linux-kernel - Re: [PATCH v3 1/3] sched/fair: fix rounding issue for asym packing

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Message-ID: <CAKfTPtBjfSTXqt4tks6eBzXi+w1wCOBQ23BJ2wcSzVWAQiW9oA@mail.gmail.com>
Date:   Thu, 20 Dec 2018 15:22:10 +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 v3 1/3] sched/fair: fix rounding issue for asym packing

On Thu, 20 Dec 2018 at 12:16, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 20/12/2018 07:55, Vincent Guittot wrote:
> > When check_asym_packing() is triggered, the imbalance is set to :
> >   busiest_stat.avg_load * busiest_stat.group_capacity / SCHED_CAPACITY_SCALE
> > But busiest_stat.avg_load equals
> >   sgs->group_load *SCHED_CAPACITY_SCALE / sgs->group_capacity
> > These divisions can generate a rounding that will make imbalance slightly
> > lower than the weighted load of the cfs_rq.
> > But this is enough to skip the rq in find_busiest_queue and prevents asym
> > migration to happen.
> >
> > Directly set imbalance to sgs->group_load to remove the rounding.
>                             ^^^^^^^^^^^^^^^
> I see where that's coming from, but using 'sgs' here is tad confusing since
> 'sds->busiest_stat' is what's actually used.
>
> Maybe just something like 'the busiest's sgs->group_load' would be good
> enough to make things explicit.
>
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >  kernel/sched/fair.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index ca46964..9b31247 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8476,9 +8476,7 @@ static int check_asym_packing(struct lb_env *env, struct sd_lb_stats *sds)
> >       if (sched_asym_prefer(busiest_cpu, env->dst_cpu))
> >               return 0;
> >
> > -     env->imbalance = DIV_ROUND_CLOSEST(
> > -             sds->busiest_stat.avg_load * sds->busiest_stat.group_capacity,
> > -             SCHED_CAPACITY_SCALE);
> > +     env->imbalance = sds->busiest_stat.avg_load;
>
> That should be group_load, not avg_load. With that fixed:

ah yes... too quick at changing it

>
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>
>
> >
> >       return 1;
> >  }
> >