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 1B7BDC433B4 for ; Mon, 12 Apr 2021 17:22:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DBEC961222 for ; Mon, 12 Apr 2021 17:22:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243915AbhDLRXF (ORCPT ); Mon, 12 Apr 2021 13:23:05 -0400 Received: from foss.arm.com ([217.140.110.172]:56364 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243299AbhDLRXE (ORCPT ); Mon, 12 Apr 2021 13:23:04 -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 3D43831B; Mon, 12 Apr 2021 10:22:46 -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 582F93F73B; Mon, 12 Apr 2021 10:22:44 -0700 (PDT) From: Valentin Schneider To: Peter Zijlstra Cc: tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de, swood@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com, qais.yousef@arm.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback In-Reply-To: References: <20210310145258.899619710@infradead.org> <20210310150109.259726371@infradead.org> <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me> Date: Mon, 12 Apr 2021 18:22:42 +0100 Message-ID: <87r1jfmn8d.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/04/21 14:03, Peter Zijlstra wrote: > On Thu, Mar 11, 2021 at 03:13:04PM +0000, Valentin Schneider wrote: >> Peter Zijlstra writes: >> > @@ -7910,6 +7908,14 @@ int sched_cpu_deactivate(unsigned int cp >> > } >> > rq_unlock_irqrestore(rq, &rf); >> > >> > + /* >> > + * From this point forward, this CPU will refuse to run any task that >> > + * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively >> > + * push those tasks away until this gets cleared, see >> > + * sched_cpu_dying(). >> > + */ >> > + balance_push_set(cpu, true); >> > + >> >> AIUI with cpu_dying_mask being flipped before even entering >> sched_cpu_deactivate(), we don't need this to be before the >> synchronize_rcu() anymore; is there more than that to why you're punting it >> back this side of it? > > I think it does does need to be like this, we need to clearly separate > the active=true and balance_push_set(). If we were to somehow observe > both balance_push_set() and active==false, we'd be in trouble. > I'm afraid I don't follow; we're replacing a read of rq->balance_push with cpu_dying(), and those are still written on the same side of the synchronize_rcu(). What am I missing? >> > #ifdef CONFIG_SCHED_SMT >> > /* >> > * When going down, decrement the number of cores with SMT present. >> >> > @@ -8206,7 +8212,7 @@ void __init sched_init(void) >> > rq->sd = NULL; >> > rq->rd = NULL; >> > rq->cpu_capacity = rq->cpu_capacity_orig = SCHED_CAPACITY_SCALE; >> > - rq->balance_callback = NULL; >> > + rq->balance_callback = &balance_push_callback; >> > rq->active_balance = 0; >> > rq->next_balance = jiffies; >> > rq->push_cpu = 0; >> > @@ -8253,6 +8259,7 @@ void __init sched_init(void) >> > >> > #ifdef CONFIG_SMP >> > idle_thread_set_boot_cpu(); >> > + balance_push_set(smp_processor_id(), false); >> > #endif >> > init_sched_fair_class(); >> > >> >> I don't get what these two changes do - the end result is the same as >> before, no? > > Not quite; we have to make sure the state of the offline CPUs matches > that of a CPU that's been offlined. For consistency if nothing else, but > it's actually important for a point below. > >> Also, AIUI this patch covers the cpu_dying -> !cpu_dying rollback case >> since balance_push gets numbed down by !cpu_dying. What about the other way >> around (hot-plug failure + rollback)? We may have allowed !pcpu tasks on the >> now-dying CPU, and we'd need to re-install the balance_push callback. > > This is in fact handled. Note how the previous point initialized the > offline CPU to have the push_callback installed. > > Also note how balance_push() re-instates itself unconditionally. > > So the thing is, we install the push callback on deactivate() and leave > it in place until activate, it is always there, regardless what way > we're moving. > > However, it is only effective whild going down, see the early exit. Oooh, I can't read, only the boot CPU gets its callback uninstalled in sched_init()! So secondaries keep push_callback installed up until sched_cpu_activate(), but as you said it's not effective unless a rollback happens. Now, doesn't that mean we should *not* uninstall the callback in sched_cpu_dying()? AFAIK it's possible for the initial secondary CPU boot to go fine, but the next offline+online cycle fails while going up - that would need to rollback with push_callback installed.