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

[Hongyan Xia]

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@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?

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