From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9863CC3A5A6 for ; Sun, 22 Sep 2019 06:20:47 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 386D72070C for ; Sun, 22 Sep 2019 06:20:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 386D72070C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 9AC336B0007; Sun, 22 Sep 2019 02:20:45 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 95CEE6B0008; Sun, 22 Sep 2019 02:20:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8726D6B000A; Sun, 22 Sep 2019 02:20:45 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0151.hostedemail.com [216.40.44.151]) by kanga.kvack.org (Postfix) with ESMTP id 65FC26B0007 for ; Sun, 22 Sep 2019 02:20:45 -0400 (EDT) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with SMTP id E9D03824376E for ; Sun, 22 Sep 2019 06:20:44 +0000 (UTC) X-FDA: 75961558008.29.knot12_238de18778515 X-HE-Tag: knot12_238de18778515 X-Filterd-Recvd-Size: 4615 Received: from mx1.suse.de (mx2.suse.de [195.135.220.15]) by imf20.hostedemail.com (Postfix) with ESMTP for ; Sun, 22 Sep 2019 06:20:44 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 7B05FAC3E; Sun, 22 Sep 2019 06:20:42 +0000 (UTC) Date: Sun, 22 Sep 2019 08:20:40 +0200 From: Michal Hocko To: Tetsuo Handa Cc: Andrew Morton , Roman Gushchin , ShakeelButt , linux-mm@kvack.org, Linus Torvalds Subject: Re: [PATCH] mm, oom: avoid printk() iteration under RCU Message-ID: <20190922062040.GA18814@dhcp22.suse.cz> References: <1563360901-8277-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20190718083014.GB30461@dhcp22.suse.cz> <7478e014-e5ce-504c-34b6-f2f9da952600@i-love.sakura.ne.jp> <20190718140224.GC30461@dhcp22.suse.cz> <4291b98c-a961-5648-34d1-6f9347e65782@i-love.sakura.ne.jp> <20190920171042.8d970f9fc6f360de9b20ebbe@linux-foundation.org> <20190921203043.GA3382@dhcp22.suse.cz> <11c42f07-74d1-d4be-99bc-ca50d7c0ec71@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <11c42f07-74d1-d4be-99bc-ca50d7c0ec71@i-love.sakura.ne.jp> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun 22-09-19 08:47:31, Tetsuo Handa wrote: > On 2019/09/22 5:30, Michal Hocko wrote: > > On Fri 20-09-19 17:10:42, Andrew Morton wrote: > >> On Sat, 20 Jul 2019 20:29:23 +0900 Tetsuo Handa wrote: > >> > >>>> > >>>>> ) under RCU and this patch is one of them (except that we can't remove > >>>>> printk() for dump_tasks() case). > >>>> > >>>> No, this one adds a complexity for something that is not clearly a huge > >>>> win or the win is not explained properly. > >>>> > >>> > >>> The win is already explained properly by the past commits. Avoiding RCU stalls > >>> (even without slow consoles) is a clear win. The duration of RCU stall avoided > >>> by this patch is roughly the same with commit b2b469939e934587. > >>> > >>> We haven't succeeded making printk() asynchronous (and potentially we won't > >>> succeed making printk() asynchronous because we need synchronous printk() > >>> when something critical is undergoing outside of out_of_memory()). Thus, > >>> bringing printk() to outside of RCU section is a clear win we can make for now. > >> > >> It's actually not a complex patch and moving all that printing outside > >> the rcu section makes sense. So I'll sit on the patch for a few more > >> days but am inclined to send it upstream. > > > > Look, I am quite tired of arguing about this and other changes following > > the similar pattern. In short a problematic code is shuffled around and > > pretend to solve some problem. In this particular case it is a RCU stall > > which in itself is not a fatal condition. Sure it sucks and the primary > > reason is that printk can take way too long. This is something that is > > currently a WIP to be address. What is more important though there is no > > sign of any _real world_ workload that would require a quick workaround > > to justify a hacky stop gap solution. > > > > So again, why do we want to add more code for something which is not > > clear to be a real life problem and that will add a maintenance burden > > for future? > > > > Enqueueing zillion printk() lines from dump_tasks() will overflow printk > buffer (i.e. leads to lost messages) if OOM killer messages were printed > asynchronously. I don't think that making printk() asynchronous will solve > this problem. I repeat again; there is no better solution than "printk() > users are careful not to exhaust the printk buffer". This patch is the > first step towards avoiding thoughtless printk(). Irrelevant because this patch doesn't reduce the amount of output. > Delay from dump_tasks() not only affects a thread holding oom_lock but also > other threads which are directly doing concurrent allocation requests or > indirectly waiting for the thread holding oom_lock. Your "it is a RCU stall > which in itself is not a fatal condition" is underestimating the _real world_ > problems (e.g. "delay can trigger watchdog timeout and cause the system to > reboot even if the administrator does not want the system to reboot"). Please back your claims by real world examples. -- Michal Hocko SUSE Labs