sched/fair: Only compute base_energy_pd if necessary

find_energy_efficient_cpu() searches the best energy CPU
to place a task on. To do so, the energy of each performance domain
(pd) is computed w/ and w/o the task placed on it.

The energy of a pd w/o the task (base_energy_pd) is computed prior
knowing whether a CPU is available in the pd.

Move the base_energy_pd computation after looping through the CPUs
of a pd and only compute it if at least one CPU is available.

Suggested-by: Xuewen Yan <xuewen.yan@unisoc.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Donnefort <vincent.donnefort@arm.com>
Link: https://lkml.kernel.org/r/20210504090743.9688-2-Pierre.Gondois@arm.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d10c6cc..b229d0c 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6687,13 +6687,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
 
        for (; pd; pd = pd->next) {
                unsigned long cur_delta, spare_cap, max_spare_cap = 0;
+               bool compute_prev_delta = false;
                unsigned long base_energy_pd;
                int max_spare_cap_cpu = -1;
 
-               /* Compute the 'base' energy of the pd, without @p */
-               base_energy_pd = compute_energy(p, -1, pd);
-               base_energy += base_energy_pd;
-
                for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
                        if (!cpumask_test_cpu(cpu, p->cpus_ptr))
                                continue;
@@ -6714,25 +6711,35 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                        if (!fits_capacity(util, cpu_cap))
                                continue;
 
-                       /* Always use prev_cpu as a candidate. */
                        if (cpu == prev_cpu) {
-                               prev_delta = compute_energy(p, prev_cpu, pd);
-                               prev_delta -= base_energy_pd;
-                               best_delta = min(best_delta, prev_delta);
-                       }
-
-                       /*
-                        * Find the CPU with the maximum spare capacity in
-                        * the performance domain
-                        */
-                       if (spare_cap > max_spare_cap) {
+                               /* Always use prev_cpu as a candidate. */
+                               compute_prev_delta = true;
+                       } else if (spare_cap > max_spare_cap) {
+                               /*
+                                * Find the CPU with the maximum spare capacity
+                                * in the performance domain.
+                                */
                                max_spare_cap = spare_cap;
                                max_spare_cap_cpu = cpu;
                        }
                }
 
-               /* Evaluate the energy impact of using this CPU. */
-               if (max_spare_cap_cpu >= 0 && max_spare_cap_cpu != prev_cpu) {
+               if (max_spare_cap_cpu < 0 && !compute_prev_delta)
+                       continue;
+
+               /* Compute the 'base' energy of the pd, without @p */
+               base_energy_pd = compute_energy(p, -1, pd);
+               base_energy += base_energy_pd;
+
+               /* Evaluate the energy impact of using prev_cpu. */
+               if (compute_prev_delta) {
+                       prev_delta = compute_energy(p, prev_cpu, pd);
+                       prev_delta -= base_energy_pd;
+                       best_delta = min(best_delta, prev_delta);
+               }
+
+               /* Evaluate the energy impact of using max_spare_cap_cpu. */
+               if (max_spare_cap_cpu >= 0) {
                        cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
                        cur_delta -= base_energy_pd;
                        if (cur_delta < best_delta) {