linux-kernel - Re: [patch 2/2] memcg: do not sleep on OOM waitqueue with full charge context

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Date:	Wed, 12 Jun 2013 13:49:47 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Johannes Weiner <hannes@...xchg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] memcg: do not sleep on OOM waitqueue with full charge
 context

On Wed, 12 Jun 2013, Michal Hocko wrote:

> The patch is a big improvement with a minimum code overhead. Blocking
> any task which sits on top of an unpredictable amount of locks is just
> broken. So regardless how many users are affected we should merge it and
> backport to stable trees. The problem is there since ever. We seem to
> be surprisingly lucky to not hit this more often.
> 

Right now it appears that that number of users is 0 and we're talking 
about a problem that was reported in 3.2 that was released a year and a 
half ago.  The rules of inclusion in stable also prohibit such a change 
from being backported, specifically "It must fix a real bug that bothers 
people (not a, "This could be a problem..." type thing)".

We have deployed memcg on a very large number of machines and I can run a 
query over all software watchdog timeouts that have occurred by 
deadlocking on i_mutex during memcg oom.  It returns 0 results.

> I am not quite sure I understand your reservation about the patch to be
> honest. Andrew still hasn't merged this one although 1/2 is in.

Perhaps he is as unconvinced?  The patch adds 100 lines of code, including 
fields to task_struct for memcg, for a problem that nobody can reproduce.  
My question still stands: can anybody, even with an instrumented kernel to 
make it more probable, reproduce the issue this is addressing?
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