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Date: Wed, 7 Jul 2021 08:49:46 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
Chris Redpath <Chris.Redpath@....com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Morten Rasmussen <morten.rasmussen@....com>,
Quentin Perret <qperret@...gle.com>,
"open list:THERMAL" <linux-pm@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Viresh Kumar <viresh.kumar@...aro.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>, segall@...gle.com,
Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
CCj.Yeh@...iatek.com
Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision
of EAS estimated energy
On 7/7/21 8:07 AM, Vincent Guittot wrote:
> On Fri, 25 Jun 2021 at 17:26, Lukasz Luba <lukasz.luba@....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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