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

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Date:	Sat, 15 Oct 2011 02:34:45 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	containers@...ts.linux-foundation.org,
	jacob.jun.pan@...ux.intel.com,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Matt Helsley <matthltc@...ibm.com>,
	linux-api@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
	lennart@...ttering.net, harald@...hat.com, david@...ar.dk,
	greg@...ah.com
Subject: 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" <kirill@...temov.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
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