Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu

[Barry Song]

On Thu, Nov 25, 2021 at 12:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 24, 2021 at 05:15:46PM +0800, Barry Song wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 6e476f6..8cd23f1 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6278,6 +6278,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >               time = cpu_clock(this);
> >       }
> >
> > +     --nr;
> >       for_each_cpu_wrap(cpu, cpus, target + 1) {
> >               if (has_idle_core) {
> >                       i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > @@ -6285,11 +6286,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >                               return i;
> >
> >               } else {
> > -                     if (!--nr)
> > -                             return -1;
> >                       idle_cpu = __select_idle_cpu(cpu, p);
> >                       if ((unsigned int)idle_cpu < nr_cpumask_bits)
> >                               break;
> > +                     if (!--nr)
> > +                             return -1;
> >               }
> >       }
>
> This way nr can never be 1 for a single iteration -- it current isn't,
> but that's besides the point.

Yep. nr=1 seems to be a corner case.
if nr=1, the original code will return -1 directly without scanning
any cpu. but the new code will scan
one  time because I haven't checked if(!--nr)  and return before
for_each_cpu_wrap(). so this changes
the behaviour for this corner case.

but if i change "--nr" to "nr--", this new code will scan nr  times
rather than nr-1, this changes the behaviour
for all cases besides nr!=1. The original code was looping nr times
but scanning nr-1 times only.

so you prefer here the codes should scan nr times and change the
scanning amount from nr-1
to nr?

Thanks
Barry