`On 07/07/2021 10:23, Lukasz Luba 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:[...]>>>> 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 ofeffective_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.`