linux-kernel - Re: [PATCH] fair/util_est: Separate util_est_dequeue() for cfs_rq_util

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Date:   Tue, 15 Dec 2020 18:56:48 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Xuewen Yan <Xuewen.Yan@...soc.com>,
        Ryan Y <xuewyan@...mail.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        王科 (Ke Wang) <Ke.Wang@...soc.com>
Subject: Re: [PATCH] fair/util_est: Separate util_est_dequeue() for
 cfs_rq_util_change

On 15/12/2020 13:56, Xuewen Yan wrote:
> On Tue, Dec 15, 2020 at 5:39 PM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
>>
>> On Mon, 14 Dec 2020 at 19:46, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>
>>> On 11/12/2020 13:03, Ryan Y wrote:

[...]

>>> It's true that 'sugov_get_util() -> cpu_util_cfs()' can use
>>> rq->cfs.avg.util_est.enqueued before _task_util_est(p) is subtracted
>>> from it.
>>>
>>> But isn't rq->cfs.avg.util_est.enqueued (in this case 601) always close
>>> to rq->cfs.avg.util_avg (597) since the task was just running?
>>> The cfs_rq utilization contains a blocked (sleeping) task.
>>
>> There will be a difference if the task alternates long and short runs
>> in which case util_avg is lower than util_est. But even in this case,
>> the freq will be update at next enqueue/dequeue/tick.
>> The only real case could be when cpu goes idle in shallow state (WFI)
>> which is impacted by the freq/voltage. But even in this case, the
>> situation should not be long
>>
>> That being said, I agree that the value used by schedutil is not
>> correct at dequeue

Ok, makes sense.

> Hi Dietmar,
> 
> as Vincent said, like the following scenario:
>                running                              sleep
> running        sleep
> |---------------------------------------|
>   |--------|
> 
>                   ^^
> at the ^^ time, the util_avg is lower than util_est.
> we hope that the CPU frequency would be reduced as soon as possible after
> the task is dequeued. But the issue affects the sensitivity of cpu
> frequency reduce.
> worse, since the time, if there is no task enqueue or other scenarios where the
> load is updated, the cpu may always maintain a high frequency.
> 
> if keep the util_est_dequeue as it is, are there other concerns,
> or this patch would affect other places of system?

I see. So essentially this could have an effect in task ramp-down and ramp-up scenarios.

periodic ramp-down task [8/16 -> 4/16 -> 1/16ms], 3 consecutive dequeue_task_fair():  

task0-0-1690  [000] 43.677788: sched_pelt_se:      cpu=0 path=(null) comm=task0-0 pid=1690 load=283 runnable=283 util=283 update_time=43333963776
task0-0-1690  [000] 43.677792: sched_pelt_cfs:     cpu=0 path=/ load=283 runnable=283 util=283 update_time=43333963776
task0-0-1690  [000] 43.677798: bprint:             sugov_get_util: CPU0 rq->cfs.avg.util_avg=283 rq->cfs.avg.util_est.enqueued=249
task0-0-1690  [000] 43.677803: sched_util_est_cfs: cpu=0 path=/ enqueued=0 ewma=0 util=283
task0-0-1690  [000] 43.677805: sched_util_est_se:  cpu=0 path=(null) comm=task0-0 pid=1690 enqueued=283 ewma=283 util=283
task0-0-1690  [000] 43.677810: bprint:             dequeue_task_fair: CPU0 [task0-0 1690] rq->cfs.avg.util_avg=[270->283] rq->cfs.avg.util_est.enqueued=[249->0]

task0-0-1690  [000] 43.698764: sched_pelt_se:      cpu=0 path=(null) comm=task0-0 pid=1690 load=247 runnable=248 util=248 update_time=43363011584
task0-0-1690  [000] 43.698768: sched_pelt_cfs:     cpu=0 path=/ load=248 runnable=248 util=248 update_time=43363011584
--> task0-0-1690  [000] 43.698774: bprint:             sugov_get_util: CPU0 rq->cfs.avg.util_avg=248 rq->cfs.avg.util_est.enqueued=283 <-- !!!
task0-0-1690  [000] 43.698778: sched_util_est_cfs: cpu=0 path=/ enqueued=0 ewma=0 util=248
task0-0-1690  [000] 43.698780: sched_util_est_se:  cpu=0 path=(null) comm=task0-0 pid=1690 enqueued=249 ewma=274 util=248
task0-0-1690  [000] 43.698785: bprint:             dequeue_task_fair: CPU0 [task0-0 1690] rq->cfs.avg.util_avg=[228->248] rq->cfs.avg.util_est.enqueued=[283->0]


task0-0-1690  [000] 43.714120: sched_pelt_se:      cpu=0 path=(null) comm=task0-0 pid=1690 load=183 runnable=183 util=183 update_time=43384443904
task0-0-1690  [000] 43.714125: sched_pelt_cfs:     cpu=0 path=/ load=183 runnable=183 util=183 update_time=43384443904
--> task0-0-1690  [000] 43.714131: bprint:             sugov_get_util: CPU0 rq->cfs.avg.util_avg=183 rq->cfs.avg.util_est.enqueued=275 <-- !!!
task0-0-1690  [000] 43.714135: sched_util_est_cfs: cpu=0 path=/ enqueued=0 ewma=0 util=183
task0-0-1690  [000] 43.714138: sched_util_est_se:  cpu=0 path=(null) comm=task0-0 pid=1690 enqueued=183 ewma=251 util=183
task0-0-1690  [000] 43.714142: bprint:             dequeue_task_fair: CPU0 [task0-0 1690] rq->cfs.avg.util_avg=[163->183] rq->cfs.avg.util_est.enqueued=[275->0]