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

**From: **Vincent Guittot

**Date: ** Wed Jul 07 2021 - 05:37:45 EST

On Wed, 7 Jul 2021 at 10:23, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:

>

>

>

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

>* > On Wed, 7 Jul 2021 at 09:49, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:*

>* >>*

>* >>*

>* >>*

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

>* >>> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@xxxxxxx> 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 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.*