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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 817A8C433B4 for ; Thu, 15 Apr 2021 08:59:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4244F6124B for ; Thu, 15 Apr 2021 08:59:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231903AbhDOJAB (ORCPT ); Thu, 15 Apr 2021 05:00:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50214 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231749AbhDOI7z (ORCPT ); Thu, 15 Apr 2021 04:59:55 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 38879C061574 for ; Thu, 15 Apr 2021 01:59:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=RzyEi5B73kAsrxZ5avxC+MpqWeclsttq4wG14cBnHr0=; b=QPFFHrF8fEBaudxi550unoleUS G3JKPZAEzk+hFV0uAn2M5KnOEr4Zi7hbz9aUIqH3wqPWWPmCM5MJQKvfyytMfCPdVoVRnHJgVaaOj 2okMXfCbZdBYAwJGh+VMdrnDlJxV8o5vjpMY3ML+LvSZ6z+HSNxgieurZL8EizrGJH2gY0blcda8N 3ankXBXrG1CnXGUSJhu/v7RhKCFMBiA4SmeBxtDC6aRtKOYlKfE5WzGgIZCU9+wRUi8gxJd84oIpI b1Adrs3pP1MsDfl2t+R54NLSV+Ml+aDbkJkdgJ+UKSc9cBOKlfj0FAs/9AFHclCTvTkD9THXtr5uu AZ5kidCg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lWxqE-00FRDH-Ro; Thu, 15 Apr 2021 08:59:15 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 10E95300033; Thu, 15 Apr 2021 10:59:13 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id C8AFC20224210; Thu, 15 Apr 2021 10:59:13 +0200 (CEST) Date: Thu, 15 Apr 2021 10:59:13 +0200 From: Peter Zijlstra To: Valentin Schneider 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 Message-ID: References: <20210310145258.899619710@infradead.org> <20210310150109.259726371@infradead.org> <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me> <87r1jfmn8d.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 13, 2021 at 08:51:23AM +0200, Peter Zijlstra wrote: > > 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? > > Yeah, I'm not sure anymnore either; I tried to work out why I'd done > that but upon closer examination everything fell flat. > > Let me try again today :-) Can't make sense of what I did.. I've removed that hunk. Patch now looks like this. --- Subject: sched: Use cpu_dying() to fix balance_push vs hotplug-rollback From: Peter Zijlstra Date: Thu Jan 21 16:09:32 CET 2021 Use the new cpu_dying() state to simplify and fix the balance_push() vs CPU hotplug rollback state. Specifically, we currently rely on notifiers sched_cpu_dying() / sched_cpu_activate() to terminate balance_push, however if the cpu_down() fails when we're past sched_cpu_deactivate(), it should terminate balance_push at that point and not wait until we hit sched_cpu_activate(). Similarly, when cpu_up() fails and we're going back down, balance_push should be active, where it currently is not. So instead, make sure balance_push is enabled between sched_cpu_deactivate() and sched_cpu_activate() (eg. when !cpu_active()), and gate it's utility with cpu_dying(). Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/core.c | 26 +++++++++++++++----------- kernel/sched/sched.h | 1 - 2 files changed, 15 insertions(+), 12 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1811,7 +1811,7 @@ static inline bool is_cpu_allowed(struct return cpu_online(cpu); /* Regular kernel threads don't get to stay during offline. */ - if (cpu_rq(cpu)->balance_push) + if (cpu_dying(cpu)) return false; /* But are allowed during online. */ @@ -7639,6 +7639,9 @@ static DEFINE_PER_CPU(struct cpu_stop_wo /* * Ensure we only run per-cpu kthreads once the CPU goes !active. + * + * This is active/set between sched_cpu_deactivate() / sched_cpu_activate(). + * But only effective when the hotplug motion is down. */ static void balance_push(struct rq *rq) { @@ -7646,12 +7649,19 @@ static void balance_push(struct rq *rq) lockdep_assert_held(&rq->lock); SCHED_WARN_ON(rq->cpu != smp_processor_id()); + /* * Ensure the thing is persistent until balance_push_set(.on = false); */ rq->balance_callback = &balance_push_callback; /* + * Only active while going offline. + */ + if (!cpu_dying(rq->cpu)) + return; + + /* * Both the cpu-hotplug and stop task are in this case and are * required to complete the hotplug process. * @@ -7704,7 +7714,6 @@ static void balance_push_set(int cpu, bo struct rq_flags rf; rq_lock_irqsave(rq, &rf); - rq->balance_push = on; if (on) { WARN_ON_ONCE(rq->balance_callback); rq->balance_callback = &balance_push_callback; @@ -7829,8 +7838,8 @@ int sched_cpu_activate(unsigned int cpu) struct rq_flags rf; /* - * Make sure that when the hotplug state machine does a roll-back - * we clear balance_push. Ideally that would happen earlier... + * Clear the balance_push callback and prepare to schedule + * regular tasks. */ balance_push_set(cpu, false); @@ -8015,12 +8024,6 @@ int sched_cpu_dying(unsigned int cpu) } rq_unlock_irqrestore(rq, &rf); - /* - * Now that the CPU is offline, make sure we're welcome - * to new tasks once we come back up. - */ - balance_push_set(cpu, false); - calc_load_migrate(rq); update_max_interval(); hrtick_clear(rq); @@ -8205,7 +8208,7 @@ 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; @@ -8252,6 +8255,7 @@ 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(); --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -983,7 +983,6 @@ struct rq { unsigned long cpu_capacity_orig; struct callback_head *balance_callback; - unsigned char balance_push; unsigned char nohz_idle_balance; unsigned char idle_balance;