Commit ad841e56 authored by Pierre Gondois's avatar Pierre Gondois Committed by Peter Zijlstra
Browse files

sched/fair: Check if prev_cpu has highest spare cap in feec() 



When evaluating the CPU candidates in the perf domain (pd) containing
the previously used CPU (prev_cpu), find_energy_efficient_cpu()
evaluates the energy of the pd:
- without the task (base_energy)
- with the task placed on prev_cpu (if the task fits)
- with the task placed on the CPU with the highest spare capacity,
  prev_cpu being excluded from this set

If prev_cpu is already the CPU with the highest spare capacity,
max_spare_cap_cpu will be the CPU with the second highest spare
capacity.

On an Arm64 Juno-r2, with a workload of 10 tasks at a 10% duty cycle,
when prev_cpu and max_spare_cap_cpu are both valid candidates,
prev_spare_cap > max_spare_cap at ~82%.
Thus the energy of the pd when placing the task on max_spare_cap_cpu
is computed with no possible positive outcome 82% most of the time.

Do not consider max_spare_cap_cpu as a valid candidate if
prev_spare_cap > max_spare_cap.

Signed-off-by: default avatarPierre Gondois <pierre.gondois@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20221006081052.3862167-2-pierre.gondois@arm.com
parent aa69c36f

kernel/sched/fair.c

+7 −6
Original line number Diff line number Diff line
@@ -7221,7 +7221,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) 
		unsigned long cur_delta, max_spare_cap = 0;

		unsigned long rq_util_min, rq_util_max;

		unsigned long util_min, util_max;

		bool compute_prev_delta = false;

		unsigned long prev_spare_cap = 0;

		int max_spare_cap_cpu = -1;

		unsigned long base_energy;
@@ -7283,18 +7283,19 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) 


			if (cpu == prev_cpu) {

				/* Always use prev_cpu as a candidate. */

				compute_prev_delta = true;

				prev_spare_cap = cpu_cap;

			} else if (cpu_cap > max_spare_cap) {

				/*

				 * Find the CPU with the maximum spare capacity

				 * in the performance domain.

				 * among the remaining CPUs in the performance

				 * domain.

				 */

				max_spare_cap = cpu_cap;

				max_spare_cap_cpu = cpu;

			}

		}



		if (max_spare_cap_cpu < 0 && !compute_prev_delta)

		if (max_spare_cap_cpu < 0 && prev_spare_cap == 0)

			continue;



		eenv_pd_busy_time(&eenv, cpus, p);
@@ -7302,7 +7303,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) 
		base_energy = compute_energy(&eenv, pd, cpus, p, -1);



		/* Evaluate the energy impact of using prev_cpu. */

		if (compute_prev_delta) {

		if (prev_spare_cap > 0) {

			prev_delta = compute_energy(&eenv, pd, cpus, p,

						    prev_cpu);

			/* CPU utilization has changed */
@@ -7313,7 +7314,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) 
		}



		/* Evaluate the energy impact of using max_spare_cap_cpu. */

		if (max_spare_cap_cpu >= 0) {

		if (max_spare_cap_cpu >= 0 && max_spare_cap > prev_spare_cap) {

			cur_delta = compute_energy(&eenv, pd, cpus, p,

						   max_spare_cap_cpu);

			/* CPU utilization has changed */