On 07/07/2021 10:23, Lukasz Luba wrote:

 
On 7/7/21 9:00 AM, Vincent Guittot wrote:

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On Wed, 7 Jul 2021 at 09:49, Lukasz Luba [off-list ref] wrote:

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On 7/7/21 8:07 AM, Vincent Guittot wrote:

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On Fri, 25 Jun 2021 at 17:26, Lukasz Luba [off-list ref] wrote:

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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).

I assume the worst case is `x * 1024` (max return value of
effective_cpu_util = effective_cpu_util()) so x ~ 6.7.

I'm not aware of any arm32 b.L. systems with > 4 CPUs in a PD.