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

Quentin Perret
2018-10-04T10:27:31Z

On Thursday 04 Oct 2018 at 11:44:12 (+0200), Peter Zijlstra wrote:
> On Wed, Sep 12, 2018 at 10:13:07AM +0100, Quentin Perret wrote:
> > + while (pd) {
> > + unsigned long cur_energy, spare_cap, max_spare_cap = 0;
> > + int max_spare_cap_cpu = -1;
> > +
> > + 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.

> > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > + continue;
> > +
> > + /* Skip CPUs that will be overutilized. */
> > + util = cpu_util_next(cpu, p, cpu);
> > + cpu_cap = capacity_of(cpu);
> > + if (cpu_cap * 1024 < util * capacity_margin)
> > + continue;
> > +
> > + /* Always use prev_cpu as a candidate. */
> > + if (cpu == prev_cpu) {
> > + prev_energy = compute_energy(p, prev_cpu, head);
> > + if (prev_energy < best_energy)
> > + best_energy = prev_energy;
>
> best_energy = min(best_energy, prev_energy);
>
> That's both shorter and clearer.

OK.

> > + continue;
> > + }
> > +
> > + /*
> > + * Find the CPU with the maximum spare capacity in
> > + * the performance domain
> > + */
> > + spare_cap = cpu_cap - util;
> > + 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 ?

> > + }
> > + pd = pd->next;
> > + }
> > +
> > + /*
> > + * Pick the best CPU if prev_cpu cannot be used, or if it saves at
> > + * least 6% of the energy used by prev_cpu.
> > + */
> > + if (prev_energy == ULONG_MAX ||
> > + (prev_energy - best_energy) > (prev_energy >> 4))
> > + return best_energy_cpu;
>
> Does that become more readable if we split that into two conditions?
>
> if (prev_energy == ULONG_MAX)
> return best_energy_cpu;
>
> if ((prev_energy - best_energy) > (prev_energy >> 4))
> return best_energy_cpu;

Yeah, why not :-)

> > + return prev_cpu;
> > +}
> > +
> > /*
> > * 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,
> > @@ -6360,13 +6468,37 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> > int want_affine = 0;
> > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> >
> > + rcu_read_lock();
> > if (sd_flag & SD_BALANCE_WAKE) {
> > record_wakee(p);
> > +
> > + /*
> > + * Forkees are not accepted in the energy-aware wake-up path
> > + * because they don't have any useful utilization data yet and
> > + * it's not possible to forecast their impact on energy
> > + * consumption. Consequently, they will be placed by
> > + * find_idlest_cpu() on the least loaded CPU, which might turn
> > + * out to be energy-inefficient in some use-cases. The
> > + * alternative would be to bias new tasks towards specific types
> > + * of CPUs first, or to try to infer their util_avg from the
> > + * parent task, but those heuristics could hurt other use-cases
> > + * too. So, until someone finds a better way to solve this,
> > + * let's keep things simple by re-using the existing slow path.
> > + */
> > + if (sched_feat(ENERGY_AWARE)) {
> > + struct root_domain *rd = cpu_rq(cpu)->rd;
> > + struct perf_domain *pd = rcu_dereference(rd->pd);
> > +
> > + if (pd && !READ_ONCE(rd->overutilized)) {
> > + new_cpu = find_energy_efficient_cpu(p, prev_cpu, pd);
> > + goto unlock;
> > + }
> > + }
> > +
> > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
> > + cpumask_test_cpu(cpu, &p->cpus_allowed);
> > }
>
> 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.

Thanks