Commit 7736ae55 authored by Vincent Guittot's avatar Vincent Guittot Committed by Ingo Molnar
Browse files

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's remove it
and make fast increase a permanent behavior.

Signed-off-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Tested-by: default avatarLukasz Luba <lukasz.luba@arm.com>
Reviewed-by: default avatarLukasz Luba <lukasz.luba@arm.com>
Reviewed-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: default avatarHongyan Xia <hongyan.xia2@arm.com>
Reviewed-by: default avatarTang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn> [for the Chinese translation]
Reviewed-by: default avatarAlex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20231201161652.1241695-2-vincent.guittot@linaro.org
parent 1f023007

Documentation/scheduler/schedutil.rst

+3 −4
Original line number Diff line number Diff line
@@ -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:

Documentation/translations/zh_CN/scheduler/schedutil.rst

+3 −4
Original line number Diff line number Diff line
@@ -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：

kernel/sched/fair.c

+3 −5
Original line number Diff line number Diff line
@@ -4870,12 +4870,10 @@ 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;

	}

	}



	/*

	 * Skip update of task's estimated utilization when its members are

kernel/sched/features.h

+0 −1
Original line number Diff line number Diff line
@@ -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)