`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 longLet's take a look at existing *10000 precision for x CPUs:cost * sum_util / scale_cpu =(64k *10000) * (x * 800) / 1024which is:x * ~500mlnSo to be close to overflowing u32 the 'x' has to be > (?=) 8(depends on sum_util).> >>>> 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.`