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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 CDC14C433E1 for ; Thu, 20 Aug 2020 12:43:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B1BAA22B4E for ; Thu, 20 Aug 2020 12:43:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730324AbgHTMnY (ORCPT ); Thu, 20 Aug 2020 08:43:24 -0400 Received: from mx2.suse.de ([195.135.220.15]:38374 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729117AbgHTMmo (ORCPT ); Thu, 20 Aug 2020 08:42:44 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id CB459AF0F; Thu, 20 Aug 2020 12:43:09 +0000 (UTC) Date: Thu, 20 Aug 2020 14:42:41 +0200 From: Michal Hocko To: "Eric W. Biederman" Cc: Suren Baghdasaryan , christian.brauner@ubuntu.com, 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: <20200820124241.GJ5033@dhcp22.suse.cz> References: <20200820002053.1424000-1-surenb@google.com> <87zh6pxzq6.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87zh6pxzq6.fsf@x220.int.ebiederm.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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") -- Michal Hocko SUSE Labs