From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932320AbdIGPLc (ORCPT ); Thu, 7 Sep 2017 11:11:32 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:60716 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932213AbdIGPLG (ORCPT ); Thu, 7 Sep 2017 11:11:06 -0400 Message-Id: <20170907150614.044460912@infradead.org> User-Agent: quilt/0.63-1 Date: Thu, 07 Sep 2017 17:03:51 +0200 From: Peter Zijlstra To: mingo@kernel.org, tglx@linutronix.de Cc: linux-kernel@vger.kernel.org, peterz@infradead.org Subject: [PATCH 2/4] sched/fair: Plug hole between hotplug and active load_balance References: <20170907150349.063852465@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline; filename=peterz-sched-active-balance-hole.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The load balancer applies cpu_active_mask to whatever sched_domains it finds, however in the case of active_balance there is a hole between setting rq->{active_balance,push_cpu} and running the stop_machine work doing the actual migration. The @push_cpu can go offline in this window, which would result in us moving a task onto a dead cpu, which is a fairly bad thing. Double check the active mask before the stop work does the migration. CPU0 CPU1 stop_machine(takedown_cpu) load_balance() cpu_stopper_thread() ... work = multi_cpu_stop stop_one_cpu_nowait( /* wait for CPU0 */ .func = active_load_balance_cpu_stop ); cpu_stopper_thread() work = multi_cpu_stop /* sync with CPU1 */ take_cpu_down() play_dead(); work = active_load_balance_cpu_stop set_task_cpu(p, CPU1); /* oops!! */ Reported-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/fair.c | 7 +++++++ 1 file changed, 7 insertions(+) --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop( struct rq_flags rf; rq_lock_irq(busiest_rq, &rf); + /* + * Between queueing the stop-work and running it is a hole in which + * CPUs can become inactive. We should not move tasks from or to + * inactive CPUs. + */ + if (!cpu_active(busiest_cpu) || !cpu_active(target_cpu)) + goto out_unlock; /* make sure the requested cpu hasn't gone down in the meantime */ if (unlikely(busiest_cpu != smp_processor_id() ||