Re: [PATCH] memcg, oom: be careful about races when warning about no reclaimable task

From: Michal Hocko <mhocko@kernel.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	linux-mm@kvack.org, Greg Thelen <gthelen@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] memcg, oom: be careful about races when warning about no reclaimable task
Date: Tue, 7 Aug 2018 13:04:05 +0200	[thread overview]
Message-ID: <20180807110405.GW10003@dhcp22.suse.cz> (raw)
In-Reply-To: <863d73ce-fae9-c117-e361-12c415c787de@i-love.sakura.ne.jp>

On Tue 07-08-18 19:15:11, Tetsuo Handa wrote:
[...]
> Of course, if the hard limit is 0, all processes will be killed after all. But
> Michal is ignoring the fact that if the hard limit were not 0, there is a chance
> of saving next process from needlessly killed if we waited until "mm of PID=23766
> completed __mmput()" or "mm of PID=23766 failed to complete __mmput() within
> reasonable period". 

This is a completely different issue IMHO. I haven't seen reports about
overly eager memcg oom killing so far.

> We can make efforts not to return false at
> 
> 	/*
> 	 * This task has already been drained by the oom reaper so there are
> 	 * only small chances it will free some more
> 	 */
> 	if (test_bit(MMF_OOM_SKIP, &mm->flags))
> 		return false;
> 
> (I admit that ignoring MMF_OOM_SKIP for once might not be sufficient for memcg
> case), and we can use feedback based backoff like
> "[PATCH 4/4] mm, oom: Fix unnecessary killing of additional processes." *UNTIL*
> we come to the point where the OOM reaper can always reclaim all memory.

The code is quite tricky and I am really reluctant to make it even more
so without seeing this is really hurting real users with real workloads.
-- 
Michal Hocko
SUSE Labs