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=-12.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 40697C56201 for ; Thu, 19 Nov 2020 16:41:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C86A22467A for ; Thu, 19 Nov 2020 16:41:41 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="wdqvydQZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727845AbgKSQll (ORCPT ); Thu, 19 Nov 2020 11:41:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:52756 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727916AbgKSQll (ORCPT ); Thu, 19 Nov 2020 11:41:41 -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 DA7022220B; Thu, 19 Nov 2020 16:41:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605804100; bh=IdNj10e/sr3b3LdQtcOLRRg2JJyeeh5XmiDCYsVk77I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=wdqvydQZQ26mI0gT3SQ5VKM5IhdXfEQ8VgwTBlmOBqVHfH0/KPdIOgwcdOGFzH6v1 /0MGPlofiGzxedOeEeBZz+RuTolxNUXr8TN05c8PqWwG+Y+YvjMckmsHgIOYykIGs2 mGxx0kU96+vE7hoxIFRJXZZoaaQu2VFmrUe6T4IE= Date: Thu, 19 Nov 2020 16:41:33 +0000 From: Will Deacon To: Valentin Schneider 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 Message-ID: <20201119164133.GD4582@willie-the-truck> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 02:54:32PM +0000, Valentin Schneider wrote: > > 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. Ah, gotcha. Still not convinced it's any better, but I see that it works. > 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). I think that makes sense for static masks, but since this is dynamic I was following the lead of sched_setaffinity(). Will