[RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up

[Dietmar Eggemann]

From: Quentin Perret <quentin.perret@xxxxxxx>

In case an energy model is available, waking tasks are re-routed into a
new energy-aware placement algorithm. The eligible CPUs to be used in the
energy-aware wakeup path are restricted to the highest non-overutilized
sched_domain containing prev_cpu and this_cpu. If no such domain is found,
the tasks go through the usual wake-up path, hence energy-aware placement
happens only in lightly utilized scenarios.

The selection of the most 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 each candidate CPU. 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 brute force approach clearly cannot scale to platforms with
numerous CPUs. This patch 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 a consequence, the scope of usability
of the energy-aware wake-up path is restricted to systems with the
SD_ASYM_CPUCAPACITY flag set. These systems not only show the most
promising opportunities for saving energy but also typically feature a
limited number of logical CPUs.

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Quentin Perret <quentin.perret@xxxxxxx>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
---
 kernel/sched/fair.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 71 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 76bd46502486..65a1bead0773 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6513,6 +6513,60 @@ static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
 	return energy;
 }
 
+static bool task_fits(struct task_struct *p, int cpu)
+{
+	unsigned long next_util = cpu_util_next(cpu, p, cpu);
+
+	return util_fits_capacity(next_util, capacity_orig_of(cpu));
+}
+
+static int find_energy_efficient_cpu(struct sched_domain *sd,
+					struct task_struct *p, int prev_cpu)
+{
+	unsigned long cur_energy, prev_energy, best_energy;
+	int cpu, best_cpu = prev_cpu;
+
+	if (!task_util(p))
+		return prev_cpu;
+
+	/* Compute the energy impact of leaving the task on prev_cpu. */
+	prev_energy = best_energy = compute_energy(p, prev_cpu);
+
+	/* Look for the CPU that minimizes the energy. */
+	for_each_cpu_and(cpu, &p->cpus_allowed, sched_domain_span(sd)) {
+		if (!task_fits(p, cpu) || cpu == prev_cpu)
+			continue;
+		cur_energy = compute_energy(p, cpu);
+		if (cur_energy < best_energy) {
+			best_energy = cur_energy;
+			best_cpu = cpu;
+		}
+	}
+
+	/*
+	 * We pick the best CPU only if it saves at least 1.5% of the
+	 * energy used by prev_cpu.
+	 */
+	if ((prev_energy - best_energy) > (prev_energy >> 6))
+		return best_cpu;
+
+	return prev_cpu;
+}
+
+static inline bool wake_energy(struct task_struct *p, int prev_cpu)
+{
+	struct sched_domain *sd;
+
+	if (!static_branch_unlikely(&sched_energy_present))
+		return false;
+
+	sd = rcu_dereference_sched(cpu_rq(prev_cpu)->sd);
+	if (!sd || sd_overutilized(sd))
+		return false;
+
+	return true;
+}
+
 /*
  * 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,
@@ -6529,18 +6583,22 @@ static int
 select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
 {
 	struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
+	struct sched_domain *energy_sd = NULL;
 	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_energy = wake_energy(p, prev_cpu);
 		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
-			      && cpumask_test_cpu(cpu, &p->cpus_allowed);
+			      && cpumask_test_cpu(cpu, &p->cpus_allowed)
+			      && !want_energy;
 	}
 
-	rcu_read_lock();
 	for_each_domain(cpu, tmp) {
 		if (!(tmp->flags & SD_LOAD_BALANCE))
 			break;
@@ -6555,6 +6613,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 			break;
 		}
 
+		/*
+		 * Energy-aware task placement is performed on the highest
+		 * non-overutilized domain spanning over cpu and prev_cpu.
+		 */
+		if (want_energy && !sd_overutilized(tmp) &&
+		    cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
+			energy_sd = tmp;
+
 		if (tmp->flags & sd_flag)
 			sd = tmp;
 		else if (!want_affine)
@@ -6586,6 +6652,8 @@ 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;
 		}
+	} else if (energy_sd) {
+		new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
 	} else {
 		new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
 	}
-- 
2.11.0