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=-13.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 0499AC2BBCD for ; Thu, 17 Dec 2020 15:01:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C1D082396F for ; Thu, 17 Dec 2020 15:01:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728740AbgLQPBk (ORCPT ); Thu, 17 Dec 2020 10:01:40 -0500 Received: from foss.arm.com ([217.140.110.172]:40786 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728086AbgLQPBj (ORCPT ); Thu, 17 Dec 2020 10:01:39 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6A48130E; Thu, 17 Dec 2020 07:00:53 -0800 (PST) Received: from e107158-lin.cambridge.arm.com (e107158-lin.cambridge.arm.com [10.1.194.78]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id F036B3F66B; Thu, 17 Dec 2020 07:00:50 -0800 (PST) Date: Thu, 17 Dec 2020 15:00:48 +0000 From: Qais Yousef To: Peter Zijlstra Cc: Will Deacon , linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Morten Rasmussen , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH v5 07/15] cpuset: Don't use the cpu_possible_mask as a last resort for cgroup v1 Message-ID: <20201217150048.d6enq5hhchvh32hz@e107158-lin.cambridge.arm.com> References: <20201208132835.6151-1-will@kernel.org> <20201208132835.6151-8-will@kernel.org> <20201217121552.ds7g2icvqp5nvtha@e107158-lin.cambridge.arm.com> <20201217134401.GY3040@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20201217134401.GY3040@hirez.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/17/20 14:44, Peter Zijlstra wrote: > On Thu, Dec 17, 2020 at 12:15:52PM +0000, Qais Yousef wrote: > > On 12/08/20 13:28, Will Deacon wrote: > > > If the scheduler cannot find an allowed CPU for a task, > > > cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask > > > if cgroup v1 is in use. > > > > > > In preparation for allowing architectures to provide their own fallback > > > mask, just return early if we're not using cgroup v2 and allow > > > select_fallback_rq() to figure out the mask by itself. > > > > > > Cc: Li Zefan > > > Cc: Tejun Heo > > > Cc: Johannes Weiner > > > Reviewed-by: Quentin Perret > > > Signed-off-by: Will Deacon > > > --- > > > kernel/cgroup/cpuset.c | 6 ++++-- > > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c > > > index 57b5b5d0a5fd..e970737c3ed2 100644 > > > --- a/kernel/cgroup/cpuset.c > > > +++ b/kernel/cgroup/cpuset.c > > > @@ -3299,9 +3299,11 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) > > > > > > void cpuset_cpus_allowed_fallback(struct task_struct *tsk) > > > { > > > + if (!is_in_v2_mode()) > > > + return; /* select_fallback_rq will try harder */ > > > + > > > rcu_read_lock(); > > > - do_set_cpus_allowed(tsk, is_in_v2_mode() ? > > > - task_cs(tsk)->cpus_allowed : cpu_possible_mask); > > > + do_set_cpus_allowed(tsk, task_cs(tsk)->cpus_allowed); > > > > Why is it safe to return that for cpuset v2? > > v1 > > Because in that case it does cpu_possible_mask, which, if you look at > select_fallback_rq(), is exactly what happens when cpuset 'fails' to > find a candidate. > > Or at least, that's how I read the patch. Okay I can see that if v2 has effectively empty mask for the 32bit tasks, then we'll fallback to the 'possible' switch case where we set task_cpu_possible_mask(). But how about when task_cs(tsk)->cpus_allowed contains partially invalid cpus? The search for a candidate cpu will return a correct dest_cpu, but the actual cpu_mask of the task will contain invalid cpus that could be picked up later, no? Shouldn't we cpumask_and(mask, task_cs(tsk)->cpus_allowed, task_cpu_possible_mask()) to remove those invalid cpus? Thanks -- Qais Yousef