From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id 78DD86B0038 for ; Fri, 1 Dec 2017 10:17:17 -0500 (EST) Received: by mail-wr0-f199.google.com with SMTP id r20so5869134wrg.23 for ; Fri, 01 Dec 2017 07:17:17 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 64si5341963edm.364.2017.12.01.07.17.15 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 01 Dec 2017 07:17:16 -0800 (PST) Date: Fri, 1 Dec 2017 16:17:15 +0100 From: Michal Hocko Subject: Re: [PATCH 1/3] mm,oom: Move last second allocation to inside the OOM killer. Message-ID: <20171201151715.yiep5wkmxmp77nxn@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171201145638.GA10280@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Tetsuo Handa , akpm@linux-foundation.org, linux-mm@kvack.org, Andrea Arcangeli 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. Anyway, if the delay is immaterial than the existing last-retry is even more pointless because it is executed right _after_ we gave up reclaim retries. Compare that to the select_bad_process time window. And really, that can take quite a lot of time. Especially in weird priority inversion situations. > > 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. > > > Nacked-by: Johannes Weiner -- Michal Hocko SUSE Labs -- 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