sched/fair: Avoid redundant EAS calculation

The EAS wake-up path computes the system energy for several CPU
candidates: the CPU with maximum spare capacity in each performance
domain, and the prev_cpu. However, if prev_cpu also happens to be the
CPU with maximum spare capacity in its performance domain, the energy
calculation is still done twice, unnecessarily.

Add a condition to filter out this corner case before doing the energy
calculation.

Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Quentin Perret <qperret@qperret.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: juri.lelli@redhat.com
Cc: morten.rasmussen@arm.com
Cc: qais.yousef@arm.com
Cc: rjw@rjwysocki.net
Cc: tkjos@google.com
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Fixes: eb92692b2544 ("sched/fair: Speed-up energy-aware wake-ups")
Link: https://lkml.kernel.org/r/20190920094115.GA11503@qperret.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dfdac90fd211648fbf8cc64593d36582ebf53d53..83ab35e2374f632e0211029d7e1baa70150c388a 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6389,7 +6389,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                }
 
                /* Evaluate the energy impact of using this CPU. */
-               if (max_spare_cap_cpu >= 0) {
+               if (max_spare_cap_cpu >= 0 && max_spare_cap_cpu != prev_cpu) {
                        cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
                        cur_delta -= base_energy_pd;
                        if (cur_delta < best_delta) {