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From:   Xuewen Yan <xuewen.yan94@gmail.com>
Date:   Fri, 22 Oct 2021 12:05:27 +0800
Message-ID: <CAB8ipk_1-ZMMcaLiLGYBT3N3S_VdeGMhYmCqGytcAwmPE4Ni2g@mail.gmail.com>
Subject: Re: [RFC PATCH] sched/fair: use max_spare_cap_cpu if it is more
 energy efficient
To:     brookxu <brookxu.cn@gmail.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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Hi Chunguang

brookxu <brookxu.cn@gmail.com> =E4=BA=8E2021=E5=B9=B410=E6=9C=8821=E6=97=A5=
=E5=91=A8=E5=9B=9B =E4=B8=8B=E5=8D=884:24=E5=86=99=E9=81=93=EF=BC=9A
>
> From: Chunguang Xu <brookxu@tencent.com>
>
> When debugging EAS, I found that if the task is migrated to
> max_spare_cap_cpu, even if the power consumption of pd is lower,
> we still put the task on prev_cpu. Maybe we should fix it.
>
> Signed-off-by: Chunguang Xu <brookxu@tencent.com>
> ---
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff69f245b939..2ae7e03de6d2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6867,8 +6867,10 @@ static int find_energy_efficient_cpu(struct task_s=
truct *p, int prev_cpu)
>                 /* Evaluate the energy impact of using max_spare_cap_cpu.=
 */
>                 if (max_spare_cap_cpu >=3D 0) {
>                         cur_delta =3D compute_energy(p, max_spare_cap_cpu=
, pd);
> -                       if (cur_delta < base_energy_pd)

this is aimed to prevent the cur_delta < 0, and usuallly, when the
task was put on the max_spare_cpu, the cur_power should be bigger than
base_pd_power,
if the cur_power < base_pd_power, the cpu util may have changed, at
this time, we should keep prev_cpu.

You can look at below discuss and patch:
https://lore.kernel.org/all/20210429101948.31224-3-Pierre.Gondois@arm.com/
https://lore.kernel.org/all/CAB8ipk_vgtg5d1obH36BYfNLZosbwr2k_U3xnAD4=3DH5u=
Zt_M_g@mail.gmail.com/


> +                       if (cur_delta < base_energy_pd) {
> +                               target =3D max_spare_cap_cpu;
>                                 goto unlock;
> +                       }
>                         cur_delta -=3D base_energy_pd;
>                         if (cur_delta < best_delta) {
>                                 best_delta =3D cur_delta;
> --
> 2.30.0
>