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

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Date:	Thu, 28 Jan 2016 18:51:10 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	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>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before
 out_of_memory

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;
 }