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

[Lukasz Luba <lukasz.luba@arm.com>]

On 7/7/21 10:37 AM, Vincent Guittot wrote:
> 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 example above shows the need of u64 return value for platforms
which are:
- 32bit
- have e.g. 16 CPUs
- has big power value e.g. ~64k mW
Then let's to the calc:
(64k * 10000) * (16 * 800) / 1024 = ~8000mln = ~8bln

The returned value after applying the whole patch set
won't fit in u32 for such cluster.

We might make *assumption* that the 32bit platforms will not
have bigger number of CPUs in the cluster or won't report
big power values. But I didn't wanted to make such assumption.