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_PATCH,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 BF49CC63697 for ; Thu, 19 Nov 2020 14:54:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6618F22261 for ; Thu, 19 Nov 2020 14:54:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727849AbgKSOyl (ORCPT ); Thu, 19 Nov 2020 09:54:41 -0500 Received: from foss.arm.com ([217.140.110.172]:59712 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727777AbgKSOyl (ORCPT ); Thu, 19 Nov 2020 09:54:41 -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 CDDAD1042; Thu, 19 Nov 2020 06:54:40 -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 7FB233F719; Thu, 19 Nov 2020 06:54:38 -0800 (PST) References: <20201113093720.21106-1-will@kernel.org> <20201113093720.21106-8-will@kernel.org> <20201119091820.GA2416649@google.com> <20201119110549.GA3946@willie-the-truck> <20201119131301.GD4331@willie-the-truck> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Will Deacon Cc: Quentin Perret , 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 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity In-reply-to: <20201119131301.GD4331@willie-the-truck> Date: Thu, 19 Nov 2020 14:54:32 +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 11:27:55AM +0000, Valentin Schneider wrote: >> >> On 19/11/20 11:05, Will Deacon wrote: >> > On Thu, Nov 19, 2020 at 09:18:20AM +0000, Quentin Perret wrote: >> >> > @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, >> >> > * OK, since we're going to drop the lock immediately >> >> > * afterwards anyway. >> >> > */ >> >> > - rq = move_queued_task(rq, &rf, p, dest_cpu); >> >> > + rq = move_queued_task(rq, rf, p, dest_cpu); >> >> > } >> >> > out: >> >> > - task_rq_unlock(rq, p, &rf); >> >> > + task_rq_unlock(rq, p, rf); >> >> >> >> And that's a little odd to have here no? Can we move it back on the >> >> caller's side? >> > >> > I don't think so, unfortunately. __set_cpus_allowed_ptr_locked() can trigger >> > migration, so it can drop the rq lock as part of that and end up relocking a >> > new rq, which it also unlocks before returning. Doing the unlock in the >> > caller is therfore even weirder, because you'd have to return the lock >> > pointer or something horrible like that. >> > >> > I did add a comment about this right before the function and it's an >> > internal function to the scheduler so I think it's ok. >> > >> >> An alternative here would be to add a new SCA_RESTRICT flag for >> __set_cpus_allowed_ptr() (see migrate_disable() faff in >> tip/sched/core). Not fond of either approaches, but the flag thing would >> avoid this "quirk". > > I tried this when I read about the migrate_disable() stuff on lwn, but I > didn't really find it any better to work with tbh. It also doesn't help > with the locking that Quentin was mentioning, does it? (i.e. you still > have to allocate). > You could keep it all bundled within __set_cpus_allowed_ptr() (i.e. not have a _locked() version) and use the flag as indicator of any extra work. Also FWIW we have this pattern of pre-allocating pcpu cpumasks (select_idle_mask, load_balance_mask), but given this is AIUI a very-not-hot path, this might be overkill (and reusing an existing one would be on the icky side of things). > Will