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:

quoted

@@ -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