Re: [RESEND PATCH] sched/fair: consider RT/IRQ pressure in select_idle_sibling

[Rohit Jain]

On 02/05/2018 10:42 PM, Joel Fernandes wrote:
> On Tue, Jan 30, 2018 at 11:47 AM, Rohit Jain <rohit.k.jain@xxxxxxxxxx> wrote:
> [...]
> > > > @@ -6102,7 +6107,8 @@ static int select_idle_core(struct task_struct *p,
> > > > struct sched_domain *sd, int
> > > >     */
> > > >    static int select_idle_smt(struct task_struct *p, struct sched_domain
> > > > *sd, int target)
> > > >    {
> > > > -       int cpu;
> > > > +       int cpu, rcpu = -1;
> > > > +       unsigned long max_cap = 0;
> > > >
> > > >           if (!static_branch_likely(&sched_smt_present))
> > > >                   return -1;
> > > > @@ -6110,11 +6116,13 @@ static int select_idle_smt(struct task_struct *p,
> > > > struct sched_domain *sd, int t
> > > >           for_each_cpu(cpu, cpu_smt_mask(target)) {
> > > >                   if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > > >                           continue;
> > > > -               if (idle_cpu(cpu))
> > > > -                       return cpu;
> > > > +               if (idle_cpu(cpu) && (capacity_of(cpu) > max_cap)) {
> > > > +                       max_cap = capacity_of(cpu);
> > > > +                       rcpu = cpu;
> > > At the SMT level, do you need to bother with choosing best capacity
> > > among threads? If RT is eating into one of the SMT thread's underlying
> > > capacity, it would eat into the other's. Wondering what's the benefit
> > > of doing this here.
> >
> > Yes, you are right because of SD_SHARE_CPUCAPACITY, however the benefit
> > is that if don't do this check, we might end up picking a SMT thread
> > which has "high" RT/IRQ activity and be on the run queue for a while,
> > till the pull side can bail us out.
> Do your tests show a difference in results though with such change
> (for select_idle_smt)?

I don't have the numbers readily available, but I did see a measurable
difference with the select_idle_smt changes.

Thanks,
Rohit

> thanks,
>
> - Joel