From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758513Ab3K1DN0 (ORCPT ); Wed, 27 Nov 2013 22:13:26 -0500 Received: from zene.cmpxchg.org ([85.214.230.12]:42575 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754667Ab3K1DNW (ORCPT ); Wed, 27 Nov 2013 22:13:22 -0500 Date: Wed, 27 Nov 2013 22:13:13 -0500 From: Johannes Weiner To: David Rientjes Cc: Andrew Morton , stable@kernel.org, Michal Hocko , azurit@pobox.sk, mm-commits@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [merged] mm-memcg-handle-non-error-oom-situations-more-gracefully.patch removed from -mm tree Message-ID: <20131128031313.GK3556@cmpxchg.org> References: <526028bd.k5qPj2+MDOK1o6ii%akpm@linux-foundation.org> <20131127233353.GH3556@cmpxchg.org> <20131128021809.GI3556@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 27, 2013 at 06:38:31PM -0800, David Rientjes wrote: > On Wed, 27 Nov 2013, Johannes Weiner wrote: > > > > The task that is bypassing the memcg charge to the root memcg may not be > > > the process that is chosen by the oom killer, and it's possible the amount > > > of memory freed by killing the victim is less than the amount of memory > > > bypassed. > > > > That's true, though unlikely. > > > > Well, the "goto bypass" allows it and it's trivial to cause by > manipulating /proc/pid/oom_score_adj values to prefer processes with very > little rss. It will just continue looping and killing processes as they > are forked and never cause the memcg to free memory below its limit. At > least the "goto nomem" allows us to free some memory instead of leaking to > the root memcg. Yes, that's the better way of doing it, I'll send the patch. Thanks. > > > Were you targeting these to 3.13 instead? If so, it would have already > > > appeared in 3.13-rc1 anyway. Is it still a work in progress? > > > > I don't know how to answer this question. > > > > It appears as though this work is being developed in Linus's tree rather > than -mm, so I'm asking if we should consider backing some of it out for > 3.14 instead. The changes fix a deadlock problem. Are they creating problems that are worse than deadlocks, that would justify their revert? > > > Should we be checking mem_cgroup_margin() here to ensure > > > task_in_memcg_oom() is still accurate and we haven't raced by freeing > > > memory? > > > > We would have invoked the OOM killer long before this point prior to > > my patches. There is a line we draw and from that point on we start > > killing things. I tried to explain multiple times now that there is > > no race-free OOM killing and I'm tired of it. Convince me otherwise > > or stop repeating this non-sense. > > > > In our internal kernel we call mem_cgroup_margin() with the order of the > charge immediately prior to sending the SIGKILL to see if it's still > needed even after selecting the victim. It makes the race smaller. > > It's obvious that after the SIGKILL is sent, either from the kernel or > from userspace, that memory might subsequently be freed or another process > might exit before the process killed could even wake up. There's nothing > we can do about that since we don't have psychic abilities. I think we > should try to reduce the chance for unnecessary oom killing as much as > possible, however. Since we can't physically draw a perfect line, we should strive for a reasonable and intuitive line. After that it's rapidly diminishing returns. Killing something after that much reclaim effort without success is a completely reasonable and intuitive line to draw. It's also the line that has been drawn a long time ago and we're not breaking this because of a micro optmimization.