From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754847Ab3ITNrV (ORCPT ); Fri, 20 Sep 2013 09:47:21 -0400 Received: from terminus.zytor.com ([198.137.202.10]:44270 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754634Ab3ITNrS (ORCPT ); Fri, 20 Sep 2013 09:47:18 -0400 Date: Fri, 20 Sep 2013 06:46:49 -0700 From: tip-bot for Vladimir Davydov Message-ID: Cc: linux-kernel@vger.kernel.org, hpa@zytor.com, mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, vdavydov@parallels.com Reply-To: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, peterz@infradead.org, tglx@linutronix.de, vdavydov@parallels.com In-Reply-To: References: To: linux-tip-commits@vger.kernel.org Subject: [tip:sched/core] sched/balancing: Fix 'local->avg_load > busiest- >avg_load' case in fix_small_imbalance() Git-Commit-ID: 3029ede39373c368f402a76896600d85a4f7121b X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.2.7 (terminus.zytor.com [127.0.0.1]); Fri, 20 Sep 2013 06:46:55 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Commit-ID: 3029ede39373c368f402a76896600d85a4f7121b Gitweb: http://git.kernel.org/tip/3029ede39373c368f402a76896600d85a4f7121b Author: Vladimir Davydov AuthorDate: Sun, 15 Sep 2013 17:49:14 +0400 Committer: Ingo Molnar CommitDate: Fri, 20 Sep 2013 11:59:38 +0200 sched/balancing: Fix 'local->avg_load > busiest->avg_load' case in fix_small_imbalance() In busiest->group_imb case we can come to fix_small_imbalance() with local->avg_load > busiest->avg_load. This can result in wrong imbalance fix-up, because there is the following check there where all the members are unsigned: if (busiest->avg_load - local->avg_load + scaled_busy_load_per_task >= (scaled_busy_load_per_task * imbn)) { env->imbalance = busiest->load_per_task; return; } As a result we can end up constantly bouncing tasks from one cpu to another if there are pinned tasks. Fix it by substituting the subtraction with an equivalent addition in the check. [ The bug can be caught by running 2*N cpuhogs pinned to two logical cpus belonging to different cores on an HT-enabled machine with N logical cpus: just look at se.nr_migrations growth. ] Signed-off-by: Vladimir Davydov Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/ef167822e5c5b2d96cf5b0e3e4f4bdff3f0414a2.1379252740.git.vdavydov@parallels.com Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0b99aae..2aedacc 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4823,8 +4823,8 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds) (busiest->load_per_task * SCHED_POWER_SCALE) / busiest->group_power; - if (busiest->avg_load - local->avg_load + scaled_busy_load_per_task >= - (scaled_busy_load_per_task * imbn)) { + if (busiest->avg_load + scaled_busy_load_per_task >= + local->avg_load + (scaled_busy_load_per_task * imbn)) { env->imbalance = busiest->load_per_task; return; }