From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f70.google.com (mail-ot1-f70.google.com [209.85.210.70]) by kanga.kvack.org (Postfix) with ESMTP id 9E5908E0002 for ; Wed, 16 Jan 2019 08:33:12 -0500 (EST) Received: by mail-ot1-f70.google.com with SMTP id q11so3191066otl.23 for ; Wed, 16 Jan 2019 05:33:12 -0800 (PST) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [202.181.97.72]) by mx.google.com with ESMTPS id z7si3144375oth.246.2019.01.16.05.33.10 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Jan 2019 05:33:11 -0800 (PST) Subject: Re: [PATCH] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj. References: <1547636121-9229-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20190116110937.GI24149@dhcp22.suse.cz> <88e10029-f3d9-5bb5-be46-a3547c54de28@I-love.SAKURA.ne.jp> <20190116121915.GJ24149@dhcp22.suse.cz> From: Tetsuo Handa Message-ID: <6118fa8a-7344-b4b2-36ce-d77d495fba69@i-love.sakura.ne.jp> Date: Wed, 16 Jan 2019 22:32:50 +0900 MIME-Version: 1.0 In-Reply-To: <20190116121915.GJ24149@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Andrew Morton , Johannes Weiner , David Rientjes , linux-mm@kvack.org, Yong-Taek Lee , Paul McKenney , Linus Torvalds On 2019/01/16 21:19, Michal Hocko wrote: > On Wed 16-01-19 20:30:25, Tetsuo Handa wrote: >> On 2019/01/16 20:09, Michal Hocko wrote: >>> On Wed 16-01-19 19:55:21, Tetsuo Handa wrote: >>>> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure >>>> processes sharing mm have same view of oom_score_adj") and commit >>>> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to >>>> close a race and reduce the latency at __set_oom_adj(), and reduces the >>>> warning at __oom_kill_process() in order to minimize the latency. >>>> >>>> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed >>>> to unmap the address space") introduced the worst case mentioned in >>>> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set, >>>> only administrators can trigger the worst case. >>>> >>>> Since 44a70adec910d692 did not take latency into account, we can hold RCU >>>> for minutes and trigger RCU stall warnings by calling printk() on many >>>> thousands of thread groups. Even without calling printk(), the latency is >>>> mentioned by Yong-Taek Lee [1]. And I noticed that 44a70adec910d692 is >>>> racy, and trying to fix the race will require a global lock which is too >>>> costly for rare events. >>>> >>>> If the worst case in 44a70adec910d692 happens, it is an administrator's >>>> request. Therefore, tolerate the worst case and speed up __set_oom_adj(). >>> >>> I really do not think we care about latency. I consider the overal API >>> sanity much more important. Besides that the original report you are >>> referring to was never exaplained/shown to represent real world usecase. >>> oom_score_adj is not really a an interface to be tweaked in hot paths. >> >> I do care about the latency. Holding RCU for more than 2 minutes is insane. > > Creating 8k threads could be considered insane as well. But more > seriously. I absolutely do not insist on holding a single RCU section > for the whole operation. But that doesn't really mean that we want to > revert these changes. for_each_process is by far not only called from > this path. Unlike check_hung_uninterruptible_tasks() where failing to resume after breaking RCU section is tolerable, failing to resume after breaking RCU section for __set_oom_adj() is not tolerable; it leaves the possibility of different oom_score_adj. Unless it is inevitable (e.g. SysRq-t), I think that calling printk() on each thread from RCU section is a poor choice. What if thousands of threads concurrently called __set_oom_adj() when each __set_oom_adj() call involves printk() on thousands of threads which can take more than 2 minutes? How long will it take to complete?