------------
-
-To start a new job that is to be contained within a cgroup, using
-the "cpuset" cgroup subsystem, the steps are something like:
-
- 1) mount -t tmpfs cgroup_root /sys/fs/cgroup
- 2) mkdir /sys/fs/cgroup/cpuset
- 3) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset
- 4) Create the new cgroup by doing mkdir's and write's (or echo's) in
- the /sys/fs/cgroup virtual file system.
- 5) Start a task that will be the "founding father" of the new job.
- 6) Attach that task to the new cgroup by writing its pid to the
- /sys/fs/cgroup/cpuset/tasks file for that cgroup.
- 7) fork, exec or clone the job tasks from this founding father task.
-
-For example, the following sequence of commands will setup a cgroup
-named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
-and then start a subshell 'sh' in that cgroup:
-
- mount -t tmpfs cgroup_root /sys/fs/cgroup
- mkdir /sys/fs/cgroup/cpuset
- mount -t cgroup cpuset -ocpuset /sys/fs/cgroup/cpuset
- cd /sys/fs/cgroup/cpuset
- mkdir Charlie
- cd Charlie
- /bin/echo 2-3 > cpuset.cpus
- /bin/echo 1 > cpuset.mems
- /bin/echo $$ > tasks
- sh
- # The subshell 'sh' is now running in cgroup Charlie
- # The next line should display '/Charlie'
- cat /proc/self/cgroup
-
-2. Usage Examples and Syntax
-============================
-
-2.1 Basic Usage
----------------
-
-Creating, modifying, using the cgroups can be done through the cgroup
-virtual filesystem.
-
-To mount a cgroup hierarchy with all available subsystems, type:
-# mount -t cgroup xxx /sys/fs/cgroup
-
-The "xxx" is not interpreted by the cgroup code, but will appear in
-/proc/mounts so may be any useful identifying string that you like.
-
-Note: Some subsystems do not work without some user input first. For instance,
-if cpusets are enabled the user will have to populate the cpus and mems files
-for each new cgroup created before that group can be used.
-
-As explained in section `1.2 Why are cgroups needed?' you should create
-different hierarchies of cgroups for each single resource or group of
-resources you want to control. Therefore, you should mount a tmpfs on
-/sys/fs/cgroup and create directories for each cgroup resource or resource
-group.
-
-# mount -t tmpfs cgroup_root /sys/fs/cgroup
-# mkdir /sys/fs/cgroup/rg1
-
-To mount a cgroup hierarchy with just the cpuset and memory
-subsystems, type:
-# mount -t cgroup -o cpuset,memory hier1 /sys/fs/cgroup/rg1
-
-To change the set of subsystems bound to a mounted hierarchy, just
-remount with different options:
-# mount -o remount,cpuset,blkio hier1 /sys/fs/cgroup/rg1
-
-Now memory is removed from the hierarchy and blkio is added.
-
-Note this will add blkio to the hierarchy but won't remove memory or
-cpuset, because the new options are appended to the old ones:
-# mount -o remount,blkio /sys/fs/cgroup/rg1
-
-To Specify a hierarchy's release_agent:
-# mount -t cgroup -o cpuset,release_agent="/sbin/cpuset_release_agent" \
- xxx /sys/fs/cgroup/rg1
-
-Note that specifying 'release_agent' more than once will return failure.
-
-Note that changing the set of subsystems is currently only supported
-when the hierarchy consists of a single (root) cgroup. Supporting
-the ability to arbitrarily bind/unbind subsystems from an existing
-cgroup hierarchy is intended to be implemented in the future.
-
-Then under /sys/fs/cgroup/rg1 you can find a tree that corresponds to the
-tree of the cgroups in the system. For instance, /sys/fs/cgroup/rg1
-is the cgroup that holds the whole system.
-
-If you want to change the value of release_agent:
-# echo "/sbin/new_release_agent" > /sys/fs/cgroup/rg1/release_agent
-
-It can also be changed via remount.
-
-If you want to create a new cgroup under /sys/fs/cgroup/rg1:
-# cd /sys/fs/cgroup/rg1
-# mkdir my_cgroup
-
-Now you want to do something with this cgroup.
-# cd my_cgroup
-
-In this directory you can find several files:
-# ls
-cgroup.procs notify_on_release tasks
-(plus whatever files added by the attached subsystems)
-
-Now attach your shell to this cgroup:
-# /bin/echo $$ > tasks
-
-You can also create cgroups inside your cgroup by using mkdir in this
-directory.
