Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932560AbcJMPxC (ORCPT ); Thu, 13 Oct 2016 11:53:02 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:48116 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756550AbcJMPwt (ORCPT ); Thu, 13 Oct 2016 11:52:49 -0400 Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes To: Vincent Guittot References: <57F7F9AF.2010609@canonical.com> <20161008080019.GE3142@twins.programming.kicks-ass.net> <20161008083936.GA13658@gmail.com> <1475927349.5573.1.camel@gmx.de> <57FE6302.6060103@canonical.com> Cc: Ingo Molnar , Peter Zijlstra , Linus Torvalds , Thomas Gleixner , LKML , Mike Galbraith From: Joseph Salisbury Message-ID: <57FFADC8.2020602@canonical.com> Date: Thu, 13 Oct 2016 11:52:40 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: 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 Content-Length: 3573 Lines: 82 On 10/13/2016 06:58 AM, Vincent Guittot wrote: > Hi, > > On 12 October 2016 at 18:21, Joseph Salisbury > wrote: >> On 10/12/2016 08:20 AM, Vincent Guittot wrote: >>> On 8 October 2016 at 13:49, Mike Galbraith wrote: >>>> On Sat, 2016-10-08 at 13:37 +0200, Vincent Guittot wrote: >>>>> On 8 October 2016 at 10:39, Ingo Molnar wrote: >>>>>> * Peter Zijlstra wrote: >>>>>> >>>>>>> On Fri, Oct 07, 2016 at 03:38:23PM -0400, Joseph Salisbury wrote: >>>>>>>> Hello Peter, >>>>>>>> >>>>>>>> A kernel bug report was opened against Ubuntu [0]. After a >>>>>>>> kernel >>>>>>>> bisect, it was found that reverting the following commit >>>>>>>> resolved this bug: >>>>>>>> >>>>>>>> commit 3d30544f02120b884bba2a9466c87dba980e3be5 >>>>>>>> Author: Peter Zijlstra >>>>>>>> Date: Tue Jun 21 14:27:50 2016 +0200 >>>>>>>> >>>>>>>> sched/fair: Apply more PELT fixes >>>>> This patch only speeds up the update of task group load in order to >>>>> reflect the new load balance but It should not change the final value >>>>> and as a result the final behavior. I will try to reproduce it in my >>>>> target later today >>>> FWIW, I tried and failed w/wo autogroup on 4.8 and master. >>> Me too >>> >>> Is it possible to get some dump of /proc/sched_debug while the problem occurs ? >>> >>> Vincent >>> >>>> -Mike >> The output from /proc/shed_debug can be seen here: >> http://paste.ubuntu.com/23312351/ > I have looked at the dump and there is something very odd for > system.slice task group where the display manager is running. > system.slice->tg_load_avg is around 381697 but tg_load_avg is > normally equal to Sum of system.slice[cpu]->tg_load_avg_contrib > whereas Sum of system.slice[cpu]->tg_load_avg_contrib = 1013 in our > case. We can have some differences because the dump of > /proc/shed_debug is not atomic and some changes can happen but nothing > like this difference. > > The main effect of this quite high value is that the weight/prio of > the sched_entity that represents system.slice in root cfs_rq is very > low (lower than task with the smallest nice prio) so the system.slice > task group will not get the CPU quite often compared to the user.slice > task group: less than 1% for the system.slice where lightDM and xorg > are running compared 99% for the user.slice where the stress tasks are > running. This is confirmed by the se->avg.util_avg value of the task > groups which reflect how much time each task group is effectively > running on a CPU: > system.slice[CPU3].se->avg.util_avg = 8 whereas > user.slice[CPU3].se->avg.util_avg = 991 > > This difference of weight/priority explains why the system becomes > unresponsive. For now, I can't explain is why > system.slice->tg_load_avg = 381697 whereas is should be around 1013 > and how the patch can generate this situation. > > Is it possible to have a dump of /proc/sched_debug before starting > stress command ? to check if the problem is there from the beginning > but not seen because not overloaded. Or if it the problem comes when > user starts to load the system Here is the dump before stress is started: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1627108/+attachment/4760437/+files/dump_nonbuggy Here it is after: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1627108/+attachment/4760436/+files/dump_buggy > > Thanks, > >> Ingo, the latest scheduler bits also still exhibit the bug: >> git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git >> >>