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 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 sum_util values that we can see for x CPUs which have scale_cap=1024
> >> can be close to 800, let's use it in the example:
> >> cost * sum_util = 64k * (x * 800), where
> >> x=4: ~200mln
> >> x=8: ~400mln
> >> x=16: ~800mln
> >> x=64: ~3200mln (last one which would fit in u32)
> >>
> >> When we increase the precision by even 100, then the above values won't
> >> fit in the u32. Even a max cost of e.g. 10k mW and 100 precision has
> >> issues:
> >> cost * sum_util = (10k *100) * (x * 800), where
> >> x=4: ~3200mln
> >> x=8: ~6400mln
> >>
> >> For *1000 precision even a power of 1Watt becomes an issue:
> >> cost * sum_util = (1k *1000) * (x * 800), where
> >> x=4: ~3200mln
> >> x=8: ~6400mln
> >>
> >> That's why to make the code safe for bigger power values, I had to use
> >> the u64 on 32bit machines.