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[202.181.97.72]) by mx.google.com with ESMTPS id a7si5610009pfc.54.2019.07.20.05.17.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 20 Jul 2019 05:17:51 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of penguin-kernel@i-love.sakura.ne.jp designates 202.181.97.72 as permitted sender) client-ip=202.181.97.72; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of penguin-kernel@i-love.sakura.ne.jp designates 202.181.97.72 as permitted sender) smtp.mailfrom=penguin-kernel@i-love.sakura.ne.jp Received: from fsav303.sakura.ne.jp (fsav303.sakura.ne.jp [153.120.85.134]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id x6KBTR7g079218; Sat, 20 Jul 2019 20:29:27 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav303.sakura.ne.jp (F-Secure/fsigk_smtp/530/fsav303.sakura.ne.jp); Sat, 20 Jul 2019 20:29:27 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/530/fsav303.sakura.ne.jp) Received: from [192.168.1.8] (softbank126012062002.bbtec.net [126.12.62.2]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id x6KBTNdY079107 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NO); Sat, 20 Jul 2019 20:29:27 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: [PATCH] mm, oom: avoid printk() iteration under RCU To: Michal Hocko Cc: Roman Gushchin , Shakeel Butt , linux-mm@kvack.org, Andrew Morton , Linus Torvalds 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> From: Tetsuo Handa Message-ID: <4291b98c-a961-5648-34d1-6f9347e65782@i-love.sakura.ne.jp> Date: Sat, 20 Jul 2019 20:29:23 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20190718140224.GC30461@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 2019/07/18 23:02, Michal Hocko wrote: > On Thu 18-07-19 22:50:14, Tetsuo Handa wrote: >> On 2019/07/18 17:30, Michal Hocko wrote: >>> On Wed 17-07-19 19:55:01, Tetsuo Handa wrote: >>>> Currently dump_tasks() might call printk() for many thousands times under >>>> RCU, which might take many minutes for slow consoles. >>> >>> Is is even wise to enable dumping tasks on systems with thousands of >>> tasks and slow consoles? I mean you still have to call printk that is >>> slow that many times. So why do we actually care? Because of RCU stall >>> warnings? >>> >> >> That's a stupid question. WE DO CARE. > > -ENOARGUMENT > >> We are making efforts for avoid calling printk() on each thread group (e.g. >> >> commit 0c1b2d783cf34324 ("mm/oom_kill: remove the wrong fatal_signal_pending() check in oom_kill_process()") > > removes fatal_signal_pending rather than focusing on printk No. Its focus is to suppress printk(), for it fixes fatal_signal_pending() test introduced by commit 840807a8f40bb25a ("mm/oom_kill.c: suppress unnecessary "sharing same memory" message"). > >> commit b2b469939e934587 ("proc, oom: do not report alien mms when setting oom_score_adj")) > > removes a printk of a dubious value. No. Its focus is to remove printk(), for that printk() allows the system to TASK_UNINTERRUPTIBLE stall for 44 days (even without slow consoles) in addition to RCU stall for 2 minutes. > >> ) 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.