Control Groups

RESOURCE MANAGEMENT GUIDE

CHAPTER 1. INTRODUCTION TO CONTROL GROUPS (CGROUPS)

1.1. What are Control Groups

Control Groups provide a way to hierarchically group and label processes, and to apply resource limits to them.
By binding the system of cgroup hierarchies with the systemd unit tree, you can manage system resources with systemctl commands, or by modifying systemd unit files.
Cgroups are similar to processes in that:

• they are hierarchical
• child cgroups inherit certain attributes from their parent cgroup

The fundamental difference is that many different hierarchies of cgroups can exist simultaneously on a system.
But, all processes descend from a single parent init, the Linux process model is a single hierarchy, or tree.
Multiple separate hierarchies of cgroups are necessary because each cgroup hierarchy is attached to one or more subsystems.
A subsystem represents a single resource, such as CPU time or memory.
Subsystems are also called resource controllers, also called a control group subsystem, is a kernel subsystem that represents a single resource, such as CPU time, memory, network bandwidth or disk I/O.
Currently mounted resource controllers can be listed in the /proc/cgroups file.
Here are a few simple rules governing the relationships between subsystems, hierarchies of cgroups, and tasks,

• A single cgroup hierarchy can have one or more subsystems attached to it.

• Any single subsystem (such as cpu) cannot be attached to more than one cgroup hierarchy if one of those hierarchies has a different subsystem attached to it already.

• A task cannot be a member of 2 different cgroups in the same cgroup hierarchy

• A child task automatically inherits the cgroup membership of its parent but can be moved to different cgroups as needed.

1.2. DEFAULT CGROUP HIERARCHIES

All processes running on the system are child processes of the systemd init process.
Systemd provides 3 unit types that are used for the purpose of resource control:

• name.service
Service is a process or a group of processes which are started by systemd based on a unit configuration file name.service. Where name stands for the name of the service.
• name.scope
Scope is a group of processes externally created through the fork() function and then registered by systemd at runtime.
• parent-name.slice
Slice is a group of hierarchically organized units. Slice is a hierarchy in which scopes and services are placed. The actual processes are contained in scopes or in services.

There are four slices created by default:

• -.slice
the root slice;
• system.slice
the default place for all system services;
• user.slice
the default place for all user sessions;
• machine.slice
the default place for all virtual machines and Linux containers.

The following tree is a simplified example of a cgroup tree listed by systemd-cgls,

Control group /:
-.slice
├─user.slice
│ ├─user-1000.slice
│ │ ├─user@1000.service
│ │ │ ├─gvfs-goa-volume-monitor.service
│ │ │ │ └─1572 /usr/lib/gvfs/gvfs-goa-volume-monitor
│ │ │ ...
│ │ │ ├─init.scope
│ │ │ │ ├─1322 /lib/systemd/systemd --user
│ │ │ │ └─1323 (sd-pam)
│ │ │ ...
│ │ └─session-2.scope
│ │   ├─1319 gdm-session-worker [pam/gdm-password]
│ │   ...
│ └─user-121.slice
│   ├─session-c1.scope
│
│   └─user@121.service
│
├─init.scope
│ └─1 /sbin/init splash
└─system.slice
  ├─irqbalance.service
  │ └─797 /usr/sbin/irqbalance --foreground
   ...
  └─systemd-logind.service
    └─813 /lib/systemd/systemd-logind

As you can see, services and scopes contain processes and are placed in slices.

1.3. RESOURCE CONTROLLERS IN LINUX KERNEL

A resource controller, also called a cgroup subsystem, represents a single resource, such as CPU time or memory.
The Linux kernel provides a range of resource controllers, that are mounted automatically by systemd.

$ cat /proc/cgroups
#subsys_name	hierarchy	num_cgroups	enabled
cpuset	            6	        1	        1
cpu	                8	        1	        1
cpuacct	            8	        1	        1
blkio	            9	        1	        1
memory	            4	        1	        1
devices	            10	        65	        1
freezer	            3	        1	        1
net_cls	            5	        1	        1
perf_event	        7	        1	        1
net_prio	        5	        1	        1
hugetlb	            2	        1	        1
pids	            12	        71	        1
rdma	            11	        1	        1

CHAPTER 2. USING CONTROL GROUPS

This guide focuses on utilities provided by systemd that are preferred as a way of cgroup management.

2.1. Creating Control Groups

From the systemd's perspective,

  a cgroup is bound to a system unit configurable with a unit file and manageable with systemd's command-line utilities.

• To create a transient cgroup for a service
Start the service with the systemd-run command.
Transient unit is removed automatically as soon as the service is stopped.