-# mkdir my_sub_cs
-
-To remove a cgroup, just use rmdir:
-# rmdir my_sub_cs
-
-This will fail if the cgroup is in use (has cgroups inside, or
-has processes attached, or is held alive by other subsystem-specific
-reference).
-
-2.2 Attaching processes
------------------------
-
-# /bin/echo PID > tasks
-
-Note that it is PID, not PIDs. You can only attach ONE task at a time.
-If you have several tasks to attach, you have to do it one after another:
-
-# /bin/echo PID1 > tasks
-# /bin/echo PID2 > tasks
- ...
-# /bin/echo PIDn > tasks
-
-You can attach the current shell task by echoing 0:
-
-# echo 0 > tasks
-
-You can use the cgroup.procs file instead of the tasks file to move all
-threads in a threadgroup at once. Echoing the pid of any task in a
-threadgroup to cgroup.procs causes all tasks in that threadgroup to be
-be attached to the cgroup. Writing 0 to cgroup.procs moves all tasks
-in the writing task's threadgroup.
-
-Note: Since every task is always a member of exactly one cgroup in each
-mounted hierarchy, to remove a task from its current cgroup you must
-move it into a new cgroup (possibly the root cgroup) by writing to the
-new cgroup's tasks file.
-
-Note: Due to some restrictions enforced by some cgroup subsystems, moving
-a process to another cgroup can fail.
-
-2.3 Mounting hierarchies by name
---------------------------------
-
-Passing the name=<x> option when mounting a cgroups hierarchy
-associates the given name with the hierarchy. This can be used when
-mounting a pre-existing hierarchy, in order to refer to it by name
-rather than by its set of active subsystems. Each hierarchy is either
-nameless, or has a unique name.
-
-The name should match [\w.-]+
-
-When passing a name=<x> option for a new hierarchy, you need to
-specify subsystems manually; the legacy behaviour of mounting all
-subsystems when none are explicitly specified is not supported when
-you give a subsystem a name.
-
-The name of the subsystem appears as part of the hierarchy description
-in /proc/mounts and /proc/<pid>/cgroups.
-
-2.4 Notification API
---------------------
-
-There is mechanism which allows to get notifications about changing
-status of a cgroup.
-
-To register new notification handler you need:
- - create a file descriptor for event notification using eventfd(2);
- - open a control file to be monitored (e.g. memory.usage_in_bytes);
- - write "<event_fd> <control_fd> <args>" to cgroup.event_control.
- Interpretation of args is defined by control file implementation;
-
-eventfd will be woken up by control file implementation or when the
-cgroup is removed.
-
-To unregister notification handler just close eventfd.
-
-NOTE: Support of notifications should be implemented for the control
-file. See documentation for the subsystem.
-
-3. Kernel API
-=============
-
-3.1 Overview
-------------
-
-Each kernel subsystem that wants to hook into the generic cgroup
-system needs to create a cgroup_subsys object. This contains
-various methods, which are callbacks from the cgroup system, along
-with a subsystem id which will be assigned by the cgroup system.
-
-Other fields in the cgroup_subsys object include:
-
-- subsys_id: a unique array index for the subsystem, indicating which
- entry in cgroup->subsys[] this subsystem should be managing.
-
-- name: should be initialized to a unique subsystem name. Should be
- no longer than MAX_CGROUP_TYPE_NAMELEN.
-
-- early_init: indicate if the subsystem needs early initialization
- at system boot.
-
-Each cgroup object created by the system has an array of pointers,
-indexed by subsystem id; this pointer is entirely managed by the
-subsystem; the generic cgroup code will never touch this pointer.
-
-3.2 Synchronization
--------------------
-
-There is a global mutex, cgroup_mutex, used by the cgroup
-system. This should be taken by anything that wants to modify a
-cgroup. It may also be taken to prevent cgroups from being
-modified, but more specific locks may be more appropriate in that
-situation.
-
-See kernel/cgroup.c for more details.
-
-Subsystems can take/release the cgroup_mutex via the functions
-cgroup_lock()/cgroup_unlock().
-
-Accessing a task's cgroup pointer may be done in the following ways:
-- while holding cgroup_mutex
-- while holding the task's alloc_lock (via task_lock())
-- inside an rcu_read_lock() section via rcu_dereference()
-
-3.3 Subsystem API
------------------
-
-Each subsystem should:
-
-- add an entry in linux/cgroup_subsys.h
-- define a cgroup_subsys object called <name>_subsys
-
-If a subsystem can be compiled as a module, it should also have in its
-module initcall a call to cgroup_load_subsys(), and in its exitcall a
-call to cgroup_unload_subsys(). It should also set its_subsys.module =
-THIS_MODULE in its .c file.
