Re: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 22-Mar 09:27, Joel Fernandes wrote:
> Hi,
> 
> On Tue, Mar 20, 2018 at 2:43 AM, Dietmar Eggemann
> <dietmar.eggemann@arm.com> wrote:
> >
> > From: Quentin Perret <quentin.perret@arm.com>

[...]

> > +static inline bool wake_energy(struct task_struct *p, int prev_cpu)
> > +{
> > +       struct sched_domain *sd;
> > +
> > +       if (!static_branch_unlikely(&sched_energy_present))
> > +               return false;
> > +
> > +       sd = rcu_dereference_sched(cpu_rq(prev_cpu)->sd);
> > +       if (!sd || sd_overutilized(sd))
> 
> Shouldn't you check for the SD_ASYM_CPUCAPACITY flag here?

I think that should be already covered by the static key check
above...

> 
> > +               return false;
> > +
> > +       return true;
> > +}
> > +
> >  /*
> >   * select_task_rq_fair: Select target runqueue for the waking task in domains
> >   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> > @@ -6529,18 +6583,22 @@ static int
> >  select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
> >  {
> >         struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
> > +       struct sched_domain *energy_sd = NULL;
> >         int cpu = smp_processor_id();
> >         int new_cpu = prev_cpu;
> > -       int want_affine = 0;
> > +       int want_affine = 0, want_energy = 0;
> >         int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> >
> > +       rcu_read_lock();
> > +
> >         if (sd_flag & SD_BALANCE_WAKE) {
> >                 record_wakee(p);
> > +               want_energy = wake_energy(p, prev_cpu);
> >                 want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > -                             && cpumask_test_cpu(cpu, &p->cpus_allowed);
> > +                             && cpumask_test_cpu(cpu, &p->cpus_allowed)
> > +                             && !want_energy;
> >         }
> >
> > -       rcu_read_lock();
> >         for_each_domain(cpu, tmp) {
> >                 if (!(tmp->flags & SD_LOAD_BALANCE))
> >                         break;
> > @@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >                         break;
> >                 }
> >
> > +               /*
> > +                * Energy-aware task placement is performed on the highest
> > +                * non-overutilized domain spanning over cpu and prev_cpu.
> > +                */
> > +               if (want_energy && !sd_overutilized(tmp) &&
> > +                   cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
> 
> Shouldn't you check for the SD_ASYM_CPUCAPACITY flag here for tmp level?

... and this then should be covered by the previous check in
wake_energy(), which sets want_energy.

> 
> > +                       energy_sd = tmp;
> > +
> >                 if (tmp->flags & sd_flag)
> >                         sd = tmp;
> >                 else if (!want_affine)
> > @@ -6586,6 +6652,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> >                         if (want_affine)
> >                                 current->recent_used_cpu = cpu;
> >                 }
> > +       } else if (energy_sd) {
> > +               new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
> 
> Even if want_affine = 0 (want_energy = 1), we can have sd = NULL if
> sd_flag and tmp->flags don't match. In this case we wont enter the EAS
> selection path because sd will be = NULL. So should you be setting sd
> = NULL explicitly if energy_sd != NULL , or rather do the if
> (energy_sd) before doing the if (!sd) ?

That's the same think I was also proposing in my reply to this patch.
But in my case the point was mainly to make the code easier to
follow... which at the end it's also to void all the consideration on
dependencies you describe above.

Joel, can you have a look at what I proposed... I was not entirely
sure that we miss some code paths doing it that way.

> If you still want to keep the logic this way, then probably you should
> also check if (tmp->flags & sd_flag) == true in the loop? That way
> energy_sd wont be set at all (Since we're basically saying we dont
> want to do wake up across this sd (in energy aware fashion in this
> case) if the domain flags don't watch the wake up sd_flag.
> 
> thanks,
> 
> - Joel

-- 
#include <best/regards.h>

Patrick Bellasi