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Commit 54655c9f authored by Quentin Perret's avatar Quentin Perret
Browse files

FROMLIST: sched/fair: Select an energy-efficient CPU on task wake-up 



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.

Change-Id: I8c6384af904668f405319ed4e05054a7fa449192
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarQuentin Perret <quentin.perret@arm.com>
Message-Id: <20181016101513.26919-15-quentin.perret@arm.com>
Signed-off-by: default avatarQuentin Perret <quentin.perret@arm.com>
parent 0d548be9

kernel/sched/fair.c

+141 −2
Original line number Diff line number Diff line
@@ -6432,6 +6432,137 @@ 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,
@@ -6455,8 +6586,16 @@ 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();