linux-kernel - [PATCH v2 1/2] sched/fair: Remove SCHED_FEAT(UTIL_EST

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Message-Id: <20231201161652.1241695-2-vincent.guittot@linaro.org>
Date:   Fri,  1 Dec 2023 17:16:51 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     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, hongyan.xia2@....com, yizhou.tang@...pee.com,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v2 1/2] sched/fair: Remove SCHED_FEAT(UTIL_EST_FASTUP, true)

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>
Reviewed-and-tested-by: Lukasz Luba <lukasz.luba@....com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Reviewed-by: Hongyan Xia <hongyan.xia2@....com>
Reviewed-by: Tang Yizhou <yizhou.tang@...pee.com>
---
 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