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SubjectRe: [PATCH v5 12/14] sched/fair: Select an energy-efficient CPU on task wake-up
On Thursday 02 Aug 2018 at 15:54:26 (+0200), Peter Zijlstra wrote:
> On Tue, Jul 24, 2018 at 01:25:19PM +0100, Quentin Perret wrote:
> > @@ -6385,18 +6492,26 @@ static int
> > select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
> > {
> > struct sched_domain *tmp, *sd = NULL;
> > + struct freq_domain *fd;
> > 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_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > - && cpumask_test_cpu(cpu, &p->cpus_allowed);
> > + fd = rd_freq_domain(cpu_rq(cpu)->rd);
> > + want_energy = fd && !READ_ONCE(cpu_rq(cpu)->rd->overutilized);
> > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
> > + cpumask_test_cpu(cpu, &p->cpus_allowed);
> > + }
> > +
> > + if (want_energy) {
> > + new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
> > + goto unlock;
> > }
> >
>
> And I suppose you rely on the compiler to optimize that for the static
> key inside rd_freq_domain()... Does it do a good job of that?

I does for sure when CONFIG_ENERGY_MODEL=n since rd_freq_domain() is
stubbed to false, but that's an easy one ;-)

>
> That is, would not something like:
>
>
> rcu_read_lock();
> if (sd_flag & SD_BALANCE_WAKE) {
> record_wakee(p);
>
> if (static_branch_unlikely(&sched_energy_present)) {
> struct root_domain *rd = cpu_rq(cpu)->rd;
> struct freq_domain *fd = rd_freq_domain(rd);
>
> if (fd && !READ_ONCE(rd->overutilized)) {
> new_cpu = find_energy_efficient_cpu(p, prev_cpu, fd);
> goto unlock;
> }
> }
>
> /* ... */
> }
>
>
> Be far more clear ?

It is clearer. Having the static key check in rd_freq_domain() makes
the change to find_busiest_group() smaller, but I can totally change it
with something like the above.

I'll do that in v6.

Thanks,
Quentin

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