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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 9E1E0C388F9 for ; Thu, 19 Nov 2020 11:07:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 39027246EF for ; Thu, 19 Nov 2020 11:07:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="he9WtmW7" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726619AbgKSLHc (ORCPT ); Thu, 19 Nov 2020 06:07:32 -0500 Received: from mail.kernel.org ([198.145.29.99]:33432 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725843AbgKSLHb (ORCPT ); Thu, 19 Nov 2020 06:07:31 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id AB98A22248; Thu, 19 Nov 2020 11:07:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605784050; bh=IFWCYkPdfqc245s/XXDJ72zIwthXd5CYEww4fQuyMKI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=he9WtmW7U8XMQ7IHFIvgoUxjzie8s4Ven1o1/QWXq+g1T/8ZhjiuayVMhWin8Iu9s 7XH5+ElgMqV7pi+NWIzowDER3ZlzIH8xTevnazQlpph+PupkOe//ZQ5PsY5adsgv/E qqTkqyrtXpxBvvfIoPb9HHSqB4k2ffoc3plB/KFU= Date: Thu, 19 Nov 2020 11:07:24 +0000 From: Will Deacon To: Quentin Perret Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH v3 11/14] sched: Reject CPU affinity changes based on arch_cpu_allowed_mask() Message-ID: <20201119110723.GE3946@willie-the-truck> References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-12-will@kernel.org> <20201119094744.GE2416649@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201119094744.GE2416649@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Nov 19, 2020 at 09:47:44AM +0000, Quentin Perret wrote: > On Friday 13 Nov 2020 at 09:37:16 (+0000), Will Deacon wrote: > > Reject explicit requests to change the affinity mask of a task via > > set_cpus_allowed_ptr() if the requested mask is not a subset of the > > mask returned by arch_cpu_allowed_mask(). This ensures that the > > 'cpus_mask' for a given task cannot contain CPUs which are incapable of > > executing it, except in cases where the affinity is forced. > > > > Signed-off-by: Will Deacon > > --- > > kernel/sched/core.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 8df38ebfe769..13bdb2ae4d3f 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1877,6 +1877,7 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p, > > struct rq_flags *rf) > > { > > const struct cpumask *cpu_valid_mask = cpu_active_mask; > > + const struct cpumask *cpu_allowed_mask = arch_cpu_allowed_mask(p); > > unsigned int dest_cpu; > > int ret = 0; > > > > @@ -1887,6 +1888,9 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p, > > * Kernel threads are allowed on online && !active CPUs > > */ > > cpu_valid_mask = cpu_online_mask; > > + } else if (!cpumask_subset(new_mask, cpu_allowed_mask)) { > > + ret = -EINVAL; > > + goto out; > > So, IIUC, this should make the sched_setaffinity() syscall fail and > return -EINVAL to userspace if it tries to put 64bits CPUs in the > affinity mask of a 32 bits task, which I think makes sense. > > But what about affinity change via cpusets? e.g., if a 32 bit task is > migrated to a cpuset with 64 bit CPUs, then the migration will be > 'successful' and the task will appear to be in the destination cgroup, > but the actual affinity of the task will be something completely > different? Yeah, the cpuset code ignores the return value of set_cpus_allowed_ptr() in update_tasks_cpumask() so the failure won't be propagated, but then again I think that might be the right thing to do. Nothing prevents 32-bit and 64-bit tasks from co-existing in the same cpuseti afaict, so forcing the 64-bit tasks onto the 32-bit-capable cores feels much worse than the approach taken here imo. Nothing says we _have_ to schedule on all of the cores in the mask. The interesting case is what happens if the cpuset for a 32-bit task is changed to contain only the 64-bit-only cores. I think that's a userspace bug, but the fallback rq selection should avert disaster. Will