Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding actual_cpu_capacity

[Vincent Guittot]

On Mon, 17 Jun 2024 at 12:53, Qais Yousef <qyousef@xxxxxxxxxxx> wrote:
>
> On 06/17/24 11:07, Vincent Guittot wrote:
>
> > > And should effective_cpu_util() return a value higher than
> > > get_actual_cpu_capacity()?
> >
> > I don't think we should because we want to return the effective
> > utilization not the actual compute capacity.
> > Having an utilization of the cpu or group of cpus above the actual
> > capacity or the original capacity mainly means that we will have to
> > run longer
> >
> > By capping the utilization we filter this information.
> >
> > capacity orig = 800
> > util_avg = 700
> >
> > if we cap the capacity to 400 the cpu is expected to run twice longer
> > for the same amount of work to be done
>
> Okay makes sense. Wouldn't the util be 'wrong' (to what degree will depend on
> min/max freq ratio) though?
>
> We cap with arch_scale_capacity() still, I guess we know at this stage it is
> 100% wrong if we allow returning higher values?

I think that capping utilization to max capacity generates some energy
estimation error because it filters the fact that we run longer in
some cases.