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

Peter Zijlstra
2018-10-04T09:51:56Z

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?

> + 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.

> + 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.

> + }
> + 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;

> + 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().