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 1B07AC83018 for ; Wed, 2 Dec 2020 14:08:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C445D2222E for ; Wed, 2 Dec 2020 14:08:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388971AbgLBOIM (ORCPT ); Wed, 2 Dec 2020 09:08:12 -0500 Received: from foss.arm.com ([217.140.110.172]:40906 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729043AbgLBOIK (ORCPT ); Wed, 2 Dec 2020 09:08:10 -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 DDDFE1063; Wed, 2 Dec 2020 06:07:24 -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 A600A3F718; Wed, 2 Dec 2020 06:07:22 -0800 (PST) Date: Wed, 2 Dec 2020 14:07:20 +0000 From: Qais Yousef To: Will Deacon 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 , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH v4 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0 Message-ID: <20201202140720.vlnpvge3bgtvn43s@e107158-lin.cambridge.arm.com> References: <20201124155039.13804-1-will@kernel.org> <20201124155039.13804-9-will@kernel.org> <20201127132306.ee4frq6ujz3fqxic@e107158-lin.cambridge.arm.com> <20201201165556.GA27783@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20201201165556.GA27783@willie-the-truck> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/01/20 16:55, Will Deacon wrote: > > > +static void adjust_compat_task_affinity(struct task_struct *p) > > > +{ > > > + cpumask_var_t cpuset_mask; > > > + const struct cpumask *possible_mask = system_32bit_el0_cpumask(); > > > + const struct cpumask *newmask = possible_mask; > > > + > > > + /* > > > + * Restrict the CPU affinity mask for a 32-bit task so that it contains > > > + * only the 32-bit-capable subset of its original CPU mask. If this is > > > + * empty, then try again with the cpuset allowed mask. If that fails, > > > + * forcefully override it with the set of all 32-bit-capable CPUs that > > > + * we know about. > > > + * > > > + * From the perspective of the task, this looks similar to what would > > > + * happen if the 64-bit-only CPUs were hot-unplugged at the point of > > > + * execve(). > > > + */ > > > + if (!restrict_cpus_allowed_ptr(p, possible_mask)) > > > + goto out; > > > + > > > + if (alloc_cpumask_var(&cpuset_mask, GFP_KERNEL)) { > > > + cpuset_cpus_allowed(p, cpuset_mask); > > > + if (cpumask_and(cpuset_mask, cpuset_mask, possible_mask)) { > > > + newmask = cpuset_mask; > > > + goto out_set_mask; > > > + } > > > + } > > > > Wouldn't it be better to move this logic to restrict_cpus_allowed_ptr()? > > I think it should always take cpusets into account and it's not special to > > this particular handling here, no? > > I did actually try this but didn't pursue it further because I was worried > that I was putting too much of the "can't run a 32-bit task on a 64-bit-only > CPU" logic into what would otherwise be a potentially useful library function > if/when other architectures want something similar. But I'll have another > look because there were a couple of ideas I didn't try out. If we improve the cpuset handling issues to take into account arch_task_cpu_possible_mask() as discussed in the other thread, I think we can drop the cpuset handling here. > > > > + if (printk_ratelimit()) { > > > + printk_deferred("Overriding affinity for 32-bit process %d (%s) to CPUs %*pbl\n", > > > + task_pid_nr(p), p->comm, cpumask_pr_args(newmask)); > > > + } > > > > We have 2 cases where the affinity could have been overridden but we won't > > print anything: > > > > 1. restrict_cpus_allowed_ptr() > > 2. intersection of cpuset_mask and possible mask drops some cpus. > > > > Shouldn't we print something in these cases too? > > I don't think so: in these cases we've found a subset of CPUs that we can > run on, and so there's no need to warn. Nothing says we _have_ to use all > the CPUs available to us. The case where we override the affinity mask > altogether, however, does warrant a warning. This is very similar to the > hotplug behaviour in select_fallback_rq(). Okay. It is just to warn when we actually break the affinity because we ended up with empty mask; not just because we changed the affinity to an intersecting one. I think this makes sense, yes. We might be able to drop this too if we improve cpuset handling. The devil is in the details I guess. Thanks -- Qais Yousef