To: mhocko@kernel.org, akpm@linux-foundation.org
Cc: tj@kernel.org, clameter@sgi.com, arekm@maven.pl, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, js1304@gmail.com, cl@linux.com
Subject: Re: [PATCH] mm, vmstat: Allow WQ concurrency to discover memory reclaim doesn't make any progress
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
References: <1447936253-18134-1-git-send-email-mhocko@kernel.org>
	<20151124154448.ac124e62528db313279224ef@linux-foundation.org>
	<20151125110705.GC27283@dhcp22.suse.cz>
In-Reply-To: <20151125110705.GC27283@dhcp22.suse.cz>
Message-Id: <201511252054.DEC87052.MSLVJHFQtOFOFO@I-love.SAKURA.ne.jp>
Date: Wed, 25 Nov 2015 20:54:13 +0900
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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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