Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy

[Vincent Guittot <vincent.guittot@linaro.org>]

On Wed, 7 Jul 2021 at 12:06, Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> On 7/7/21 10:56 AM, Vincent Guittot wrote:
> > On Wed, 7 Jul 2021 at 11:48, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>
> >>
> >>
> >> On 7/7/21 10:37 AM, Vincent Guittot wrote:
> >>> On Wed, 7 Jul 2021 at 10:23, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 7/7/21 9:00 AM, Vincent Guittot wrote:
> >>>>> On Wed, 7 Jul 2021 at 09:49, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 7/7/21 8:07 AM, Vincent Guittot wrote:
> >>>>>>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@arm.com> wrote:
> >>>>>>>>
> >>>>>>>> The Energy Aware Scheduler (EAS) tries to find best CPU for a waking up
> >>>>>>>> task. It probes many possibilities and compares the estimated energy values
> >>>>>>>> for different scenarios. For calculating those energy values it relies on
> >>>>>>>> Energy Model (EM) data and em_cpu_energy(). The precision which is used in
> >>>>>>>> EM data is in milli-Watts (or abstract scale), which sometimes is not
> >>>>>>>> sufficient. In some cases it might happen that two CPUs from different
> >>>>>>>> Performance Domains (PDs) get the same calculated value for a given task
> >>>>>>>> placement, but in more precised scale, they might differ. This rounding
> >>>>>>>> error has to be addressed. This patch prepares EAS code for better
> >>>>>>>> precision in the coming EM improvements.
> >>>>>>>
> >>>>>>> Could you explain why 32bits results are not enough and you need to
> >>>>>>> move to 64bits ?
> >>>>>>>
> >>>>>>> Right now the result is in the range [0..2^32[ mW. If you need more
> >>>>>>> precision and you want to return uW instead, you will have a result in
> >>>>>>> the range  [0..4kW[ which seems to be still enough
> >>>>>>>
> >>>>>>
> >>>>>> Currently we have the max value limit for 'power' in EM which is
> >>>>>> EM_MAX_POWER 0xffff (64k - 1). We allow to register such big power
> >>>>>> values ~64k mW (~64Watts) for an OPP. Then based on 'power' we
> >>>>>> pre-calculate 'cost' fields:
> >>>>>> cost[i] = power[i] * freq_max / freq[i]
> >>>>>> So, for max freq the cost == power. Let's use that in the example.
> >>>>>>
> >>>>>> Then the em_cpu_energy() calculates as follow:
> >>>>>> cost * sum_util / scale_cpu
> >>>>>> We are interested in the first part - the value of multiplication.
> >>>>>
> >>>>> But all these are internal computations of the energy model. At the
> >>>>> end, the computed energy that is returned by compute_energy() and
> >>>>> em_cpu_energy(), fits in a long
> >>>>
> >>>> Let's take a look at existing *10000 precision for x CPUs:
> >>>> cost * sum_util / scale_cpu =
> >>>> (64k *10000) * (x * 800) / 1024
> >>>> which is:
> >>>> x * ~500mln
> >>>>
> >>>> So to be close to overflowing u32 the 'x' has to be > (?=) 8
> >>>> (depends on sum_util).
> >>>
> >>> Sorry but I don't get your point.
> >>> This patch is about the return type of compute_energy() and
> >>> em_cpu_energy(). And even if we decide to return uW instead of mW,
> >>> there is still a lot of margin.
> >>>
> >>> It's not because you need u64 for computing intermediate value that
> >>> you must returns u64
> >>
> >> The example above shows the need of u64 return value for platforms
> >> which are:
> >> - 32bit
> >> - have e.g. 16 CPUs
> >> - has big power value e.g. ~64k mW
> >> Then let's to the calc:
> >> (64k * 10000) * (16 * 800) / 1024 = ~8000mln = ~8bln
> >
> > so you return a power consumption of 8kW !!!
> >
>
> No. It's in 0.1uW scale, so 800Watts. Which is 16 CPUs * 64Watts

Oh! you want 0.1uW precision .... This doesn't seem realistic at all.
I'm not even sure that the power model can even reach an accuracy of
1mW

> each at max freq and 80% load.
>
> Max power can be < 64Watts, which is 64k milli-Watts (< EM_MAX_POWER)
> 64k mW * 10000 --> is the 0.1uW precision
>