Hi Pavan, On Friday 20 Sep 2019 at 08:32:15 (+0530), Pavan Kondeti wrote: > Earlier, we are not checking the spare capacity for the prev_cpu. Now that the > continue statement is removed, prev_cpu could also be the max_spare_cap_cpu. > Actually that makes sense. Because there is no reason why we want to select > another CPU which has less spare capacity than previous CPU. > > Is this behavior intentional?
The intent was indeed to not compute the energy for another CPU in prev_cpu's perf domain if prev_cpu is the one with max spare cap -- it is useless to do so since this other CPU cannot 'beat' prev_cpu and will never be chosen in the end. But I did miss that we'd end up computing the energy for prev_cpu twice ... Harmless but useless. So yeah, let's optimize that case too :) > When prev_cpu == max_spare_cap_cpu, we are evaluating the energy again for the > same CPU below. That could have been skipped by returning prev_cpu when > prev_cpu == max_spare_cap_cpu. Right, something like the patch below ? My test results are still looking good with it applied. Thanks for the careful review, Quentin --- >From 7b8258287f180a2c383ebe397e8129f5f898ffbe Mon Sep 17 00:00:00 2001 From: Quentin Perret <[email protected]> Date: Fri, 20 Sep 2019 09:07:20 +0100 Subject: [PATCH] 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 <[email protected]> Signed-off-by: Quentin Perret <[email protected]> --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d4bbf68c3161..7399382bc291 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6412,7 +6412,7 @@ 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) { -- 2.22.1
