linux-kernel - Re: [PATCH v2 1/3] sched/fair: fix rounding issue for asym packing

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Date:   Wed, 19 Dec 2018 16:54:02 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2 1/3] sched/fair: fix rounding issue for asym packing

On Wed, 19 Dec 2018 at 16:30, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 19/12/2018 15:20, Vincent Guittot wrote:
> [...]
> >> Oh yes, I never said it didn't work - I was doing some investigation on
> >> the reason as to why we'd need this fix, because it's wasn't explicit from
> >> the commit message.
> >>
> >> The rounding errors are countered by the +1, yes, but I'd rather remove
> >> the errors altogether and go for the snippet I suggested in my previous
> >> reply.
> >
> > except that you don't always want to migrate all group load.
> > I prefer keeping current algorithm and fix it for now. Trying "new"
> > thing can come in a 2nd step
>
> We already set the imbalance as the whole group load, we just introduce
> rounding errors inbetween. As I've already said:
>
> in update_sg_lb_stats() we do:
>
>         sgs->avg_load = (sgs->group_load*SCHED_CAPACITY_SCALE) / sgs->group_capacity;
>
> and in check_asym_packing() we do:
>
>         env->imbalance = DIV_ROUND_CLOSEST(
>                 sds->busiest_stat.avg_load * sds->busiest_stat.group_capacity,
>                 SCHED_CAPACITY_SCALE)
>
> So we end up with something like:
>
>                     group_load * SCHED_CAPACITY_SCALE * group_capacity
>         imbalance = --------------------------------------------------
>                             group_capacity * SCHED_CAPACITY_SCALE
>
> Which we could reduce down to:
>
>         imbalance = group_load

ah yes, i thought the nr_running was involved but it's not the case.
This looks better indeed

>
> and not get any rounding errors.