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=-7.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 92A17C2B9F4 for ; Tue, 22 Jun 2021 13:51:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 71C476128C for ; Tue, 22 Jun 2021 13:51:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231301AbhFVNyD (ORCPT ); Tue, 22 Jun 2021 09:54:03 -0400 Received: from mail.kernel.org ([198.145.29.99]:38416 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229907AbhFVNyC (ORCPT ); Tue, 22 Jun 2021 09:54:02 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 976D361363; Tue, 22 Jun 2021 13:51:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1624369906; bh=EIxfbfEg1HQQFiuv3Jaw3D2aNsSQzOEzYeGe/1N7ulU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=sdSfYpEYJ6dO2WISawBmvvmoCu6D0SZIkdcB6QvneSXC54Z1COG7+GMAM6cMWmJXO qj86JA/5jNCAnXIoCMzwGNiLkxMcTwkgNbZvxE+5ZdiCS7eZNOOJ473y6Llb6j3+JH zTpaA1ljPgmhsySeEel+TvkeLZnj3k1qjPCN3M1ApZLlEncmOsQTXfxniFk5vRxaP3 d0NgrNHRyfFN1t8bIvRxOWcCT3HN9DllGrXW0Q9Yz8V+p482guEwgoLrvFmY697ti/ EGeJEjA+U5kfE1jEFF70WmQIdoJc0nZt4CmLCCk2klrRvrABJj/21dyT54LYz8BnBP KxAwGhSYw6U5Q== Date: Tue, 22 Jun 2021 14:51:41 +0100 From: Will Deacon To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Morten Rasmussen , Qais Yousef , Dietmar Eggemann , Quentin Perret , Juri Lelli , Vincent Guittot , kernel-team@android.com Subject: Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and migration_cpu_stop() Message-ID: <20210622135140.GA31071@willie-the-truck> References: <20210526205751.842360-1-valentin.schneider@arm.com> <20210526205751.842360-3-valentin.schneider@arm.com> <87h7ihfrlf.mognet@arm.com> <20210602152629.GF31179@willie-the-truck> <87bl8of3x8.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87bl8of3x8.mognet@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Valentin, I've been looking at this on and off and I'm afraid I'm still not convinced, almost certainly due to my own ignorance, but hey :) On Wed, Jun 02, 2021 at 08:43:31PM +0100, Valentin Schneider wrote: > On 02/06/21 16:26, Will Deacon wrote: > > On Tue, Jun 01, 2021 at 05:59:56PM +0100, Valentin Schneider wrote: > >> On 26/05/21 21:57, Valentin Schneider wrote: > >> > + dest_cpu = arg->dest_cpu; > >> > + if (task_on_rq_queued(p)) { > >> > + /* > >> > + * A hotplug operation could have happened between > >> > + * set_cpus_allowed_ptr() and here, making dest_cpu no > >> > + * longer allowed. > >> > + */ > >> > + if (!is_cpu_allowed(p, dest_cpu)) > >> > + dest_cpu = select_fallback_rq(cpu_of(rq), p); > >> > + /* > >> > + * dest_cpu can be victim of hotplug between is_cpu_allowed() > >> > + * and here. However, per the synchronize_rcu() in > >> > + * sched_cpu_deactivate(), it can't have gone lower than > >> > + * CPUHP_AP_ACTIVE, so it's safe to punt it over and let > >> > + * balance_push() route it elsewhere. > >> > + */ > >> > + update_rq_clock(rq); > >> > + rq = move_queued_task(rq, &rf, p, dest_cpu); > >> > >> So, while digesting this I started having doubts vs pcpu kthreads since > >> they're allowed on online CPUs. The bogus scenario here would be picking a > >> !active && online CPU, and see it go !online before the move_queued_task(). > >> > >> Now, to transition from online -> !online, we have to go through > >> take_cpu_down() which is issued via a stop_machine() call. This means the > >> transition can't happen until all online CPUs are running the stopper task > >> and reach MULTI_STOP_RUN. > >> > >> migration_cpu_stop() being already a stopper callback should thus make it > >> "atomic" vs takedown_cpu(), meaning the above should be fine. > > > > I'd be more inclined to agree with your reasoning if migration_cpu_stop() > > couldn't itself call stop_one_cpu_nowait() to queue more work for the > > stopper thread. What guarantees that takedown_cpu() can't queue its stopper > > work in the middle of that? > > > > Harumph... > > So something like all CPUs but one are running their take_cpu_down() > callback because one is still running migration_cpu_stop(), i.e.: > > CPU0 CPU1 ... CPUN > > migration_cpu_stop() take_cpu_down()@MULTI_STOP_PREPARE take_cpu_down()@MULTI_STOP_PREPARE > > If CPU0 hits that else if (pending) condition, it'll queue a > migration_cpu_stop() elsewhere (say CPU1), then run the take_cpu_down() > callback which follows in its work->list. > > If the CPU being brought down is anything else than CPU1, it shouldn't > really matter. If it *is* CPU1, then I think we've got some guarantees. > > Namely, there's no (read: there shouldn't be any) way for a task to > still be on CPU1 at this point; per sched_cpu_wait_empty(), > migration-disabled tasks and pcpu kthreads must vacate the CPU before it > then (migrate_disable regions must be bounded, and pcpu kthreads are > expected to be moved away by their respective owners). I agree with that, but the stopper is still running on CPU1 and so migration_cpu_stop() could still queue work there after sched_cpu_wait_empty() has returned but before stop_machine_park(), afaict. Actually, it looks like migration_cpu_stop() ignores the return value of stop_one_cpu_nowait(), so if the stopper thread has been parked I think we'll quietly do nothing there as well. > With the above, I don't really see how migration_cpu_stop() could queue > itself again on an about-to-die CPU (thus racing with the take_cpu_down() > callback), and even if it did, then we'd still run the callback before > parking the stopper thread (no surprise callback persisting during > offline), in which case we'd either see the task as having moved somewhere > sensible, or we'll queue yet another callback. In this case, I think we'd run the migration_cpu_stop() callback, but then __migrate_task() would fail to move the task because the CPU would be !online. Will