Re: [PATCH] sched/fair: prefer prev cpu in asymmetric wakeup path

[Vincent Guittot]

On Thu, 22 Oct 2020 at 19:46, Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
>
> On 22/10/20 16:33, Vincent Guittot wrote:
> > On Thu, 22 Oct 2020 at 16:53, Valentin Schneider
> > <valentin.schneider@xxxxxxx> wrote:
> >> > @@ -6170,7 +6170,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >> >   * maximize capacity.
> >> >   */
> >> >  static int
> >> > -select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >> > +select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int prev, int target)
> >> >  {
> >> >       unsigned long best_cap = 0;
> >> >       int cpu, best_cpu = -1;
> >> > @@ -6178,9 +6178,22 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> >> >
> >> >       sync_entity_load_avg(&p->se);
> >> >
> >> > +     if ((available_idle_cpu(target) || sched_idle_cpu(target)) &&
> >> > +         task_fits_capacity(p, capacity_of(target)))
> >> > +             return target;
> >> > +
> >>
> >> I think we still need to check for CPU affinity here.
> >
> > yes good point
> >
> >>
> >> >       cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> >> >       cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >> >
> >> > +     /*
> >> > +      * If the previous CPU belongs to this asymmetric domain and is idle,
> >> > +      * check it 1st as it's the best candidate.
> >> > +      */
> >> > +     if (prev != target && cpumask_test_cpu(prev, cpus) &&
> >> > +         (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> >> > +         task_fits_capacity(p, capacity_of(prev)))
> >> > +             return prev;
> >> > +
> >> >       for_each_cpu_wrap(cpu, cpus, target) {
> >>
> >> So we prioritize target over prev, like the rest of the
> >> select_idle_sibling() family. Here however we apply the same acceptability
> >> function to target, prev and the loop body, so perhaps we could simplify
> >> this to:
> >
> > My 1st implementation was similar to you proposal below but i finally
> > decided to strictly follow the same sequence as symmetric which:
> > - checks target
> > - then prev cpu
> > - and finally uses target as the starting point of the loop for
> > looking for another cpu
> >
> > Using prev as the starting point of the loop will change which cpu
> > will be selected but I don't have a strong opinion if this will make a
> > real difference at the end because bit position doesn't imply any
> > relation with others cpus.
> >
>
> Yep, also one difference with the symmetric path here is that the first
> checks against target & prev use exactly the same criteria as the loop
> body, so we shouldn't feel shy about doing this here.
>
> > So I'm fine to go with your proposal below
> >
> > Also I wonder if i should also add the test of p->recent_used_cpu and
> > the per cpu kthread optimization, which benefit XFS IIRC.
> >
>
> If we head down that route it would be nice to reuse the existing
> conditions (rather than copy and tweak them) and move the asymmetric loop
> further down. Maybe with something like (with a better name though):

Yes, That would ensure that asymmetric will stay align symmetric.

I 'm going to look at this for the next version

>
>   static inline bool asym_task_fits_capacity(struct task_struct *p, int cpu)
>   {
>           if (!static_branch_unlikely(&sched_asym_cpucapacity))
>                   return true;
>
>           return task_fits_capacity(p, capacity_of(cpu));
>   }
>
> and we could && that to the existing cases. Food for thought.
>
> >>
> >>   if (accept(target))
> >>       return target;
> >>
> >>   ...
> >>
> >>   for_each_cpu_wrap(cpu, cpus, prev) {
> >>       ...
> >>   }
> >>
> >> That way we evaluate target twice only if it isn't a direct candidate
> >> (but might be a fallback one).
> >>
> >> >               unsigned long cpu_cap = capacity_of(cpu);
> >> >
> >> > @@ -6223,7 +6236,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >> >               if (!sd)
> >> >                       goto symmetric;
> >> >
> >> > -             i = select_idle_capacity(p, sd, target);
> >> > +             i = select_idle_capacity(p, sd, prev, target);
> >> >               return ((unsigned)i < nr_cpumask_bits) ? i : target;
> >> >       }