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 8F071C388F9 for ; Thu, 19 Nov 2020 20:39:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3173D221FE for ; Thu, 19 Nov 2020 20:39:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="EbG09We/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726154AbgKSUjP (ORCPT ); Thu, 19 Nov 2020 15:39:15 -0500 Received: from mail.kernel.org ([198.145.29.99]:51404 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725887AbgKSUjP (ORCPT ); Thu, 19 Nov 2020 15:39:15 -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 0012720888; Thu, 19 Nov 2020 20:39:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605818354; bh=fvpCr+a9O+Y1yL2+2TReoWD/IuwSGHB9+sXcjjtLcvw=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=EbG09We/Nz78MMVDFtrYLXZNMaT4FSWbbbgJ9mXUy+AjoYVLyZerlm09pJPcfPX3C t75DYlftONIZEBuSVS+s+HjC/T2Te+j1Oxz25IxbEPqYKnW+4PnnxwKXR3uSbWWgID VHYhM7e4DHbqpdDV5/QH2K70rFz1tBkL0pGUESqQ= Date: Thu, 19 Nov 2020 20:39:07 +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 10/14] sched: Introduce arch_cpu_allowed_mask() to limit fallback rq selection Message-ID: <20201119203906.GA5099@willie-the-truck> References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-11-will@kernel.org> <20201119093850.GD2416649@google.com> <20201119110709.GD3946@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201119110709.GD3946@willie-the-truck> 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 11:07:09AM +0000, Will Deacon wrote: > On Thu, Nov 19, 2020 at 09:38:50AM +0000, Quentin Perret wrote: > > On Friday 13 Nov 2020 at 09:37:15 (+0000), 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 an arch_cpu_allowed_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. > > > > > > Signed-off-by: Will Deacon > > > --- > > > kernel/sched/core.c | 13 ++++++++++--- > > > 1 file changed, 10 insertions(+), 3 deletions(-) > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index 818c8f7bdf2a..8df38ebfe769 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -1696,6 +1696,11 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags) > > > > > > #ifdef CONFIG_SMP > > > > > > +/* Must contain at least one active CPU */ > > > +#ifndef arch_cpu_allowed_mask > > > +#define arch_cpu_allowed_mask(p) cpu_possible_mask > > > +#endif > > > + > > > /* > > > * Per-CPU kthreads are allowed to run on !active && online CPUs, see > > > * __set_cpus_allowed_ptr() and select_fallback_rq(). > > > @@ -1708,7 +1713,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu) > > > if (is_per_cpu_kthread(p)) > > > return cpu_online(cpu); > > > > > > - return cpu_active(cpu); > > > + if (!cpu_active(cpu)) > > > + return false; > > > + > > > + return cpumask_test_cpu(cpu, arch_cpu_allowed_mask(p)); > > > } > > > > > > /* > > > @@ -2361,10 +2369,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p) > > > } > > > fallthrough; > > > case possible: > > > - do_set_cpus_allowed(p, cpu_possible_mask); > > > + do_set_cpus_allowed(p, arch_cpu_allowed_mask(p)); > > > > Nit: I'm wondering if this should be called arch_cpu_possible_mask() > > instead? > > I'm open to renaming it, so if nobody else has any better ideas then I'll > go with this. Ah, so in doing this I realised I don't like arch_cpu_possible_mask() so much because it makes it sound like a back-end to cpu_possible_mask, but the two are really different things. arch_task_cpu_possible_mask() might work? Will