linux-kernel - Re: [PATCH] sched/fair: Speed-up energy-aware wake-ups

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Date:   Fri, 20 Sep 2019 11:41:15 +0200
From:   Quentin Perret <qperret@...rret.net>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, juri.lelli@...hat.com, rjw@...ysocki.net,
        morten.rasmussen@....com, valentin.schneider@....com,
        qais.yousef@....com, tkjos@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Speed-up energy-aware wake-ups

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 <qperret@...rret.net>
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 <pkondeti@...eaurora.org>
Signed-off-by: Quentin Perret <qperret@...rret.net>
---
 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