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SubjectRe: [PATCH v3 1/3] sched/fair: fix rounding issue for asym packing
On Thu, 20 Dec 2018 at 12:16, Valentin Schneider
<valentin.schneider@arm.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@linaro.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@arm.com>
>
> >
> > return 1;
> > }
> >

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