• To assign a persistent cgroup to a service
Edit its unit configuration file.
The configuration is preserved after the system reboot, so it can be used to manage services that are started automatically.
To configure a unit to be started automatically on system boot, execute the systemctl enable name.service command.
Running this command automatically creates a unit file in the /usr/lib/systemd/system/ directory.

2.2. REMOVING CONTROL GROUPS

• Transient cgroups are released automatically as soon as the processes they contain finish.
• Persistent cgroups are released when the unit is disabled and its configuration file is deleted by running:
systemctl disable name.service

Control groups series by Neil Brown

Thread Scheduling in Android

a thread scheduler is the part of the operating system in charge of deciding which threads in the system should run, when, and for how long. Android’s thread scheduler uses two main factors to determine how threads are scheduled across the entire system: nice values and cgroups.

Nice values

Nice values in Android are used as a measure of a thread’s priority.
A higher nice value (i.e., lower priority, as in they are being “nice” to other threads in the system) will run less often than those with lower nice values (i.e., higher priority).

Control Groups

Reference: https://android.googlesource.com/kernel/common/+/android-trusty-3.10/Documentation/cgroups/cgroups.txt

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. Extended attributes usage
5. 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 behavior.

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) allow
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
up 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 goes
into the 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),
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///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 let's 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 student's 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//tasks
       (after some time)
       # echo pid > /sys/fs/cgroup/network//tasks

Without this ability, the administrator 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 thread group IDs 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, otherwise 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 settings. The default value of
a cgroup hierarchy's release_agent path is empty.
1.5 What does clone_children do ?
---------------------------------
This flag only affects the cpuset controller. If the clone_children
flag is enabled (1) in a cgroup, a new cpuset cgroup will copy its
configuration from the parent during initialization.
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 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
While remounting cgroups is currently supported, it is not recommend
to use it. Remounting allows changing bound subsystems and
release_agent. Rebinding is hardly useful as it only works when the
hierarchy is empty and release_agent itself should be replaced with
conventional fsnotify. The support for remounting will be removed in
the future.
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
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= 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= 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//cgroups.
2.4 Notification API
--------------------
There is mechanism which allows to get notifications about changing
status of a cgroup.
To register a new notification handler you need to:
 - create a file descriptor for event notification using eventfd(2);
 - open a control file to be monitored (e.g. memory.usage_in_bytes);
 - write "  " 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 a 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 _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 css_alloc/free. Any others that are null are presumed to
be successful no-ops.
struct cgroup_subsys_state *css_alloc(struct cgroup *cgrp)
(cgroup_mutex held by caller)
Called to allocate 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
ERR_PTR() value. 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.
int css_online(struct cgroup *cgrp)
(cgroup_mutex held by caller)
Called after @cgrp successfully completed all allocations and made
visible to cgroup_for_each_child/descendant_*() iterators. The
subsystem may choose to fail creation by returning -errno. This
callback can be used to implement reliable state sharing and
propagation along the hierarchy. See the comment on
cgroup_for_each_descendant_pre() for details.
void css_offline(struct cgroup *cgrp);
(cgroup_mutex held by caller)
This is the counterpart of css_online() and called iff css_online()
has succeeded on @cgrp. This signifies the beginning of the end of
@cgrp. @cgrp is being removed and the subsystem should start dropping
all references it's holding on @cgrp. When all references are dropped,
cgroup removal will proceed to the next step - css_free(). After this
callback, @cgrp should be considered dead to the subsystem.
void css_free(struct cgroup *cgrp)
(cgroup_mutex held by caller)
The cgroup system is about to free @cgrp; the subsystem should free
its subsystem state object. By the time this method is called, @cgrp
is completely unused; @cgrp->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 allow_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
(cgroup_mutex held by caller)
Called prior to moving a task into a cgroup; if the subsystem
returns an error, this will abort the attach operation.  Used
to extend the permission checks - if all subsystems in a cgroup
return 0, the attach will be allowed to proceed, even if the
default permission check (root or same user) fails.
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.
void bind(struct cgroup *root)
(cgroup_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. Extended attribute usage
===========================
cgroup filesystem supports certain types of extended attributes in its
directories and files.  The current supported types are:
 - Trusted (XATTR_TRUSTED)
 - Security (XATTR_SECURITY)
Both require CAP_SYS_ADMIN capability to set.
Like in tmpfs, the extended attributes in cgroup filesystem are stored
using kernel memory and it's advised to keep the usage at minimum.  This
is the reason why user defined extended attributes are not supported, since
any user can do it and there's no limit in the value size.
The current known users for this feature are SELinux to limit cgroup usage
in containers and systemd for assorted meta data like main PID in a cgroup
(systemd creates a cgroup per service).
5. 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.