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=-5.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 A7230C282CB for ; Tue, 5 Feb 2019 11:49:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7BD052080D for ; Tue, 5 Feb 2019 11:49:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728674AbfBELtR (ORCPT ); Tue, 5 Feb 2019 06:49:17 -0500 Received: from mail-wm1-f67.google.com ([209.85.128.67]:55980 "EHLO mail-wm1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726065AbfBELtR (ORCPT ); Tue, 5 Feb 2019 06:49:17 -0500 Received: by mail-wm1-f67.google.com with SMTP id y139so3249943wmc.5 for ; Tue, 05 Feb 2019 03:49:16 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=3gDZxDFSfWTkr+lS6LUYByDQNMKy0cJHL0A7+onN9kQ=; b=GtKAIBv/DR+HjXRGWY50xVFXXcgUD6h52DtE1VkGcxKAUjHrUqE8jYsHlTigonq9Uz 8kL6q6Br3trdyZxKLG0ToELsyKSDA6YNQLqO1qQ9tluSGkiJRCyTNjAuPfrLGR82lhak Y2mB0jiatbAWEUgu0K1P+jji+UATaJzClxrWQT8rJTVHPojbsJcNo/m3HyrJTQjAWW6N IUgZRQCEYYYTXBEpNiqiLnrd5P3r+Ni/fpL2/9HmEiJe4MgDY/G2h71VjCJNuhYGSovg Vc9XU5jX9CAGyoIWnoKsIIcEe7b/1t/d/7hysVh4gYqYAUqPvUC0v9Qx+K2J39RBaICo HJug== X-Gm-Message-State: AHQUAubUBTZ8EFXVov1XwrulMzR+jJt3DZrIT26CkE/sdFu3ba2M89gu gsTnTkW0LGKswmQ3JjEmQ2cnBA== X-Google-Smtp-Source: AHgI3IYm4FaifIHrb/iw/TnzrmqPWj5mL6lSkjvLDNWg6zBEanNVruF7EeAdQS8DQrAcb3yLaAcm4g== X-Received: by 2002:a7b:cd14:: with SMTP id f20mr3167734wmj.93.1549367355373; Tue, 05 Feb 2019 03:49:15 -0800 (PST) Received: from localhost.localdomain ([151.15.206.34]) by smtp.gmail.com with ESMTPSA id a4sm4303105wmm.22.2019.02.05.03.49.13 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 05 Feb 2019 03:49:14 -0800 (PST) Date: Tue, 5 Feb 2019 12:49:11 +0100 From: Juri Lelli To: Peter Zijlstra Cc: mingo@redhat.com, rostedt@goodmis.org, tj@kernel.org, linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it, claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it, bristot@redhat.com, mathieu.poirier@linaro.org, lizefan@huawei.com, cgroups@vger.kernel.org Subject: Re: [PATCH v6 4/5] sched/core: Prevent race condition between cpuset and __sched_setscheduler() Message-ID: <20190205114911.GH30905@localhost.localdomain> References: <20190117084739.17078-1-juri.lelli@redhat.com> <20190117084739.17078-5-juri.lelli@redhat.com> <20190204121029.GD17550@hirez.programming.kicks-ass.net> <20190205095143.GG30905@localhost.localdomain> <20190205112049.GO17528@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190205112049.GO17528@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/02/19 12:20, Peter Zijlstra wrote: > On Tue, Feb 05, 2019 at 10:51:43AM +0100, Juri Lelli wrote: > > On 04/02/19 13:10, Peter Zijlstra wrote: > > > On Thu, Jan 17, 2019 at 09:47:38AM +0100, Juri Lelli wrote: > > > > No synchronisation mechanism exists between the cpuset subsystem and calls > > > > to function __sched_setscheduler(). As such, it is possible that new root > > > > domains are created on the cpuset side while a deadline acceptance test > > > > is carried out in __sched_setscheduler(), leading to a potential oversell > > > > of CPU bandwidth. > > > > > > > > Grab callback_lock from core scheduler, so to prevent situations such as > > > > the one described above from happening. > > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > > index f5263383170e..d928a42b8852 100644 > > > > --- a/kernel/sched/core.c > > > > +++ b/kernel/sched/core.c > > > > @@ -4224,6 +4224,13 @@ static int __sched_setscheduler(struct task_struct *p, > > > > rq = task_rq_lock(p, &rf); > > > > update_rq_clock(rq); > > > > > > > > + /* > > > > + * Make sure we don't race with the cpuset subsystem where root > > > > + * domains can be rebuilt or modified while operations like DL > > > > + * admission checks are carried out. > > > > + */ > > > > + cpuset_read_only_lock(); > > > > + > > > > /* > > > > * Changing the policy of the stop threads its a very bad idea: > > > > */ > > > > @@ -4285,6 +4292,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > /* Re-check policy now with rq lock held: */ > > > > if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) { > > > > policy = oldpolicy = -1; > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > goto recheck; > > > > } > > > > @@ -4342,6 +4350,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > > > > > /* Avoid rq from going away on us: */ > > > > preempt_disable(); > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > > > > > if (pi) > > > > @@ -4354,6 +4363,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > return 0; > > > > > > > > unlock: > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > return retval; > > > > } > > > > > > Why take callback_lock inside rq->lock and not the other way around? > > > AFAICT there is no pre-existing order so we can pick one here. > > > > I dediced to go for this order because if we do the other way around > > grabbing callback_lock should have to also disable irqs, no? And I > > didn't want to modify task_rq_lock; or at least this approach seemed > > less intrusive code-wide. > > Ah, but this way around we add the wait-time of callback_lock to > rq_lock, which seems undesirable because rq_lock is a far hotter lock in > general, right? Eh, indeed. OK, I'll work on it.