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: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20221006081052.3862167-2-pierre.gondois@arm.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 919d016c5d77d7580bf4b24863aa055923824e5c..4cc56c91e06e610592b00ce3c946db08ecf87b57 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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 */