[PATCH v9 14/15] sched/fair: Select an energy-efficient CPU on task wake-up

[Quentin Perret]

If an Energy Model (EM) is available and if the system isn't
overutilized, re-route waking tasks into an energy-aware placement
algorithm. The selection of an energy-efficient CPU for a task
is achieved by estimating the impact on system-level active energy
resulting from the placement of the task on the CPU with the highest
spare capacity in each performance domain. This strategy spreads tasks
in a performance domain and avoids overly aggressive task packing. The
best CPU energy-wise is then selected if it saves a large enough amount
of energy with respect to prev_cpu.

Although it has already shown significant benefits on some existing
targets, this approach cannot scale to platforms with numerous CPUs.
This is an attempt to do something useful as writing a fast heuristic
that performs reasonably well on a broad spectrum of architectures isn't
an easy task. As such, the scope of usability of the energy-aware
wake-up path is restricted to systems with the SD_ASYM_CPUCAPACITY flag
set, and where the EM isn't too complex.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Quentin Perret <quentin.perret@xxxxxxx>
---
 kernel/sched/fair.c | 143 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 141 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a20018ad9236..d73e7db5976a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6453,6 +6453,137 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
 	return energy;
 }
 
+/*
+ * find_energy_efficient_cpu(): Find most energy-efficient target CPU for the
+ * waking task. find_energy_efficient_cpu() looks for the CPU with maximum
+ * spare capacity in each performance domain and uses it as a potential
+ * candidate to execute the task. Then, it uses the Energy Model to figure
+ * out which of the CPU candidates is the most energy-efficient.
+ *
+ * The rationale for this heuristic is as follows. In a performance domain,
+ * all the most energy efficient CPU candidates (according to the Energy
+ * Model) are those for which we'll request a low frequency. When there are
+ * several CPUs for which the frequency request will be the same, we don't
+ * have enough data to break the tie between them, because the Energy Model
+ * only includes active power costs. With this model, if we assume that
+ * frequency requests follow utilization (e.g. using schedutil), the CPU with
+ * the maximum spare capacity in a performance domain is guaranteed to be among
+ * the best candidates of the performance domain.
+ *
+ * In practice, it could be preferable from an energy standpoint to pack
+ * small tasks on a CPU in order to let other CPUs go in deeper idle states,
+ * but that could also hurt our chances to go cluster idle, and we have no
+ * ways to tell with the current Energy Model if this is actually a good
+ * idea or not. So, find_energy_efficient_cpu() basically favors
+ * cluster-packing, and spreading inside a cluster. That should at least be
+ * a good thing for latency, and this is consistent with the idea that most
+ * of the energy savings of EAS come from the asymmetry of the system, and
+ * not so much from breaking the tie between identical CPUs. That's also the
+ * reason why EAS is enabled in the topology code only for systems where
+ * SD_ASYM_CPUCAPACITY is set.
+ *
+ * NOTE: 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.
+ */
+
+static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
+{
+	unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX;
+	struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
+	int cpu, best_energy_cpu = prev_cpu;
+	struct perf_domain *head, *pd;
+	unsigned long cpu_cap, util;
+	struct sched_domain *sd;
+
+	rcu_read_lock();
+	pd = rcu_dereference(rd->pd);
+	if (!pd || READ_ONCE(rd->overutilized))
+		goto fail;
+	head = pd;
+
+	/*
+	 * Energy-aware wake-up happens on the lowest sched_domain starting
+	 * from sd_asym_cpucapacity spanning over this_cpu and prev_cpu.
+	 */
+	sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity));
+	while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
+		sd = sd->parent;
+	if (!sd)
+		goto fail;
+
+	sync_entity_load_avg(&p->se);
+	if (!task_util_est(p))
+		goto unlock;
+
+	for (; pd; pd = pd->next) {
+		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)) {
+			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);
+				best_energy = min(best_energy, prev_energy);
+				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;
+			}
+		}
+
+		/* 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;
+			}
+		}
+	}
+unlock:
+	rcu_read_unlock();
+
+	/*
+	 * 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)
+		return best_energy_cpu;
+
+	if ((prev_energy - best_energy) > (prev_energy >> 4))
+		return best_energy_cpu;
+
+	return prev_cpu;
+
+fail:
+	rcu_read_unlock();
+
+	return -1;
+}
+
 /*
  * 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,
@@ -6476,8 +6607,16 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 
 	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);
+
+		if (static_branch_unlikely(&sched_energy_present)) {
+			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
+			if (new_cpu >= 0)
+				return new_cpu;
+			new_cpu = prev_cpu;
+		}
+
+		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
+			      cpumask_test_cpu(cpu, &p->cpus_allowed);
 	}
 
 	rcu_read_lock();
-- 
2.19.1