Re: [PATCH] sched/fair: Remove the duplicate check from group_has_capacity()

[Qi Zheng]

On 2020/8/11 上午2:33, Valentin Schneider wrote:
> On 10/08/20 02:00, Qi Zheng wrote:
> > 1. The group_has_capacity() function is only called in
> >     group_classify().
> > 2. The following inequality has already been checked in
> >     group_is_overloaded() which was also called in
> >     group_classify().
> >
> >        (sgs->group_capacity * imbalance_pct) <
> >                          (sgs->group_runnable * 100)
>
> Consider group_is_overloaded() returns false because of the first
> condition:
>
>          if (sgs->sum_nr_running <= sgs->group_weight)
>                  return false;
>
> then group_has_capacity() would be the first place where the group_runnable
> vs group_capacity comparison would be done.
>
> Now in that specific case we'll actually only check it if
>
>    sgs->sum_nr_running == sgs->group_weight
>
> and the only case where the runnable vs capacity check can fail here is if
> there's significant capacity pressure going on. TBH this capacity pressure
> could be happening even when there are fewer tasks than CPUs, so I'm not
> sure how intentional that corner case is.

Maybe some cpus in sg->cpumask are no longer active at the == case,
which causes the significant capacity pressure?

> For the
>
>      sgs->sum_nr_running > sgs->group_weight
>
> case I agree with your patch, there just is that oddity at the == case.
>
> > So just remove the duplicate check from group_has_capacity().
> >
> > Signed-off-by: Qi Zheng <arch0.zheng@xxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 4 ----
> >   1 file changed, 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 2ba8f230feb9..a41903fb327a 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8234,10 +8234,6 @@ group_has_capacity(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
> >        if (sgs->sum_nr_running < sgs->group_weight)
> >                return true;
> >
> > -	if ((sgs->group_capacity * imbalance_pct) <
> > -			(sgs->group_runnable * 100))
> > -		return false;
> > -
> >        if ((sgs->group_capacity * 100) >
> >                        (sgs->group_util * imbalance_pct))
> >                return true;