Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755285Ab1DKLBI (ORCPT ); Mon, 11 Apr 2011 07:01:08 -0400 Received: from casper.infradead.org ([85.118.1.10]:33187 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753341Ab1DKLBH convert rfc822-to-8bit (ORCPT ); Mon, 11 Apr 2011 07:01:07 -0400 Subject: Re: [tip:sched/urgent] sched: Fix sched-domain avg_load calculation From: Peter Zijlstra To: mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org, kenchen@google.com, stable@kernel.org, tglx@linutronix.de, mingo@elte.hu Cc: linux-tip-commits@vger.kernel.org In-Reply-To: References: <20110408002322.3A0D812217F@elm.corp.google.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Mon, 11 Apr 2011 13:00:48 +0200 Message-ID: <1302519648.2388.19.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2070 Lines: 46 On Mon, 2011-04-11 at 10:46 +0000, tip-bot for Ken Chen wrote: > Commit-ID: b0432d8f162c7d5d9537b4cb749d44076b76a783 > Gitweb: http://git.kernel.org/tip/b0432d8f162c7d5d9537b4cb749d44076b76a783 > Author: Ken Chen > AuthorDate: Thu, 7 Apr 2011 17:23:22 -0700 > Committer: Ingo Molnar > CommitDate: Mon, 11 Apr 2011 11:08:54 +0200 > > sched: Fix sched-domain avg_load calculation > > In function find_busiest_group(), the sched-domain avg_load isn't > calculated at all if there is a group imbalance within the domain. This > will cause erroneous imbalance calculation. > > The reason is that calculate_imbalance() sees sds->avg_load = 0 and it > will dump entire sds->max_load into imbalance variable, which is used > later on to migrate entire load from busiest CPU to the puller CPU. > > This has two really bad effect: > > 1. stampede of task migration, and they won't be able to break out > of the bad state because of positive feedback loop: large load > delta -> heavier load migration -> larger imbalance and the cycle > goes on. > > 2. severe imbalance in CPU queue depth. This causes really long > scheduling latency blip which affects badly on application that > has tight latency requirement. > > The fix is to have kernel calculate domain avg_load in both cases. This > will ensure that imbalance calculation is always sensible and the target > is usually half way between busiest and puller CPU. > > Signed-off-by: Ken Chen > Signed-off-by: Peter Zijlstra > Cc: > Link: http://lkml.kernel.org/r/20110408002322.3A0D812217F@elm.corp.google.com > Signed-off-by: Ingo Molnar This was caused by 866ab43ef (sched: Fix the group_imb logic) which is only in .39-rc. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/