linux-kernel - Re: [PATCH 05/10] Make total

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Message-ID: <20111012125857.GE14968@somewhere>
Date:	Wed, 12 Oct 2011 14:59:00 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, paul@...lmenage.org,
	lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	jbottomley@...allels.com
Subject: Re: [PATCH 05/10] Make total_forks per-cgroup

On Wed, Oct 12, 2011 at 11:35:50AM +0400, Glauber Costa wrote:
> On 10/12/2011 03:45 AM, Frederic Weisbecker wrote:
> >On Wed, Oct 05, 2011 at 04:12:00PM +0400, Glauber Costa wrote:
> >>On 10/05/2011 01:05 PM, Peter Zijlstra wrote:
> >>>On Sun, 2011-10-02 at 23:21 +0400, Glauber Costa wrote:
> >>>>This patch counts the total number of forks per-cgroup.
> >>>>The information is propagated to the parent, so the total
> >>>>number of forks in the system, is the parent cgroup's one.
> >>>>
> >>>>To achieve that, total_forks is made per-cpu. There is no
> >>>>particular reason to do that, but by doing this, we are
> >>>>able to bundle it inside the cpustat structure already
> >>>>present.
> >>>
> >>>I think fweisbec is also doing something with forks and cgroups.
> >>
> >>I am all ears...
> >>
> >>Frederic, does it conflict with what you're doing ?
> >
> >I don't know if that really conflicts but I'm working
> >on a cgroup subsystem that is able to control the number
> >of tasks running in a subsystem.
> >
> >It consists in two new files added:
> >
> >* tasks.usage
> >* tasks.limit
> >
> >The subsystem rejects any new fork or migration into the
> >cgroup when tasks.usage>  tasks.limit
> >
> >So tasks.usage can inform you about the number of tasks
> >running into the cgroup. It's not strictly the number
> >of forks because it also counts the tasks that have been
> >attached to the cgroup.
> >
> >But something like a tasks.fork file could be implemented
> >in that subsystem as well.
> >
> >It depends on what you need.
> 
> So the specific piece I am working on, is to display /proc/stat
> information per-cgroup. One of the many fields it has, is
> total_forks.
> (it is actually just a small part of the series)
> So instead of tracking how many forks the system has in total, I'll
> track it per-cpucgroup.
> 
> So I don't think we conflict at all. At the very least, IIUC, you
> are planning to account and check *before* a fork happens, right?
> This particular stat is incremented after it already succeeded.

That doesn't make much difference since the accounting is cancelled
in case the fork is finally rejected.

But probably having a simple accouting like you do involves less
overhead than the whole task counter subsystem.

Is your counting propagated to the parents in a hierarchy?
For example if A is parent cgroup of B and C, does A account the
forks happening in B and C?
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