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=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 4B24BC433E1 for ; Thu, 20 Aug 2020 13:35:32 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0C28222B3F for ; Thu, 20 Aug 2020 13:35:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0C28222B3F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ubuntu.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 6E33E8D0022; Thu, 20 Aug 2020 09:35:31 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 693868D0002; Thu, 20 Aug 2020 09:35:31 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5A9D48D0022; Thu, 20 Aug 2020 09:35:31 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0246.hostedemail.com [216.40.44.246]) by kanga.kvack.org (Postfix) with ESMTP id 457CE8D0002 for ; Thu, 20 Aug 2020 09:35:31 -0400 (EDT) Received: from smtpin19.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id D129D2C37 for ; Thu, 20 Aug 2020 13:35:30 +0000 (UTC) X-FDA: 77171044020.19.tax32_1517a3127031 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin19.hostedemail.com (Postfix) with ESMTP id 7BF611AD31E for ; Thu, 20 Aug 2020 13:35:30 +0000 (UTC) X-HE-Tag: tax32_1517a3127031 X-Filterd-Recvd-Size: 6172 Received: from youngberry.canonical.com (youngberry.canonical.com [91.189.89.112]) by imf41.hostedemail.com (Postfix) with ESMTP for ; Thu, 20 Aug 2020 13:35:28 +0000 (UTC) Received: from ip5f5af70b.dynamic.kabel-deutschland.de ([95.90.247.11] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1k8kiV-0006vu-SQ; Thu, 20 Aug 2020 13:34:55 +0000 Date: Thu, 20 Aug 2020 15:34:54 +0200 From: Christian Brauner To: Michal Hocko Cc: "Eric W. Biederman" , Suren Baghdasaryan , mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, esyr@redhat.com, christian@kellner.me, areber@redhat.com, shakeelb@google.com, cyphar@cyphar.com, oleg@redhat.com, adobriyan@gmail.com, akpm@linux-foundation.org, gladkov.alexey@gmail.com, walken@google.com, daniel.m.jordan@oracle.com, avagin@gmail.com, bernd.edlinger@hotmail.de, john.johansen@canonical.com, laoar.shao@gmail.com, timmurray@google.com, minchan@kernel.org, kernel-team@android.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary Message-ID: <20200820133454.ch24kewh42ax4ebl@wittgenstein> References: <20200820002053.1424000-1-surenb@google.com> <87zh6pxzq6.fsf@x220.int.ebiederm.org> <20200820124241.GJ5033@dhcp22.suse.cz> <87lfi9xz7y.fsf@x220.int.ebiederm.org> <87d03lxysr.fsf@x220.int.ebiederm.org> <20200820132631.GK5033@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200820132631.GK5033@dhcp22.suse.cz> X-Rspamd-Queue-Id: 7BF611AD31E X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Thu, Aug 20, 2020 at 03:26:31PM +0200, Michal Hocko wrote: > On Thu 20-08-20 07:54:44, Eric W. Biederman wrote: > > ebiederm@xmission.com (Eric W. Biederman) writes: > > > > 2> Michal Hocko writes: > > > > > >> On Thu 20-08-20 07:34:41, Eric W. Biederman wrote: > > >>> Suren Baghdasaryan writes: > > >>> > > >>> > Currently __set_oom_adj loops through all processes in the system to > > >>> > keep oom_score_adj and oom_score_adj_min in sync between processes > > >>> > sharing their mm. This is done for any task with more that one mm_users, > > >>> > which includes processes with multiple threads (sharing mm and signals). > > >>> > However for such processes the loop is unnecessary because their signal > > >>> > structure is shared as well. > > >>> > Android updates oom_score_adj whenever a tasks changes its role > > >>> > (background/foreground/...) or binds to/unbinds from a service, making > > >>> > it more/less important. Such operation can happen frequently. > > >>> > We noticed that updates to oom_score_adj became more expensive and after > > >>> > further investigation found out that the patch mentioned in "Fixes" > > >>> > introduced a regression. Using Pixel 4 with a typical Android workload, > > >>> > write time to oom_score_adj increased from ~3.57us to ~362us. Moreover > > >>> > this regression linearly depends on the number of multi-threaded > > >>> > processes running on the system. > > >>> > Mark the mm with a new MMF_PROC_SHARED flag bit when task is created with > > >>> > CLONE_VM and !CLONE_SIGHAND. Change __set_oom_adj to use MMF_PROC_SHARED > > >>> > instead of mm_users to decide whether oom_score_adj update should be > > >>> > synchronized between multiple processes. To prevent races between clone() > > >>> > and __set_oom_adj(), when oom_score_adj of the process being cloned might > > >>> > be modified from userspace, we use oom_adj_mutex. Its scope is changed to > > >>> > global and it is renamed into oom_adj_lock for naming consistency with > > >>> > oom_lock. Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely > > >>> > used the additional mutex lock in that path of the clone() syscall should > > >>> > not affect its overall performance. Clearing the MMF_PROC_SHARED flag > > >>> > (when the last process sharing the mm exits) is left out of this patch to > > >>> > keep it simple and because it is believed that this threading model is > > >>> > rare. Should there ever be a need for optimizing that case as well, it > > >>> > can be done by hooking into the exit path, likely following the > > >>> > mm_update_next_owner pattern. > > >>> > With the combination of CLONE_VM and !CLONE_SIGHAND being quite rare, the > > >>> > regression is gone after the change is applied. > > >>> > > >>> So I am confused. > > >>> > > >>> Is there any reason why we don't simply move signal->oom_score_adj to > > >>> mm->oom_score_adj and call it a day? > > >> > > >> Yes. Please read through 44a70adec910 ("mm, oom_adj: make sure processes > > >> sharing mm have same view of oom_score_adj") > > > > > > That explains why the scores are synchronized. > > > > > > It doesn't explain why we don't do the much simpler thing and move > > > oom_score_adj from signal_struct to mm_struct. Which is my question. > > > > > > Why not put the score where we need it to ensure that the oom score > > > is always synchronized? AKA on the mm_struct, not the signal_struct. > > > > Apologies. That 44a70adec910 does describe that some people have seen > > vfork users set oom_score. No details unfortunately. > > > > I will skip the part where posix calls this undefined behavior. It > > breaks userspace to change. > > > > It still seems like the code should be able to buffer oom_adj during > > vfork, and only move the value onto mm_struct during exec. > > If you can handle vfork by other means then I am all for it. There were > no patches in that regard proposed yet. Maybe it will turn out simpler > then the heavy lifting we have to do in the oom specific code. Eric's not wrong. I fiddled with this too this morning but since oom_score_adj is fiddled with in a bunch of places this seemed way more code churn then what's proposed here. Christian