linux-kernel - Re: [PATCH 1/2] sched/fair: Remove SCHED_FEAT(UTIL_EST

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Message-ID: <8009d5c4-4012-44a2-883c-0d7f74c4a2c1@arm.com>
Date:   Thu, 30 Nov 2023 20:30:01 +0000
From:   Hongyan Xia <hongyan.xia2@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        corbet@....net, alexs@...nel.org, siyanteng@...ngson.cn,
        qyousef@...alina.io, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     lukasz.luba@....com
Subject: Re: [PATCH 1/2] sched/fair: Remove SCHED_FEAT(UTIL_EST_FASTUP, true)

On 27/11/2023 14:32, Vincent Guittot 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@...aro.org>
> ---
> 
> I haven't updated the chinese documentation which also refers to
> UTIL_EST_FASTUP. IIUC, this will be updated afterward by the
> Translations' maintainers.

Here it is in case you have to provide the translation:

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：

> 
>   Documentation/scheduler/schedutil.rst | 7 +++----
>   kernel/sched/fair.c                   | 8 +++-----
>   kernel/sched/features.h               | 1 -
>   3 files changed, 6 insertions(+), 10 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/kernel/sched/fair.c b/kernel/sched/fair.c
> index 34fe6e9490c2..146329678cb8 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)
>   

Reviewed-by: Hongyan Xia <hongyan.xia2@....com>