RFC a new optional function for task assignment to cgroup

Hi -

This is a patch of a new optional function for task assignment to cgroup, RFC.

== Purpose =================================================

To provide the function that leads a task, corresponding to the conditions specified beforehand, to a specific cgroup directory.

== Description =============================================

This patch provides the function that leads a task, corresponding to the conditions specified beforehand, to a specific cgroup directory.

Currently, this patch uses user-id as a condition to lead a task. On its I/F, specifies user-id of a task and a cgroup directory.

The task set to specified user-id will automatically lead to the cgroup directory. (it is attached to specific cgroup)

This function makes possible to attach a task to cgroup automatically when specific user logs in, also to attach a task of a service which is set to specific effective user-id to specific cgroup mechanically.

This function is just option, all the functions of cgroup are the same. Also the migration of a task between cgroup directories can do by rewriting pid of a control tasks file, including a task leading by this option.

It is able to enter two or more set of user-id and cgroup directory. Specified cgroup directory may be the same or that may not be. But it's not able to enter same user-id to plural cgroup directories to lead.

== Interface ===============================================

/lead_option - control file of this option

[example for reading a configuration]

       # cat /cgroup/lead_option

uid:202 leadto:/cpuset/bar_cg
uid:201 leadto:/cpuset/foo_cg

* nothing appears before assignment.

[example for adding an entry]
- To lead a task(uid 201) to /cgroup/foo_cg

       # echo uid:201 leadto:/cpuset/foo_cg > /cpuset/lead_option

* set a uid of task and cgroup dirctory to lead.
* Remake an entry uid to cgroup directory if set uid already exists.

[example for delete an entry]
- To delete an entry of uid

       # echo uid:201 > /cpuset/lead_option

* To delete a registration, omit "leadto:" token.

== Operation example (chronological order) ==================

The follows is an example of the operation.

# ############################
# # Various confirmation before testing
# ############################

# id
uid=0(root) gid=0(root) groups=0(root)
# df /cpuset
Filesystem           1K-blocks      Used Available Use% Mounted on
none                         0         0         0   -  /cpuset
# more /proc/self/cgroup
cpuset:/
# id foo
uid=201(foo) gid=100(users) groups=100(users)
# id bar
uid=202(foo) gid=100(users) groups=100(users)
# ############################
# # Add an entry of user foo,bar
# ############################
# echo uid:201 leadto:/cpuset/foo_cg > /cpuset/lead_option
# echo uid:202 leadto:/cpuset/bar_cg > /cpuset/lead_option
# more /cpuset/lead_option
uid:202        leadto:/cpuset/bar_cg
uid:201        leadto:/cpuset/foo_cg
# ############################
# # Confirmation of the assignment of user foo,bar - (1)
# ############################
# su - foo
$ more /proc/$$/cgroup
cpuset:/foo_cg
$ more /proc/self/cgroup
cpuset:/foo_cg
$ su bar --command "more /proc/self/cgroup"
cpuset:/bar_cg
$ exit
# ############################
# # Delete an entry of user foo,bar
# ############################
# echo uid:201 > /cpuset/lead_option
# echo uid:202 > /cpuset/lead_option
# more /cpuset/lead_option
# ############################
# # Confirmation of the assignment of user foo,bar - (2)
# ############################
# su - foo
$ more /proc/$$/cgroup
cpuset:/
$ su bar --command "more /proc/self/cgroup"
cpuset:/
$

Thanks,
- Kazunaga Ikeno.