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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 12322C63697 for ; Thu, 19 Nov 2020 12:48:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B7FB2246E8 for ; Thu, 19 Nov 2020 12:48:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727292AbgKSMrk (ORCPT ); Thu, 19 Nov 2020 07:47:40 -0500 Received: from foss.arm.com ([217.140.110.172]:56420 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726719AbgKSMrj (ORCPT ); Thu, 19 Nov 2020 07:47:39 -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 D7FD21396; Thu, 19 Nov 2020 04:47:38 -0800 (PST) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8C0093F718; Thu, 19 Nov 2020 04:47:36 -0800 (PST) References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider 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 , Qais Yousef , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity In-reply-to: <20201113093720.21106-8-will@kernel.org> Date: Thu, 19 Nov 2020 12:47:34 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/11/20 09:37, Will Deacon wrote: > Asymmetric systems may not offer the same level of userspace ISA support > across all CPUs, meaning that some applications cannot be executed by > some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do > not feature support for 32-bit applications on both clusters. > > Although userspace can carefully manage the affinity masks for such > tasks, one place where it is particularly problematic is execve() > because the CPU on which the execve() is occurring may be incompatible > with the new application image. In such a situation, it is desirable to > restrict the affinity mask of the task and ensure that the new image is > entered on a compatible CPU. > From userspace's point of view, this looks the same as if the > incompatible CPUs have been hotplugged off in its affinity mask. {pedantic reading warning} Hotplugged CPUs *can* be set in a task's affinity mask, though interact weirdly with cpusets [1]. Having it be the same as hotplug would mean keeping incompatible CPUs allowed in the affinity mask, but preventing them from being picked via e.g. is_cpu_allowed(). I was actually hoping this could be a feasible approach, i.e. have an extra CPU active mask filter for any task: cpu_active_mask & arch_cpu_allowed_mask(p) rather than fiddle with task affinity. Sadly this would also require fixing up pretty much any place that uses cpu_active_mask(), and probably places that use p->cpus_ptr as well. RT / DL balancing comes to mind, because that uses either a task's affinity or a CPU's root domain (which reflects the cpu_active_mask) to figure out where to push a task. So while I'm wary of hacking up affinity, I fear it might be the lesser evil :( [1]: https://lore.kernel.org/lkml/1251528473.590671.1579196495905.JavaMail.zimbra@efficios.com/ > In preparation for restricting the affinity mask for compat tasks on > arm64 systems without uniform support for 32-bit applications, introduce > a restrict_cpus_allowed_ptr(), which allows the current affinity mask > for a task to be shrunk to the intersection of a parameter mask. > > Signed-off-by: Will Deacon