Re: [PATCH v5 15/23] PM: EM: Optimize em_cpu_energy() and remove division

[Lukasz Luba]

On 1/15/24 12:21, Qais Yousef wrote:
> On 01/10/24 13:53, Lukasz Luba wrote:
> > On 1/4/24 19:23, Qais Yousef wrote:
> > > On 01/02/24 11:47, Lukasz Luba wrote:
> > > > > Did you see a problem or just being extra cautious here?
> > > >
> > > > There is no problem, 'cost' is a private coefficient for EAS only.
> > >
> > > Let me  ask differently, what goes wrong if you don't increase the resolution
> > > here? Why is it necessary?
> >
> >
> > When you have 800mW at CPU capacity 1024, then the value is small (below
> > 1 thousand).
> > Example:
> > power = 800000 uW
> > cost = 800000 / 1024 = 781
> >
> > While I know from past that sometimes OPPs might have close voltage
> > values and a rounding could occur and make some OPPs inefficient
> > while they aren't.
> >
> > This is what would happen when we have the 1x resolution:
> > /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:551
> > /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:644
> > /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:744
> > /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:851
> > /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:493
> > /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:493
> > /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:493
> > The bottom 3 OPPs have the same 'cost' thus 2 OPPs are in-efficient,
> > which is not true (see below).
> >
> > This is what would happen when we have the 10x resolution:
> > /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:5513
> > /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:6443
> > /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:7447
> > /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:8514
> > /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:4934
> > /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:4933
> > /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:4934
> > Here the OPP with 600MHz is more efficient than 408MHz,
> > which is true. So only 408MHz will be marked as in-efficient OPP.
> >
> >
> > This is what would happen when we have the 100x resolution:
> > /sys/kernel/debug/energy_model/cpu4/ps:1008000/cost:55137
> > /sys/kernel/debug/energy_model/cpu4/ps:1200000/cost:64433
> > /sys/kernel/debug/energy_model/cpu4/ps:1416000/cost:74473
> > /sys/kernel/debug/energy_model/cpu4/ps:1512000/cost:85140
> > /sys/kernel/debug/energy_model/cpu4/ps:408000/cost:49346
> > /sys/kernel/debug/energy_model/cpu4/ps:600000/cost:49331
> > /sys/kernel/debug/energy_model/cpu4/ps:816000/cost:49346
> > The higher (100x) resolution does not bring that much in
> > practice.
>
> So it seems a uW is not sufficient enough. We moved from mW because of
> resolution already. Shall we make it nW then and multiply by 1000 always? The
> choice of 10 looks arbitrary IMHO

No, there is no need of nW in the 'power' field for this.
You've missed the point.