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                                CGROUPS

                                -------

Written by Paul Menage  based on Documentation/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 

Modified by Christoph Lameter 

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 How do I use cgroups ?

2. Usage Examples and Syntax

  2.1 Basic Usage

  2.2 Attaching processes

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 hierachies 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/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

                  |              |

               (Profs)         (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 : Prof (50%), students (30%), system (20%)

       Network : WWW browsing (20%), Network File System (60%), others (20%)

                               / \

                       Prof (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 > /mnt///tasks

With only a single hierarchy, he now would potentially have to create

a separate cgroup for every browser launched and associate it with

approp 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, its just a

matter of :

       # echo pid > /mnt/network//tasks

       (after some time)

       # echo pid > /mnt/network//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

 - notify_on_release flag: run /sbin/cgroup_release_agent on exit?

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. Note that the current implementation uses a

linear search to locate an appropriate existing css_set, so isn't

very efficient. A future version will use a hash table for better

performance.

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 cont_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 ?

------------------------------------

*** notify_on_release is disabled in the current patch set. It will be

*** reactivated in a future patch in a less-intrusive manner

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 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) mkdir /dev/cgroup

 2) mount -t cgroup -ocpuset cpuset /dev/cgroup

 3) Create the new cgroup by doing mkdir's and write's (or echo's) in

    the /dev/cgroup virtual file system.

 4) Start a task that will be the "founding father" of the new job.

 5) Attach that task to the new cgroup by writing its pid to the

    /dev/cgroup tasks file for that cgroup.

 6) 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 cgroup cpuset -ocpuset /dev/cgroup

  cd /dev/cgroup

  mkdir Charlie

  cd Charlie

  /bin/echo 2-3 > cpus

  /bin/echo 1 > 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 will all available subsystems, type:

# mount -t cgroup xxx /dev/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.

To mount a cgroup hierarchy with just the cpuset and numtasks

subsystems, type:

# mount -t cgroup -o cpuset,numtasks hier1 /dev/cgroup

To change the set of subsystems bound to a mounted hierarchy, just

remount with different options:

# mount -o remount,cpuset,ns  /dev/cgroup

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 /dev/cgroup you can find a tree that corresponds to the

tree of the cgroups in the system. For instance, /dev/cgroup

is the cgroup that holds the whole system.

If you want to create a new cgroup under /dev/cgroup:

# cd /dev/cgroup

# mkdir my_cgroup

Now you want to do something with this cgroup.

# cd my_cgroup

In this directory you can find several files:

# ls

notify_on_release release_agent tasks

(plus whatever files are 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

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. Initialized by cgroup_register_subsys(); prior to this

  it should be initialized to -1

- hierarchy: an index indicating which hierarchy, if any, this

  subsystem is currently attached to. If this is -1, then the

  subsystem is not attached to any hierarchy, and all tasks should be

  considered to be members of the subsystem's top_cgroup. It should

  be initialized to -1.

- name: should be initialized to a unique subsystem name prior to

  calling cgroup_register_subsystem. Should be no longer than

  MAX_CGROUP_TYPE_NAMELEN

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(), and can

take/release the callback_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

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 *cont)

LL=cgroup_mutex

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 *cont)

LL=cgroup_mutex

The cgroup system is about to destroy the passed cgroup; the

subsystem should do any necessary cleanup

int can_attach(struct cgroup_subsys *ss, struct cgroup *cont,

               struct task_struct *task)

LL=cgroup_mutex

Called prior to moving a task into a cgroup; if the subsystem

returns an error, this will abort the attach operation.  If a NULL

task is passed, then a successful result indicates that *any*

unspecified task can be moved into the cgroup. 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.

void attach(struct cgroup_subsys *ss, struct cgroup *cont,

            struct cgroup *old_cont, struct task_struct *task)

LL=cgroup_mutex

Called after the task has been attached to the cgroup, to allow any

post-attachment activity that requires memory allocations or blocking.

void fork(struct cgroup_subsy *ss, struct task_struct *task)

LL=callback_mutex, maybe read_lock(tasklist_lock)

Called when a task is forked into a cgroup. Also called during

registration for all existing tasks.

void exit(struct cgroup_subsys *ss, struct task_struct *task)

LL=callback_mutex

Called during task exit

int populate(struct cgroup_subsys *ss, struct cgroup *cont)

LL=none

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_subsys *ss, struct cgroup *cont)

Called at the end of cgroup_clone() to do any paramater

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_subsys *ss, struct cgroup *root)

LL=callback_mutex

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.