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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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" <kirill@shutemov.name> wrote:
> 
> > 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