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

[Vincent Guittot]

On Thu, 30 Nov 2023 at 21:30, Hongyan Xia <hongyan.xia2@xxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > ---
> >
> > 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：
>

Thanks.
I will add it to the next version

> >
> >   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@xxxxxxx>