linux-kernel - Re: [PATCH] sched/documentation: elaborate on uclamp limitations

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Date:   Thu, 18 May 2023 13:42:39 +0100
From:   Hongyan Xia <hongyan.xia2@....com>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     Jonathan Corbet <corbet@....net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched/documentation: elaborate on uclamp limitations

Hi Qais,

On 2023-05-18 12:30, Qais Yousef wrote:
> Please CC sched maintainers (Ingo + Peter) next time as they should pick this
> up ultimately and they won't see it from the list only.

Will do. I was using the get_maintainers script and I thought that gave 
me all the CCs.

> On 05/05/23 16:24, Hongyan Xia wrote:
>> The story in 5.2 about util_avg abruptly jumping from 300 when
>> Fmax/Fmin == 3 to 1024 when Fmax/Fmin == 4 hides some details about how
>> clock_pelt works behind the scenes. Explicitly mention it to make it
>> easier for readers to follow.
>>
>> Signed-off-by: Hongyan Xia <hongyan.xia2@....com>
>> Cc: Qais Yousef <qyousef@...alina.io>
>> Cc: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>   Documentation/scheduler/sched-util-clamp.rst | 17 +++++++++++++++++
>>   1 file changed, 17 insertions(+)
>>
>> diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
>> index 74d5b7c6431d..524df07bceba 100644
>> --- a/Documentation/scheduler/sched-util-clamp.rst
>> +++ b/Documentation/scheduler/sched-util-clamp.rst
>> @@ -669,6 +669,19 @@ but not proportional to Fmax/Fmin.
>>   
>>           p0->util_avg = 300 + small_error
>>   
>> +The reason why util_avg is around 300 even though it runs for 900 at Fmin is:
>> +Although running at Fmin reduces the rate of rq_clock_pelt() to 1/3 thus
>> +accumulates util_sum at 1/3 of the rate at Fmax, the clock period
>> +(rq_clock_pelt() now minus previous rq_clock_pelt()) in:
>> +
>> +::
>> +
>> +        util_sum / clock period = util_avg
>> +
>> +does not shrink to 1/3, since rq->clock_pelt is periodically synchronized with
>> +rq->clock_task as long as there's idle time. As a result, we get util_avg of
>> +about 300, not 900.
>> +
> 
> I feel neutral about these changes. It does answer some questions, but poses
> more questions like what is clock_pelt. So we might end up in recursive
> regression of explaining the explanation.
> 
> I don't think we have a doc about clock_pelt. Worth adding one and just add
> a reference to it from here for those interested in understanding more details
> on why we need to go to idle to correct util_avg? I think our code has
> explanation, a reference to update_rq_clock_pelt() might suffice too.
> 
> Vincent, do you have an opinion here?

Sounds reasonable. I don't mind drafting a doc or just a couple of 
paragraphs for clock_pelt (or all the different clocks like clock, 
clock_task, clock_idle_*), if that's what we can agree on.

Hongyan