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().