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

[Qais Yousef]

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.

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@xxxxxxx>
> Cc: Qais Yousef <qyousef@xxxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  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?


Thanks!

--
Qais Yousef

>  Now if the ratio of Fmax/Fmin is 4, the maximum value becomes:
>  
>  ::
> @@ -682,6 +695,10 @@ this happens, then the _actual_ util_avg will become:
>  
>          p0->util_avg = 1024
>  
> +This is because rq->clock_pelt is no longer synchronized with the task clock.
> +The clock period therefore is proportionally shrunk by the same ratio of
> +(Fmax/Fmin), giving us a maximal util_avg of 1024.
> +
>  If task p1 wakes up on this CPU, which have:
>  
>  ::
> -- 
> 2.34.1
>