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,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 3A5C0C4743D for ; Fri, 4 Jun 2021 17:10:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 225B961405 for ; Fri, 4 Jun 2021 17:10:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229958AbhFDRMn (ORCPT ); Fri, 4 Jun 2021 13:12:43 -0400 Received: from foss.arm.com ([217.140.110.172]:43740 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229690AbhFDRMn (ORCPT ); Fri, 4 Jun 2021 13:12:43 -0400 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 D020B1063; Fri, 4 Jun 2021 10:10:56 -0700 (PDT) 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 11AEF3F73D; Fri, 4 Jun 2021 10:10:53 -0700 (PDT) From: Valentin Schneider To: Will Deacon , linux-arm-kernel@lists.infradead.org Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon , Catalin Marinas , Marc Zyngier , Greg Kroah-Hartman , Peter Zijlstra , Morten Rasmussen , Qais Yousef , Suren Baghdasaryan , Quentin Perret , Tejun Heo , Johannes Weiner , Ingo Molnar , Juri Lelli , Vincent Guittot , "Rafael J. Wysocki" , Dietmar Eggemann , Daniel Bristot de Oliveira , kernel-team@android.com Subject: Re: [PATCH v8 05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection In-Reply-To: <20210602164719.31777-6-will@kernel.org> References: <20210602164719.31777-1-will@kernel.org> <20210602164719.31777-6-will@kernel.org> Date: Fri, 04 Jun 2021 18:10:46 +0100 Message-ID: <878s3peesp.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On 02/06/21 17:47, 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. > > On such a system, we must take care not to migrate a task to an > unsupported CPU when forcefully moving tasks in select_fallback_rq() > in response to a CPU hot-unplug operation. > > Introduce a task_cpu_possible_mask() hook which, given a task argument, > allows an architecture to return a cpumask of CPUs that are capable of > executing that task. The default implementation returns the > cpu_possible_mask, since sane machines do not suffer from per-cpu ISA > limitations that affect scheduling. The new mask is used when selecting > the fallback runqueue as a last resort before forcing a migration to the > first active CPU. > Nit: Some uses of this mask (cpu_is_allowed(), __set_cpus_allowed_ptr()) don't apply to kthreads. This makes sense for the 32-bit@EL0 faff, but it wouldn't hurt to point this out somewhere IMO. Also, that's an odd place for the definitions, but IIRC there isn't a much better choice. Reviewed-by: Valentin Schneider