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 06074C63697 for ; Thu, 19 Nov 2020 14:55:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9ED6022261 for ; Thu, 19 Nov 2020 14:55:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727777AbgKSOyx (ORCPT ); Thu, 19 Nov 2020 09:54:53 -0500 Received: from foss.arm.com ([217.140.110.172]:59750 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728044AbgKSOyw (ORCPT ); Thu, 19 Nov 2020 09:54:52 -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 1F87911D4; Thu, 19 Nov 2020 06:54:52 -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 C423A3F719; Thu, 19 Nov 2020 06:54:49 -0800 (PST) References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> <20201119131319.GE4331@willie-the-truck> 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: <20201119131319.GE4331@willie-the-truck> Date: Thu, 19 Nov 2020 14:54:47 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 19/11/20 13:13, Will Deacon wrote: > On Thu, Nov 19, 2020 at 12:47:34PM +0000, Valentin Schneider wrote: >> >> 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(). > > Sure, but I was talking about what userspace sees, and I don't think it ever > sees CPUs that have been hotplugged off, right? That is, sched_getaffinity() > masks its result with the active_mask. > Right, this wasn't pedantic reading, but reading between the lines!