linux-kernel - Re: [RFC 1/4] memcg: provide root figures from system totals

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Date:	Tue, 2 Oct 2012 11:34:33 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, devel@...nvz.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC 1/4] memcg: provide root figures from system totals

On Tue 02-10-12 13:15:43, Glauber Costa wrote:
> On 10/01/2012 09:00 PM, Michal Hocko wrote:
> > On Tue 25-09-12 12:52:50, Glauber Costa wrote:
> >> > For the root memcg, there is no need to rely on the res_counters.
> > This is true only if there are no children groups but once there is at
> > least one we have to move global statistics into root res_counter and
> > start using it since then. This is a tricky part because it has to be
> > done atomically so that we do not miss anything.
> > 
> Why can't we shortcut it all the time?

Because it has its own tasks and we are still not at use_hierarchy := 1

> It makes a lot of sense to use the root cgroup as the sum of everything,
> IOW, global counters. Otherwise you are left in a situation where you
> had global statistics, and all of a sudden, when a group is created, you
> start having just a subset of that, excluding the tasks in root.

Yes because if there are no other tasks then, well, global == root. Once
you have more groups (with tasks of course) then it depends on our
favorite use_hierarchy buddy.

> If we can always assume root will have the sum of *all* tasks, including
> the ones in root, we should never need to rely on root res_counters.

but we are not there yet.

-- 
Michal Hocko
SUSE Labs
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