linux-kernel - Re: [PATCH, v3 2/2] cgroups: introduce timer slack subsystem

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Date:	Fri, 4 Feb 2011 15:34:39 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Matt Helsley <matthltc@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH, v3 2/2] cgroups: introduce timer slack subsystem

On Thu, Feb 03, 2011 at 11:57:43AM -0800, Jacob Pan wrote:
> On Thu, 3 Feb 2011 10:12:51 -0800
> Paul Menage <menage@...gle.com> wrote:
> 
> > On Thu, Feb 3, 2011 at 9:51 AM, Jacob Pan
> > <jacob.jun.pan@...ux.intel.com> wrote:
> > >
> > > I think this logic defeats the purpose of having timer_slack
> > > subsystem in the first place. IMHO, the original intention was to
> > > have grouping effect of tasks in the cgroup.
> > 
> > You can get the semantics you want by just setting min_slack_ns =
> > max_slack_ns.
> > 
> true. it will just make set fail when min = max. it is awkward and
> counter intuitive when you want to change the group timer_slack. you
> will have to move both min and max to clamp the value, where set
> function can not be used.

Interface is very similar to /sys/devices/system/cpu/cpuX/cpufreq.
I think it's sane. If you want some extention, you can do it with
userspace helper.

> In addition, when a parent changes min = max, I don't see the current
> code enforce new settings on the children. Am i missing something?

I've missed it. I'll fix.

> In my use case, i want to put some apps into a managed group where
> relaxed slack value is used, but when time comes to move the app out of
> that cgroup, we would like to resore the original timer slack. I see
> having a current value per cgroup can be useful if we let timer code
> pick whether to use task slack value or the cgroup slack value.
> Or we have to cache the old value per task

What's mean "original timer slack" if you are free to move a task
between a lot of cgroups and process itself free to change it anytime?

-- 
 Kirill A. Shutemov
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