Re: [PATCH] base: arch_topology: Use policy->max to calculate freq_factor

[Lukasz Luba]

On 11/17/21 12:49 PM, Rafael J. Wysocki wrote:
> On Wed, Nov 17, 2021 at 11:46 AM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > Hi Rafael,
> >
> > On 11/16/21 7:05 PM, Rafael J. Wysocki wrote:
> > > On Mon, Nov 15, 2021 at 9:10 PM Thara Gopinath
> > > <thara.gopinath@xxxxxxxxxx> wrote:
> > > > cpuinfo.max_freq can reflect boost frequency if enabled during boot.  Since
> > > > we don't consider boost frequencies while calculating cpu capacities, use
> > > > policy->max to populate the freq_factor during boot up.
> > >
> > > I'm not sure about this.  schedutil uses cpuinfo.max_freq as the max frequency.
> >
> > Agree it's tricky how we treat the boost frequencies and also combine
> > them with thermal pressure.
> > We probably would have consider these design bits:
> > 1. Should thermal pressure include boost frequency?
>
> Well, I guess so.
>
> Running at a boost frequency certainly increases thermal pressure.
>
> > 2. Should max capacity 1024 be a boost frequency so scheduler
> >      would see it explicitly?
>
> That's what it is now if cpuinfo.max_freq is a boost frequency.
>
> > - if no, then schedutil could still request boost freq thanks to
> >     map_util_perf() where we add 25% to the util and then
> >     map_util_freq() would return a boost freq when util was > 1024
> >
> >
> > I can see in schedutil only one place when cpuinfo.max_freq is used:
> > get_next_freq(). If the value stored in there is a boost,
> > then don't we get a higher freq value for the same util?
>
> Yes. we do, which basically is my point.
>
> The schedutil's response is proportional to cpuinfo.max_freq and that
> needs to be taken into account for the results to be consistent.

This boost thing wasn't an issue for us, because we didn't have
platforms which come with it (till recently). I've checked that you have
quite a few CPUs which support huge boost freq, e.g. 5GHz vs. 3.6GHz
nominal max freq [1]. Am I reading this correctly as kernel boost freq?
Do you represent this 5GHz as 1024 capacity?
From this schedutil get_next_freq() I would guess yes.

I cannot find if you use thermal pressure, could you help me with this,
please?


[1] 
https://ark.intel.com/content/www/us/en/ark/products/186605/intel-core-i99900k-processor-16m-cache-up-to-5-00-ghz.html