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

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Date:   Fri, 14 Oct 2016 17:18:27 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Joseph Salisbury <joseph.salisbury@...onical.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Galbraith <efault@....de>
Subject: Re: [v4.8-rc1 Regression] sched/fair: Apply more PELT fixes

Le Friday 14 Oct 2016 à 14:10:07 (+0100), Dietmar Eggemann a écrit :
> On 14/10/16 09:24, Vincent Guittot wrote:
> > On 13 October 2016 at 23:34, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> >> On 13 October 2016 at 20:49, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>> On 13/10/16 17:48, Vincent Guittot wrote:
> >>>> On 13 October 2016 at 17:52, Joseph Salisbury
> >>>> <joseph.salisbury@...onical.com> wrote:
> >>>>> On 10/13/2016 06:58 AM, Vincent Guittot wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> On 12 October 2016 at 18:21, Joseph Salisbury
> >>>>>> <joseph.salisbury@...onical.com> wrote:
> >>>>>>> On 10/12/2016 08:20 AM, Vincent Guittot wrote:
> >>>>>>>> On 8 October 2016 at 13:49, Mike Galbraith <efault@....de> wrote:
> >>>>>>>>> On Sat, 2016-10-08 at 13:37 +0200, Vincent Guittot wrote:
> >>>>>>>>>> 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:
> 
> [...]
> 
> >>> When I create a tg_root/tg_x/tg_y_1 and a tg_root/tg_x/tg_y_2 group, the tg_x->load_avg
> >>> becomes > 6*1024 before any tasks ran in it.
> >>
> >> This is normal as se->avg.load_avg is initialized to
> >> scale_load_down(se->load.weight) and this se->avg.load_avg will be
> >> added to tg_x[cpu]->cfs_rq->avg.load_avg when attached to the cfs_rq
> 
> Yeah, you right, even when I've created 50 second level groups,
> tg_x->load_avg is ~6800.
> 
> Could it have something to do with the fact that .se->load.weight = 2
> for all these task groups? on a 64bit system?

I don't think so, the problem really comes from tg->load_avg = 381697
but sum of cfs_rq[cpu]->tg_load_avg_contrib = 1013 which is << tg->load_avg
and cfs_rq[cpu]->tg_load_avg_contrib == cfs_rq[cpu]->avg.load_avg so we can't 
expect any negative delta to remove this large value

> 
> In case we call  __update_load_avg(..., se->on_rq *
> scale_load_down(se->load.weight), ...) we pass a weight argument of 0
> for these se's.
> 
> Does not happen with:
> 
> -       if (shares < MIN_SHARES)
> -               shares = MIN_SHARES;
> +       if (shares < scale_load(MIN_SHARES))
> +               shares = scale_load(MIN_SHARES);
> 
> in  calc_cfs_shares().
> 
> [...]
> 
>