From: Pavan Kondeti <pkondeti@codeaurora.org>
To: Quentin Perret <qperret@qperret.net>
Cc: peterz@infradead.org, mingo@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
juri.lelli@redhat.com, rjw@rjwysocki.net,
morten.rasmussen@arm.com, valentin.schneider@arm.com,
qais.yousef@arm.com, tkjos@google.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair: Speed-up energy-aware wake-ups
Date: Fri, 20 Sep 2019 16:03:38 +0530 [thread overview]
Message-ID: <20190920103338.GB20250@codeaurora.org> (raw)
In-Reply-To: <20190920094115.GA11503@qperret.net>
Hi Quentin,
On Fri, Sep 20, 2019 at 11:41:15AM +0200, Quentin Perret wrote:
> 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.
Yes. Selecting the prev_cpu is the correct decision.
>
> 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@qperret.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@codeaurora.org>
> Signed-off-by: Quentin Perret <qperret@qperret.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
>
+1. Looks good to me.
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
next prev parent reply other threads:[~2019-09-20 10:33 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-12 9:44 [PATCH] sched/fair: Speed-up energy-aware wake-ups Quentin Perret
2019-09-13 22:43 ` [tip: sched/core] " tip-bot2 for Quentin Perret
2019-09-20 3:02 ` [PATCH] " Pavan Kondeti
2019-09-20 9:41 ` Quentin Perret
2019-09-20 10:33 ` Pavan Kondeti [this message]
2019-09-20 11:23 ` Quentin Perret
2019-09-25 8:41 ` Peter Zijlstra
2019-09-27 8:10 ` [tip: sched/urgent] sched/fair: Avoid redundant EAS calculation tip-bot2 for Quentin Perret
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