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 A00F0C433E1 for ; Thu, 20 Aug 2020 11:54:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8996B2078B for ; Thu, 20 Aug 2020 11:54:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729342AbgHTLs1 (ORCPT ); Thu, 20 Aug 2020 07:48:27 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:40951 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730554AbgHTLr3 (ORCPT ); Thu, 20 Aug 2020 07:47:29 -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 1k8j26-0001SH-AW; Thu, 20 Aug 2020 11:47:02 +0000 Date: Thu, 20 Aug 2020 13:47:00 +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: <20200820114700.bmla72v3t4ux7gsm@wittgenstein> References: <20200820002053.1424000-1-surenb@google.com> <20200820105555.GA4546@redhat.com> <20200820111349.GE5033@dhcp22.suse.cz> <20200820112932.GG5033@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200820112932.GG5033@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 01:29:32PM +0200, Michal Hocko wrote: > On Thu 20-08-20 13:13:55, 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? > > > > > > --- 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. > > I have tried to remember but I have to say that after reading man page I > am still confused. So what is the actual difference between CLONE_THREAD > and CLONE_SIGHAND? Essentially all we care about from the OOM (and CLONE_THREAD implies CLONE_SIGHAND CLONE_SIGHAND implies CLONE_VM but CLONE_SIGHAND doesn't imply CLONE_THREAD. > oom_score_adj) POV is that signals are delivered to all entities and > that thay share signal struct. copy_signal is checking for CLONE_THREAD If a thread has a separate sighand struct it can have separate handlers (Oleg will correct me if wrong.). But fatal signals will take the whole thread-group down and can't be ignored which is the only thing you care about with OOM afair. What you care about is that the oom_score_adj{_min} settings are shared and they live in struct signal_struct and whether that's shared or not is basically guided by CLONE_THREAD. > but CLONE_THREAD requires CLONE_SIGHAND AFAIU. So is there any cae where > checking for CLONE_SIGHAND would wrong for our purpose? Without having spent a long time thinking deeply about this it likely wouldn't. But using CLONE_SIGHAND is very irritating since it doesn't clearly express what you want this for. Especially since there's now a difference between the check in copy_signal() and copy_mm() and a disconnect to what is expressed in the commit message too, imho. Christian