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

>
> 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.

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