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Subject[PATCH v2 1/2] sched/fair: Only compute base_energy_pd if necessary
Date
From: Pierre Gondois <Pierre.Gondois@arm.com>

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 in each pd.

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 computes 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>
---
kernel/sched/fair.c | 38 ++++++++++++++++++++++----------------
1 file changed, 22 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0dba0ebc3657..c5351366fb93 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6620,13 +6620,10 @@ 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;
@@ -6647,25 +6644,34 @@ 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;
}
}

+ 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;
+
+ 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 this CPU. */
- if (max_spare_cap_cpu >= 0 && max_spare_cap_cpu != prev_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) {
--
2.17.1
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