From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755477AbbHYKTk (ORCPT ); Tue, 25 Aug 2015 06:19:40 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:44386 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751291AbbHYKTj (ORCPT ); Tue, 25 Aug 2015 06:19:39 -0400 Date: Tue, 25 Aug 2015 12:19:35 +0200 From: Peter Zijlstra To: Rik van Riel Cc: linux-kernel@vger.kernel.org, mingo@kernel.org Subject: Re: [PATCH,RFC] sched: fix "impossible" load balancing oops Message-ID: <20150825101935.GP16853@twins.programming.kicks-ass.net> References: <20150821142138.57b5a7c3@riellap.home.surriel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150821142138.57b5a7c3@riellap.home.surriel.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 21, 2015 at 02:21:22PM -0400, Rik van Riel wrote: > The load balancing code can run into the situation where > the source and destination runqueues are the same, in > the active balancing code. > > /* > * This condition is "impossible", if it occurs > * we need to fix it. Originally reported by > * Bjorn Helgaas on a 128-cpu setup. > */ > BUG_ON(busiest_rq == target_rq); > > This happens despite not triggering the BUG_ON(busiest == env.dst_rq) > line after find_busiest_queue. > > From code inspection, it appears there is a condition where this can happen. > > Specifically, if we encounter only pinned tasks on a CPU, can_migrate_task > will set env->new_dst_cpu to a CPU in the env->dst_grpmask. If the group > includes the source cpu, we may end up setting env.dst_cpu to the same > as dst.src_cpu. > > The fix would be to clear the source cpu from env.dst_grpmask, to ensure > we never select the source cpu as the destination. > > Signed-off-by: Rik van Riel > --- > kernel/sched/fair.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d113c3b..514a369 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7172,6 +7172,8 @@ redo: > env.src_cpu = busiest->cpu; > env.src_rq = busiest; > > + cpumask_clear_cpu(busiest->cpu, env.dst_grpmask); > + > ld_moved = 0; > if (busiest->nr_running > 1) { > /* Right, so real problem 'recently' introduced by: 88b8dac0a14c ("sched: Improve balance_cpu() to consider other cpus in its group as target of (pinned) task") But I think there's a wee problem with the solution, if I'm not still entirely asleep, it appears dst_grpmask might be NULL in case of CPU_NEWLY_IDLE, which would make that cpumask_clear_cpu() do something naughty.