linux-kernel - Re: [PATCH] mm, vmstat: Allow WQ concurrency to discover memory reclaim doesn't make any progress

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Message-Id: <201511252054.DEC87052.MSLVJHFQtOFOFO@I-love.SAKURA.ne.jp>
Date:	Wed, 25 Nov 2015 20:54:13 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...nel.org, akpm@...ux-foundation.org
Cc:	tj@...nel.org, clameter@....com, arekm@...en.pl,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, js1304@...il.com,
	cl@...ux.com
Subject: Re: [PATCH] mm, vmstat: Allow WQ concurrency to discover memory reclaim doesn't make any progress

Michal Hocko wrote:
> Anyway I think that the issue is not solely theoretical. WQ_MEM_RECLAIM
> is simply not working if the allocation path doesn't sleep currently and
> my understanding of what Tejun claims [2] is that that reimplementing WQ
> concurrency would be too intrusive and lacks sufficient justification
> because other kernel paths do sleep. This patch tries to reduce the
> sleep only to worker threads which should not cause any problems to
> regular tasks.

I received many unexplained hangup/reboot reports from customers when I was
working at support center. But we can't answer whether real people ever hit
this problem because we have no watchdog for memory allocation stalls.
I want one like http://lkml.kernel.org/r/201511250024.AAE78692.QVOtFFOSFOMLJH@I-love.SAKURA.ne.jp
as I wrote off-list ( "mm,oom: The reason why I continue proposing timeout
based approach." ). It will help with judging when we tackle TIF_MEMDIE
livelock problem.

What I can say is that RHEL6 (a 2.6.32-based distro) backported the
wait_iff_congested() changes and therefore people might really hit
this problem.
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