Re: [PATCH v7 12/14] sched/fair: Select an energy-efficient CPU on task wake-up

Peter Zijlstra
2018-10-04T10:41:34Z

On Thu, Oct 04, 2018 at 11:27:22AM +0100, Quentin Perret wrote:
> > > + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
> >
> > Which of the two masks do we expect to be the smallest?
>
> Typically, perf_domain_span is smaller.

OK, then the above expression is in the right order :-)

> > > + if (spare_cap > max_spare_cap) {
> > > + max_spare_cap = spare_cap;
> > > + max_spare_cap_cpu = cpu;
> > > + }
> >
> > Sometimes I wonder if something like:
> >
> > #define min_filter(varp, val) \
> > ({ \
> > typeof(varp) _varp = (varp); \
> > typeof(val) _val = (val); \
> > bool f = false; \
> > \
> > if (_val < *_varp) { \
> > *_varp = _val; \
> > f = true; \
> > } \
> > \
> > f; \
> > })
> >
> > and the corresponding max_filter() are worth the trouble; it would allow
> > writing:
> >
> > if (max_filter(&max_spare_cap, spare_cap))
> > max_spare_cap_cpu = cpu;
> >
> > and:
> >
> > > + }
> > > +
> > > + /* Evaluate the energy impact of using this CPU. */
> > > + if (max_spare_cap_cpu >= 0) {
> > > + cur_energy = compute_energy(p, max_spare_cap_cpu, head);
> > > + if (cur_energy < best_energy) {
> > > + best_energy = cur_energy;
> > > + best_energy_cpu = max_spare_cap_cpu;
> > > + }
> >
> > if (min_filter(&best_energy, cur_energy))
> > best_energy_cpu = max_spare_cap_cpu;
> >
> > But then I figure, it is not... dunno. We do lots of this stuff.
>
> If there are occurrences of this stuff all over the place, we could do
> that in a separate clean-up patch that does just that, for the entire
> file. Or maybe more ?

Sure, not something that needs done now. I just always think of this
when I see this pattern repeated, but never seem to get around to doing
anything about it.

I figured I'd mention it ;-)

> > I would much prefer this to be something like:
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index a8f601edd958..5475a885ec9f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6299,12 +6299,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> > {
> > struct sched_domain *tmp, *sd = NULL;
> > int cpu = smp_processor_id();
> > - int new_cpu = prev_cpu;
> > + unsigned int new_cpu = prev_cpu;
> > int want_affine = 0;
> > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> >
> > if (sd_flag & SD_BALANCE_WAKE) {
> > record_wakee(p);
> > +
> > + if (static_branch_unlikely(sched_eas_balance)) {
> > + new_cpu = select_task_rq_eas(p, prev_cpu, sd_flags, wake_flags);
> > + if (new_cpu < nr_cpu_ids)
> > + return new_cpu;
> > + }
> > +
> > want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> > && cpumask_test_cpu(cpu, &p->cpus_allowed);
> > }
> > and then hide everything (including that giant comment) in
> > select_task_rq_eas().
>
> So you think we should rename find_energy_efficient_cpu and put all the
> checks in there ? Or should select_task_rq_eas do the checks and then
> call find_energy_efficient_cpu ?
>
> Not a huge deal, but that'll save some time if we agree on that one
> upfront.

Not sure, see what it looks like ;-) My main concern here was to get rid
of that giant blob in select_task_rq_fair().