Messages in this thread | | | Subject | Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy | From | Lukasz Luba <> | Date | Wed, 7 Jul 2021 08:49:46 +0100 |
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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.
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.
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