Re: [PATCH 5/5] sched/fair: Merge select_idle_core/cpu()

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

On Wed, Jan 20, 2021 at 09:54:20AM +0000, Mel Gorman wrote:
> On Wed, Jan 20, 2021 at 10:21:47AM +0100, Vincent Guittot wrote:
> > On Wed, 20 Jan 2021 at 10:12, Mel Gorman <mgorman@techsingularity.net> wrote:
> > >
> > > On Wed, Jan 20, 2021 at 02:00:18PM +0530, Gautham R Shenoy wrote:
> > > > > @@ -6157,18 +6169,31 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> > > > >     }
> > > > >
> > > > >     for_each_cpu_wrap(cpu, cpus, target) {
> > > > > -           if (!--nr)
> > > > > -                   return -1;
> > > > > -           if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> > > > > -                   break;
> > > > > +           if (smt) {
> > > > > +                   i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > > > > +                   if ((unsigned int)i < nr_cpumask_bits)
> > > > > +                           return i;
> > > > > +
> > > > > +           } else {
> > > > > +                   if (!--nr)
> > > > > +                           return -1;
> > > > > +                   i = __select_idle_cpu(cpu);
> > > > > +                   if ((unsigned int)i < nr_cpumask_bits) {
> > > > > +                           idle_cpu = i;
> > > > > +                           break;
> > > > > +                   }
> > > > > +           }
> > > > >     }
> > > > >
> > > > > -   if (sched_feat(SIS_PROP)) {
> > > > > +   if (smt)
> > > > > +           set_idle_cores(this, false);
> > > >
> > > > Shouldn't we set_idle_cores(false) only if this was the last idle
> > > > core in the LLC ?
> > > >
> > >
> > > That would involve rechecking the cpumask bits that have not been
> > > scanned to see if any of them are an idle core. As the existance of idle
> > > cores can change very rapidly, it's not worth the cost.
> > 
> > But don't we reach this point only if we scanned all CPUs and didn't
> > find an idle core ?

Indeed, I missed that part that we return as soon as we find an idle
core in the for_each_cpu_wrap() loop above. So here we clear the
"has_idle_cores" when there are no longer any idle-cores. Sorry for
the noise!


> 
> Yes, but my understanding of Gauthams suggestion was to check if an
> idle core found was the last idle core available and set has_idle_cores
> to false in that case.

That would have been nice, but since we do not keep a count of idle
cores, it is probably not worth the effort as you note below.

> I think this would be relatively expensive and
> possibly futile as returning the last idle core for this wakeup does not
> mean there will be no idle core on the next wakeup as other cores may go
> idle between wakeups.


> 
> -- 
> Mel Gorman
> SUSE Labs

--
Thanks and Regards
gautham.