linux-kernel - Re: [PATCH 1/2] mm, vmscan: prevent kswapd livelock due to pfmemalloc-throttled process being killed

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Date:	Fri, 19 Dec 2014 16:57:47 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/2] mm, vmscan: prevent kswapd livelock due to
 pfmemalloc-throttled process being killed

On Fri 19-12-14 14:01:55, Vlastimil Babka wrote:
> Charles Shirron and Paul Cassella from Cray Inc have reported kswapd stuck
> in a busy loop with nothing left to balance, but kswapd_try_to_sleep() failing
> to sleep. Their analysis found the cause to be a combination of several
> factors:
> 
> 1. A process is waiting in throttle_direct_reclaim() on pgdat->pfmemalloc_wait
> 
> 2. The process has been killed (by OOM in this case), but has not yet been
>    scheduled to remove itself from the waitqueue and die.

pfmemalloc_wait is used as wait_event and that one uses
autoremove_wake_function for wake ups so the task shouldn't stay on the
queue if it was woken up. Moreover pfmemalloc_wait sleeps are killable
by the OOM killer AFAICS.

$ git grep "wait_event.*pfmemalloc_wait"
mm/vmscan.c:
wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
mm/vmscan.c:    wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,))

So OOM killer would wake it up already and kswapd shouldn't see this
task on the waitqueue anymore.
-- 
Michal Hocko
SUSE Labs
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