Re: [PATCH, v10 3/3] cgroups: introduce timer slack controller

On Fri, Oct 14, 2011 at 03:43:48PM -0700, Andrew Morton wrote:

On Tue, 11 Oct 2011 19:15:29 +0300
"Kirill A. Shutemov" [off-list ref] wrote:

quoted

Every task_struct has timer_slack_ns value. This value uses to round up
poll() and select() timeout values. This feature can be useful in
mobile environment where combined wakeups are desired.

Originally, prctl() was the only way to change timer slack value of
a process. So you was not able change timer slack value of another
process.

cgroup subsys "timer_slack" implements timer slack controller. It
provides a way to set minimal timer slack value for a group of tasks.
If a task belongs to a cgroup with minimal timer slack value higher than
task's value, cgroup's value will be applied.

Timer slack controller allows to implement setting timer slack value of
a process based on a policy. For example, you can create foreground and
background cgroups and move tasks between them based on system state.

I'm having trouble understanding the value of this feature.  Users can
presently control the timer-slack of a group of processes via
inherit-over-fork.

Perhaps there's a case for providing a way for process A to set process
B's slack.  And perhaps B's children.  That would be a simpler patch
and would have the considerable advantage that it doesn't require
cgroups.

So.... why should we merge this?

Putting a task to a cgroup isn't change task's timer slack, it may affect
"effective timer slack", if min timer slack for the cgroup > task's timer
slack.

Since we don't touch task's slack value we can drag tasks between cgroups
and always get the most relaxed slack value for a task without
saving/restoring it in userspace.

-- 
 Kirill A. Shutemov