linux-kernel - Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before out_of

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Date:	Fri, 29 Jan 2016 16:32:50 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before
 out_of_memory

On Thu 28-01-16 18:51:10, Johannes Weiner wrote:
> On Thu, Jan 28, 2016 at 03:19:08PM -0800, David Rientjes wrote:
> > On Thu, 28 Jan 2016, Johannes Weiner wrote:
> > 
> > > The check has to happen while holding the OOM lock, otherwise we'll
> > > end up killing much more than necessary when there are many racing
> > > allocations.
> > > 
> > 
> > Right, we need to try with ALLOC_WMARK_HIGH after oom_lock has been 
> > acquired.
> > 
> > The situation is still somewhat fragile, however, but I think it's 
> > tangential to this patch series.  If the ALLOC_WMARK_HIGH allocation fails 
> > because an oom victim hasn't freed its memory yet, and then the TIF_MEMDIE 
> > thread isn't visible during the oom killer's tasklist scan because it has 
> > exited, we still end up killing more than we should.  The likelihood of 
> > this happening grows with the length of the tasklist.
> > 
> > Perhaps we should try testing watermarks after a victim has been selected 
> > and immediately before killing?  (Aside: we actually carry an internal 
> > patch to test mem_cgroup_margin() in the memcg oom path after selecting a 
> > victim because we have been hit with this before in the memcg path.)
> > 
> > I would think that retrying with ALLOC_WMARK_HIGH would be enough memory 
> > to deem that we aren't going to immediately reenter an oom condition so 
> > the deferred killing is a waste of time.
> > 
> > The downside is how sloppy this would be because it's blurring the line 
> > between oom killer and page allocator.  We'd need the oom killer to return 
> > the selected victim to the page allocator, try the allocation, and then 
> > call oom_kill_process() if necessary.
> 
> https://lkml.org/lkml/2015/3/25/40
> 
> We could have out_of_memory() wait until the number of outstanding OOM
> victims drops to 0. Then __alloc_pages_may_oom() doesn't relinquish
> the lock until its kill has been finalized:
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 914451a..4dc5b9d 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -892,7 +892,9 @@ bool out_of_memory(struct oom_control *oc)
>  		 * Give the killed process a good chance to exit before trying
>  		 * to allocate memory again.
>  		 */
> -		schedule_timeout_killable(1);
> +		if (!test_thread_flag(TIF_MEMDIE))
> +			wait_event_timeout(oom_victims_wait,
> +					   !atomic_read(&oom_victims), HZ);
>  	}
>  	return true;
>  }

Yes this makes sense to me
-- 
Michal Hocko
SUSE Labs