From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755569AbbHYSDa (ORCPT ); Tue, 25 Aug 2015 14:03:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38834 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751400AbbHYSD3 (ORCPT ); Tue, 25 Aug 2015 14:03:29 -0400 Message-ID: <55DCADEF.1030004@redhat.com> Date: Tue, 25 Aug 2015 14:03:27 -0400 From: Rik van Riel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: Peter Zijlstra CC: linux-kernel@vger.kernel.org, mingo@kernel.org Subject: Re: [PATCH,RFC] sched: fix "impossible" load balancing oops References: <20150821142138.57b5a7c3@riellap.home.surriel.com> <20150825101935.GP16853@twins.programming.kicks-ass.net> In-Reply-To: <20150825101935.GP16853@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/25/2015 06:19 AM, Peter Zijlstra wrote: > 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. Ahhh, so that's what I've been seeing! Also, I am not sure that find_busiest_group will actually return the group the CPU is in, so the patch may be wrong in another way too. I have yet to wrap my head around the sd & sg stuff (again), to figure out that bit... -- All rights reversed