linux-kernel - Re: [PATCH v2] memcg: oom: fix totalpages calculation for memory.swappiness==0

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Message-ID: <20121108083454.GA30792@dhcp22.suse.cz>
Date:	Thu, 8 Nov 2012 09:35:55 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] memcg: oom: fix totalpages calculation for
 memory.swappiness==0

On Wed 07-11-12 14:53:40, Andrew Morton wrote:
> On Wed, 7 Nov 2012 23:46:40 +0100
> Michal Hocko <mhocko@...e.cz> wrote:
> 
> > > Realistically, is anyone likely to hurt from this?
> > 
> > The primary motivation for the fix was a real report by a customer.
> 
> Describe it please and I'll copy it to the changelog.

The original issue (a wrong tasks get killed in a small group and memcg
swappiness=0) has been reported on top of our 3.0 based kernel (with
fe35004f backported). I have tried to replicate it by the test case
mentioned https://lkml.org/lkml/2012/10/10/223.

As David correctly pointed out (https://lkml.org/lkml/2012/10/10/418)
the significant role played the fact that all the processes in the group
have CAP_SYS_ADMIN but oom_score_adj has the similar effect. 
Say there is 2G of swap space which is 524288 pages. If you add
CAP_SYS_ADMIN bonus then you have -15728 score for the bias. This means
that all tasks with less than 60M get the minimum score and it is tasks
ordering which determines who gets killed as a result.

To summarize it. Users of small groups (relatively to the swap size)
with CAP_SYS_ADMIN tasks resp. oom_score_adj are affected the most
others might see an unexpected oom_badness calculation.
Whether this is a workload which is representative, I don't know but
I think that it is worth fixing and pushing to stable as well.
-- 
Michal Hocko
SUSE Labs
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