Re: [PATCH v2 1/2] sched/fair: Remove SCHED_FEAT(UTIL_EST_FASTUP, true)

[Alex Shi]

Nice cleanup.

Reviewed-by: Alex Shi <alexs@xxxxxxxxxx>

On Sat, Dec 2, 2023 at 12:17 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> sched_feat(UTIL_EST_FASTUP) has been added to easily disable the feature
> in order to check for possibly related regressions. After 3 years, it has
> never been used and no regression has been reported. Let remove it
> and make fast increase a permanent behavior.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Reviewed-and-tested-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Reviewed-by: Hongyan Xia <hongyan.xia2@xxxxxxx>
> Reviewed-by: Tang Yizhou <yizhou.tang@xxxxxxxxxx>
> ---
>  Documentation/scheduler/schedutil.rst                    | 7 +++----
>  Documentation/translations/zh_CN/scheduler/schedutil.rst | 7 +++----
>  kernel/sched/fair.c                                      | 8 +++-----
>  kernel/sched/features.h                                  | 1 -
>  4 files changed, 9 insertions(+), 14 deletions(-)
>
> diff --git a/Documentation/scheduler/schedutil.rst b/Documentation/scheduler/schedutil.rst
> index 32c7d69fc86c..803fba8fc714 100644
> --- a/Documentation/scheduler/schedutil.rst
> +++ b/Documentation/scheduler/schedutil.rst
> @@ -90,8 +90,8 @@ For more detail see:
>   - Documentation/scheduler/sched-capacity.rst:"1. CPU Capacity + 2. Task utilization"
>
>
> -UTIL_EST / UTIL_EST_FASTUP
> -==========================
> +UTIL_EST
> +========
>
>  Because periodic tasks have their averages decayed while they sleep, even
>  though when running their expected utilization will be the same, they suffer a
> @@ -99,8 +99,7 @@ though when running their expected utilization will be the same, they suffer a
>
>  To alleviate this (a default enabled option) UTIL_EST drives an Infinite
>  Impulse Response (IIR) EWMA with the 'running' value on dequeue -- when it is
> -highest. A further default enabled option UTIL_EST_FASTUP modifies the IIR
> -filter to instantly increase and only decay on decrease.
> +highest. UTIL_EST filters to instantly increase and only decay on decrease.
>
>  A further runqueue wide sum (of runnable tasks) is maintained of:
>
> diff --git a/Documentation/translations/zh_CN/scheduler/schedutil.rst b/Documentation/translations/zh_CN/scheduler/schedutil.rst
> index d1ea68007520..7c8d87f21c42 100644
> --- a/Documentation/translations/zh_CN/scheduler/schedutil.rst
> +++ b/Documentation/translations/zh_CN/scheduler/schedutil.rst
> @@ -89,16 +89,15 @@ r_cpu被定义为当前CPU的最高性能水平与系统中任何其它CPU的最
>   - Documentation/translations/zh_CN/scheduler/sched-capacity.rst:"1. CPU Capacity + 2. Task utilization"
>
>
> -UTIL_EST / UTIL_EST_FASTUP
> -==========================
> +UTIL_EST
> +========
>
>  由于周期性任务的平均数在睡眠时会衰减，而在运行时其预期利用率会和睡眠前相同，
>  因此它们在再次运行后会面临（DVFS）的上涨。
>
>  为了缓解这个问题，（一个默认使能的编译选项）UTIL_EST驱动一个无限脉冲响应
>  （Infinite Impulse Response，IIR）的EWMA，“运行”值在出队时是最高的。
> -另一个默认使能的编译选项UTIL_EST_FASTUP修改了IIR滤波器，使其允许立即增加，
> -仅在利用率下降时衰减。
> +UTIL_EST滤波使其在遇到更高值时立刻增加，而遇到低值时会缓慢衰减。
>
>  进一步，运行队列的（可运行任务的）利用率之和由下式计算：
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bcea3d55d95d..e94d65da8d66 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4870,11 +4870,9 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
>          * to smooth utilization decreases.
>          */
>         ue.enqueued = task_util(p);
> -       if (sched_feat(UTIL_EST_FASTUP)) {
> -               if (ue.ewma < ue.enqueued) {
> -                       ue.ewma = ue.enqueued;
> -                       goto done;
> -               }
> +       if (ue.ewma < ue.enqueued) {
> +               ue.ewma = ue.enqueued;
> +               goto done;
>         }
>
>         /*
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index a3ddf84de430..143f55df890b 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -83,7 +83,6 @@ SCHED_FEAT(WA_BIAS, true)
>   * UtilEstimation. Use estimated CPU utilization.
>   */
>  SCHED_FEAT(UTIL_EST, true)
> -SCHED_FEAT(UTIL_EST_FASTUP, true)
>
>  SCHED_FEAT(LATENCY_WARN, false)
>
> --
> 2.34.1
>