[RFC PATCH v4 10/12] sched/fair: Select an energy-efficient CPU on task wake-up

[Quentin Perret <quentin.perret@arm.com>]

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 frequency domain. This strategy spreads tasks in
a frequency 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@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
---
 kernel/sched/fair.c | 121 ++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 117 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f50c4e83a488..e85fc017cc22 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6690,6 +6690,110 @@ static long compute_energy(struct task_struct *p, int dst_cpu,
 	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 frequency 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 frequency 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 frequency domain is guaranteed to be among
+ * the best candidates of the frequency 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.
+ */
+static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu,
+							struct freq_domain *fd)
+{
+	unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX;
+	int cpu, best_energy_cpu = prev_cpu;
+	struct freq_domain *head = fd;
+	unsigned long cpu_cap, util;
+	struct sched_domain *sd;
+
+	sync_entity_load_avg(&p->se);
+
+	if (!task_util_est(p))
+		return prev_cpu;
+
+	/*
+	 * Energy-aware wake-up happens on the lowest sched_domain starting
+	 * from sd_ea spanning over this_cpu and prev_cpu.
+	 */
+	sd = rcu_dereference(*this_cpu_ptr(&sd_ea));
+	while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
+		sd = sd->parent;
+	if (!sd)
+		return prev_cpu;
+
+	while (fd) {
+		unsigned long cur_energy, spare_cap, max_spare_cap = 0;
+		int max_spare_cap_cpu = -1;
+
+		for_each_cpu_and(cpu, freq_domain_span(fd), 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);
+				if (prev_energy < best_energy)
+					best_energy = prev_energy;
+				continue;
+			}
+
+			/* Find the CPU with the max spare cap the fd. */
+			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;
+			}
+		}
+		fd = fd->next;
+	}
+
+	/*
+	 * We pick the best CPU only if it saves at least 6% of the
+	 * energy used by prev_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,
@@ -6706,18 +6810,26 @@ 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;
 	}
 
-	rcu_read_lock();
 	for_each_domain(cpu, tmp) {
 		if (!(tmp->flags & SD_LOAD_BALANCE))
 			break;
@@ -6752,6 +6864,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 		if (want_affine)
 			current->recent_used_cpu = cpu;
 	}
+unlock:
 	rcu_read_unlock();
 
 	return new_cpu;
-- 
2.17.1