From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id D5A656B0038 for ; Fri, 1 Dec 2017 10:57:20 -0500 (EST) Received: by mail-wr0-f200.google.com with SMTP id t92so6090005wrc.13 for ; Fri, 01 Dec 2017 07:57:20 -0800 (PST) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id v12si5461429edk.355.2017.12.01.07.57.19 for (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 01 Dec 2017 07:57:19 -0800 (PST) Date: Fri, 1 Dec 2017 15:57:11 +0000 From: Johannes Weiner Subject: Re: [PATCH 1/3] mm,oom: Move last second allocation to inside the OOM killer. Message-ID: <20171201155711.GA11057@cmpxchg.org> References: <1511607169-5084-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20171201143317.GC8097@cmpxchg.org> <20171201144634.sc4cn6hyyt6zawms@dhcp22.suse.cz> <20171201145638.GA10280@cmpxchg.org> <20171201151715.yiep5wkmxmp77nxn@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171201151715.yiep5wkmxmp77nxn@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Tetsuo Handa , akpm@linux-foundation.org, linux-mm@kvack.org, Andrea Arcangeli On Fri, Dec 01, 2017 at 04:17:15PM +0100, Michal Hocko wrote: > On Fri 01-12-17 14:56:38, Johannes Weiner wrote: > > On Fri, Dec 01, 2017 at 03:46:34PM +0100, Michal Hocko wrote: > > > On Fri 01-12-17 14:33:17, Johannes Weiner wrote: > > > > On Sat, Nov 25, 2017 at 07:52:47PM +0900, Tetsuo Handa wrote: > > > > > @@ -1068,6 +1071,17 @@ bool out_of_memory(struct oom_control *oc) > > > > > } > > > > > > > > > > select_bad_process(oc); > > > > > + /* > > > > > + * Try really last second allocation attempt after we selected an OOM > > > > > + * victim, for somebody might have managed to free memory while we were > > > > > + * selecting an OOM victim which can take quite some time. > > > > > > > > Somebody might free some memory right after this attempt fails. OOM > > > > can always be a temporary state that resolves on its own. > > > > > > > > What keeps us from declaring OOM prematurely is the fact that we > > > > already scanned the entire LRU list without success, not last second > > > > or last-last second, or REALLY last-last-last-second allocations. > > > > > > You are right that this is inherently racy. The point here is, however, > > > that the race window between the last check and the kill can be _huge_! > > > > My point is that it's irrelevant. We already sampled the entire LRU > > list; compared to that, the delay before the kill is immaterial. > > Well, I would disagree. I have seen OOM reports with a free memory. > Closer debugging shown that an existing process was on the way out and > the oom victim selection took way too long and fired after a large > process manage. There were different hacks^Wheuristics to cover those > cases but they turned out to just cause different corner cases. Moving > the existing last moment allocation after a potentially very time > consuming action is relatively cheap and safe measure to cover those > cases without any negative side effects I can think of. An existing process can exit right after you pull the trigger. How big is *that* race window? By this logic you could add a sleep(5) before the last-second allocation because it would increase the likelihood of somebody else exiting voluntarily. This patch is making the time it takes to select a victim an integral part of OOM semantics. Think about it: if somebody later speeds up the OOM selection process, they shrink the window in which somebody could volunteer memory for the last-second allocation. By optimizing that code, you're probabilistically increasing the rate of OOM kills. A guaranteed 5 second window would in fact be better behavior. This is bananas. I'm sticking with my nak. > > > Another argument is that the allocator itself could have changed its > > > allocation capabilities - e.g. become the OOM victim itself since the > > > last time it the allocator could have reflected that fact. > > > > Can you outline how this would happen exactly? > > http://lkml.kernel.org/r/20171101135855.bqg2kuj6ao2cicqi@dhcp22.suse.cz > > As I try to explain the workload is really pathological but this (resp. > the follow up based on this patch) as a workaround is moderately ugly > wrt. it actually can help. That's not a real case which matters. It's really unfortunate how much churn the OOM killer has been seeing based on artificial stress tests. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org