From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f199.google.com (mail-pf1-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id 5CE4F6B7D5A for ; Fri, 7 Sep 2018 04:27:49 -0400 (EDT) Received: by mail-pf1-f199.google.com with SMTP id x19-v6so7235909pfh.15 for ; Fri, 07 Sep 2018 01:27:49 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id c17-v6si7231976pgp.299.2018.09.07.01.27.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Sep 2018 01:27:47 -0700 (PDT) Date: Fri, 7 Sep 2018 10:27:45 +0200 From: Michal Hocko Subject: Re: [PATCH] mm, oom: Introduce time limit for dump_tasks duration. Message-ID: <20180907082745.GB19621@dhcp22.suse.cz> References: <201809060553.w865rmpj036017@www262.sakura.ne.jp> <58aa0543-86d0-b2ad-7fb9-9bed7c6a1f6c@i-love.sakura.ne.jp> <20180906112306.GO14951@dhcp22.suse.cz> <1611e45d-235e-67e9-26e3-d0228255fa2f@i-love.sakura.ne.jp> <20180906115320.GS14951@dhcp22.suse.cz> <7f50772a-f2ef-d16e-4d09-7f34f4bf9227@i-love.sakura.ne.jp> <20180906143905.GC14951@dhcp22.suse.cz> <32c58019-5e2d-b3a1-a6ad-ea374ccd8b60@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <32c58019-5e2d-b3a1-a6ad-ea374ccd8b60@i-love.sakura.ne.jp> Sender: owner-linux-mm@kvack.org List-ID: To: Tetsuo Handa Cc: Dmitry Vyukov , Andrew Morton , David Rientjes , syzbot , 'Dmitry Vyukov' via syzkaller-upstream-moderation , linux-mm On Fri 07-09-18 05:58:06, Tetsuo Handa wrote: > On 2018/09/06 23:39, Michal Hocko wrote: > >>>> I know /proc/sys/vm/oom_dump_tasks . Showing some entries while not always > >>>> printing all entries might be helpful. > >>> > >>> Not really. It could be more confusing than helpful. The main purpose of > >>> the listing is to double check the list to understand the oom victim > >>> selection. If you have a partial list you simply cannot do that. > >> > >> It serves as a safeguard for avoiding RCU stall warnings. > >> > >>> > >>> If the iteration takes too long and I can imagine it does with zillions > >>> of tasks then the proper way around it is either release the lock > >>> periodically after N tasks is processed or outright skip the whole thing > >>> if there are too many tasks. The first option is obviously tricky to > >>> prevent from duplicate entries or other artifacts. > >>> > >> > >> Can we add rcu_lock_break() like check_hung_uninterruptible_tasks() does? > > > > This would be a better variant of your timeout based approach. But it > > can still produce an incomplete task list so it still consumes a lot of > > resources to print a long list of tasks potentially while that list is not > > useful for any evaluation. Maybe that is good enough. I don't know. I > > would generally recommend to disable the whole thing with workloads with > > many tasks though. > > > > The "safeguard" is useful when there are _unexpectedly_ many tasks (like > syzbot in this case). Why not to allow those who want to avoid lockup to > avoid lockup rather than forcing them to disable the whole thing? So you get an rcu lockup splat and what? Unless you have panic_on_rcu_stall then this should be recoverable thing (assuming we cannot really livelock as described by Dmitry). -- Michal Hocko SUSE Labs