-
-Each subsystem may export the following methods. The only mandatory
-methods are create/destroy. Any others that are null are presumed to
-be successful no-ops.
-
-struct cgroup_subsys_state *create(struct cgroup *cgrp)
-(cgroup_mutex held by caller)
-
-Called to create a subsystem state object for a cgroup. The
-subsystem should allocate its subsystem state object for the passed
-cgroup, returning a pointer to the new object on success or a
-negative error code. On success, the subsystem pointer should point to
-a structure of type cgroup_subsys_state (typically embedded in a
-larger subsystem-specific object), which will be initialized by the
-cgroup system. Note that this will be called at initialization to
-create the root subsystem state for this subsystem; this case can be
-identified by the passed cgroup object having a NULL parent (since
-it's the root of the hierarchy) and may be an appropriate place for
-initialization code.
-
-void destroy(struct cgroup *cgrp)
-(cgroup_mutex held by caller)
-
-The cgroup system is about to destroy the passed cgroup; the subsystem
-should do any necessary cleanup and free its subsystem state
-object. By the time this method is called, the cgroup has already been
-unlinked from the file system and from the child list of its parent;
-cgroup->parent is still valid. (Note - can also be called for a
-newly-created cgroup if an error occurs after this subsystem's
-create() method has been called for the new cgroup).
-
-int pre_destroy(struct cgroup *cgrp);
-
-Called before checking the reference count on each subsystem. This may
-be useful for subsystems which have some extra references even if
-there are not tasks in the cgroup. If pre_destroy() returns error code,
-rmdir() will fail with it. From this behavior, pre_destroy() can be
-called multiple times against a cgroup.
-
-int can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
-(cgroup_mutex held by caller)
-
-Called prior to moving one or more tasks into a cgroup; if the
-subsystem returns an error, this will abort the attach operation.
-@tset contains the tasks to be attached and is guaranteed to have at
-least one task in it.
-
-If there are multiple tasks in the taskset, then:
- - it's guaranteed that all are from the same thread group
- - @tset contains all tasks from the thread group whether or not
- they're switching cgroups
- - the first task is the leader
-
-Each @tset entry also contains the task's old cgroup and tasks which
-aren't switching cgroup can be skipped easily using the
-cgroup_taskset_for_each() iterator. Note that this isn't called on a
-fork. If this method returns 0 (success) then this should remain valid
-while the caller holds cgroup_mutex and it is ensured that either
-attach() or cancel_attach() will be called in future.
-
-void cancel_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
-(cgroup_mutex held by caller)
-
-Called when a task attach operation has failed after can_attach() has succeeded.
-A subsystem whose can_attach() has some side-effects should provide this
-function, so that the subsystem can implement a rollback. If not, not necessary.
-This will be called only about subsystems whose can_attach() operation have
-succeeded. The parameters are identical to can_attach().
-
-void attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
-(cgroup_mutex held by caller)
-
-Called after the task has been attached to the cgroup, to allow any
-post-attachment activity that requires memory allocations or blocking.
-The parameters are identical to can_attach().
-
-void fork(struct task_struct *task)
-
-Called when a task is forked into a cgroup.
-
-void exit(struct task_struct *task)
-
-Called during task exit.
-
-int populate(struct cgroup *cgrp)
-(cgroup_mutex held by caller)
-
-Called after creation of a cgroup to allow a subsystem to populate
-the cgroup directory with file entries. The subsystem should make
-calls to cgroup_add_file() with objects of type cftype (see
-include/linux/cgroup.h for details). Note that although this
-method can return an error code, the error code is currently not
-always handled well.
-
-void post_clone(struct cgroup *cgrp)
-(cgroup_mutex held by caller)
-
-Called during cgroup_create() to do any parameter
-initialization which might be required before a task could attach. For
-example in cpusets, no task may attach before 'cpus' and 'mems' are set
-up.
-
-void bind(struct cgroup *root)
-(cgroup_mutex and ss->hierarchy_mutex held by caller)
-
-Called when a cgroup subsystem is rebound to a different hierarchy
-and root cgroup. Currently this will only involve movement between
-the default hierarchy (which never has sub-cgroups) and a hierarchy
-that is being created/destroyed (and hence has no sub-cgroups).
-
-4. Questions
-============
-
-Q: what's up with this '/bin/echo' ?
-A: bash's builtin 'echo' command does not check calls to write() against
- errors. If you use it in the cgroup file system, you won't be
- able to tell whether a command succeeded or failed.
-
-Q: When I attach processes, only the first of the line gets really attached !
-A: We can only return one error code per call to write(). So you should also
- put only ONE pid.
-