Subject: Re: [PATCH 2/4] sched/fair: Move avg_scan_cost calculations under SIS_PROP

From: Mel Gorman <[email-masked]>
Date: Wed, 9 Dec 2020 11:33:00 +0000

On Wed, Dec 09, 2020 at 07:07:11PM +0800, Li, Aubrey wrote:
> On 2020/12/9 17:05, Mel Gorman wrote:
> > On Wed, Dec 09, 2020 at 01:28:11PM +0800, Li, Aubrey wrote:
> >>>>                         nr = div_u64(span_avg, avg_cost);
> >>>>                 else
> >>>>                         nr = 4;
> >>>> -       }
> >>>> -
> >>>> -       time = cpu_clock(this);
> >>>>
> >>>> -       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >>>> +               time = cpu_clock(this);
> >>>> +       }
> >>>>
> >>>>         for_each_cpu_wrap(cpu, cpus, target) {
> >>>>                 if (!--nr)
> >>
> >> nr is the key of this throttling mechanism, need to be placed under sched_feat(SIS_PROP) as well.
> >>
> > 
> > It isn't necessary as nr in initialised to INT_MAX if !SIS_PROP.
> >If !SIS_PROP, nr need to -1 then tested in the loop, instead of testing directly.
> But with SIS_PROP, need adding a test in the loop.
> Since SIS_PROP is default true, I think it's okay to keep the current way.
> 

It's because it's default true and the cost is negligible that I'm leaving
it alone. The branch cost and nr accounting cost is negligible and it
avoids peppering select_idle_cpu() with too many SIS_PROP checks.

-- 
Mel Gorman
SUSE Labs

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