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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 2110DC433DF for ; Thu, 20 Aug 2020 11:39:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 04C72204EA for ; Thu, 20 Aug 2020 11:39:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729133AbgHTLjP (ORCPT ); Thu, 20 Aug 2020 07:39:15 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:40639 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730558AbgHTLat (ORCPT ); Thu, 20 Aug 2020 07:30:49 -0400 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 1k8im0-0000Eh-8L; Thu, 20 Aug 2020 11:30:24 +0000 Date: Thu, 20 Aug 2020 13:30:23 +0200 From: Christian Brauner To: Michal Hocko Cc: Oleg Nesterov , 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, adobriyan@gmail.com, akpm@linux-foundation.org, ebiederm@xmission.com, 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: <20200820113023.rjxque4jveo4nj5o@wittgenstein> References: <20200820002053.1424000-1-surenb@google.com> <20200820105555.GA4546@redhat.com> <20200820111349.GE5033@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200820111349.GE5033@dhcp22.suse.cz> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 20, 2020 at 01:13:49PM +0200, Michal Hocko wrote: > On Thu 20-08-20 12:55:56, Oleg Nesterov wrote: > > On 08/19, Suren Baghdasaryan wrote: > > > > > > 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. > > > > vfork() ? > > Could you be more specific? vfork() implies CLONE_VM but !CLONE_THREAD. The way this patch is written the mutex lock will be taken every time you do a vfork(). (It's honestly also debatable whether it's that rare. For one, userspace stuff I maintain uses it too (see [1]). [1]: https://github.com/lxc/lxc/blob/9d3b7c97f0443adc9f0b0438437657ab42f5a1c3/src/lxc/start.c#L1676 ) > > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk) > > > if (clone_flags & CLONE_VM) { > > > mmget(oldmm); > > > mm = oldmm; > > > + if (!(clone_flags & CLONE_SIGHAND)) { > > > > I agree with Christian, you need CLONE_THREAD > > This was my suggestion to Suren, likely because I've misrememberd which > clone flag is responsible for the signal delivery. But now, after double > checking we do explicitly disallow CLONE_SIGHAND && !CLONE_VM. So > CLONE_THREAD is the right thing to check. > > > > + /* We need to synchronize with __set_oom_adj */ > > > + mutex_lock(&oom_adj_lock); > > > + set_bit(MMF_PROC_SHARED, &mm->flags); > > > + /* Update the values in case they were changed after copy_signal */ > > > + tsk->signal->oom_score_adj = current->signal->oom_score_adj; > > > + tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min; > > > + mutex_unlock(&oom_adj_lock); > > > > I don't understand how this can close the race with __set_oom_adj... > > > > What if __set_oom_adj() is called right after mutex_unlock() ? It will see > > MMF_PROC_SHARED, but for_each_process() won't find the new child until > > copy_process() does list_add_tail_rcu(&p->tasks, &init_task.tasks) ? > > Good point. Then we will have to move this thing there. I was toying with moving this into sm like: static inline copy_oom_score(unsigned long flags, struct task_struct *tsk) trying to rely on set_bit() and test_bit() in copy_mm() being atomic and then calling it where Oleg said after the point of no return. Christian