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

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Date:   Tue, 7 Aug 2018 13:04:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        Greg Thelen <gthelen@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] memcg, oom: be careful about races when warning about no
 reclaimable task

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