Re: [PATCH v3 3/3] [RFC] CPUFreq: Add support for cpu-perf-dependencies

[Lukasz Luba]

On 11/17/20 10:11 AM, Viresh Kumar wrote:
> On 16-11-20, 11:33, Lukasz Luba wrote:
> > On 11/9/20 6:57 AM, Viresh Kumar wrote:
> > > On 06-11-20, 11:14, Lukasz Luba wrote:
> > > > I also had similar doubts, because if we make frequency requests
> > > > independently for each CPU, why not having N cooling devs, which
> > > > will set independently QoS max freq for them...
> > > >
> > > > What convinced me:
> > > > EAS and FIE would know the 'real' frequency of the cluster, IPA
> > > > can use it also and have only one cooling device per cluster.
> > > >
> > > > We would like to keep this old style 'one cooling device per cpuset'.
> > > > I don't have strong opinion and if it would appear that there are
> > > > some errors in freq estimation for cluster, then maybe it does make
> > > > more sense to have cdev per CPU...
> > >
> > > Let me rephrase my question. What is it that doesn't work _correctly_
> > > with cdev per cpufreq policy in your case? What doesn't work well if
> > > the thermal stuff keeps looking at only the related_cpus thing and not
> > > the cpu-perf-dependencies thing?
> >
> > We don't have a platform which would be this per-cpu freq request, yet.
> > Thus it's hard to answer your question. The EAS would work in 'old
> > style' - cluster mode. I don't know how IPA would work on such HW
> > and SW configuration. To figure this out I need a real platform.
>
> Hmm, so who are going to be the users of this new stuff (dependent
> CPUs) ? I don't think cpufreq-cooling should be updated, unless there
> is a compelling reason to.

Fair enough. What if we come back with experiments results in future?
Currently we are trying to conduct experiments with Nicola on modified 
Juno firmware and kernel)

> The other one in energy model ? Why does it need this information ?

The energy model needs this information, because it's used during the
sched domain re-build. The sched domain is then used in the EAS main
functions: feec() [1] and compute_energy() [2].
What EAS normally does is 'trying different options to put a task
on different CPUs and observe the potential energy costs'.
Example, we need the 'cluster' frequency, because when you put a task on
a CPU its freq might need to be set a bit higher. This would affect all
CPUs in the cluster not only one and we capture that energy cost. Then,
we try to put a task on other CPU in that cluster and if it appears to
be no need of rising freq for the whole cluster, then it wins.


> Who else ?

FIE..