linux-kernel - Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

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Message-ID: <CAKfTPtDbqLGRG1iD7NbTOrrg-9RqfFpt6JN_uB9FcWBSWkzrGw@mail.gmail.com>
Date:   Sat, 8 Oct 2016 13:37:57 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Joseph Salisbury <joseph.salisbury@...onical.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

On 8 October 2016 at 10:39, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Peter Zijlstra <peterz@...radead.org> 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 <peterz@...radead.org>
>> > 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

>>
>> That commit doesn't revert cleanly, did you take out more?
>
> Note that it reverts cleanly from v4.8 - while it does to revert from current
> upstream that did more changes in that area.
>
> I suspect Josheph tested a v4.8-ish kernel.
>
>> I'll try and have a look asap, but I'm traveling next week so it might a
>> tad slower than normal.
>>
>> If you could provide a /proc/sched_debug dump while the thing is running
>> that'd might be useful.
>
> Also, running the latest scheduler bits would be useful:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
>
> ... just on the off chance that another change in this area fixed the bug, plus to
> make it easier to send test patches.
>
> Upstream merge commit af79ad2b1f33 should also be pretty safe to try - it's v4.8
> with the v4.9 scheduler bits applied.
>
> Thanks,
>
>         Ingo