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 4:02 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 25, 2021 at 01:02:00AM +1300, Barry Song wrote:
> > On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> > > Let me make it clearer. if nr=5, the original code will  loop 5 times,
> > > but in the 5th loop, it returns directly, so  __select_idle_cpu is
> > > only done 4 times.
> > >
> > > if nr=1, the original code will  loop 1 time, but in the 1st loop,
> > > it returns directly, so  __select_idle_cpu is  done 0 times.
> >
> > this is also why in the first version of patch, i did this:
> >                 span_avg = sd->span_weight * avg_idle;
> >                 if (span_avg > 4*avg_cost)
> > -                       nr = div_u64(span_avg, avg_cost);
> > +                       nr = div_u64(span_avg, avg_cost) - 1;
> >                 else
> > -                       nr = 4;
> > +                       nr = 3;
> >
> > because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1
> > times but not 4 times or div_u64(span_avg, avg_cost) times.
>
> It still is confusing, because > 4*span -> nr = avg/span, very much
> implies we want to bottom out at 4.
>
> > this is not confusing at all. the only thing which is confusing is the original
> > code.
>
> But yes, it seems a whole lot of confusion stacked together. Let make it
> sane and say that we do 'nr' iterations, because clearly that was the
> intent :-)

yes. It seems this is much more sensible to do iterations in the
number of nr rather than
nr-1.  we can achieve this goal by two ways:

(1)
        nr--;

        for_each_cpu_wrap(cpu, cpus, target + 1) {
                _select_idle_cpu()....
                if (!nr--)
                                return;
        }

(2)
        for_each_cpu_wrap(cpu, cpus, target + 1) {
                _select_idle_cpu()....
                if (!--nr)
                                return;
        }

it seems the second way is still better as we don't need the "nr--" before
for_each_cpu_wrap() ?

Thanks
Barry