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

[Lukasz Luba]

On 7/7/21 10:56 AM, Vincent Guittot wrote:
> On Wed, 7 Jul 2021 at 11:48, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > On 7/7/21 10:37 AM, Vincent Guittot wrote:
> > > 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 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
>
> so you return a power consumption of 8kW !!!

No. It's in 0.1uW scale, so 800Watts. Which is 16 CPUs * 64Watts
each at max freq and 80% load.

Max power can be < 64Watts, which is 64k milli-Watts (< EM_MAX_POWER)
64k mW * 10000 --> is the 0.1uW precision