From: Lukasz Luba <lukasz.luba@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
Chris Redpath <Chris.Redpath@arm.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Morten Rasmussen <morten.rasmussen@arm.com>,
Quentin Perret <qperret@google.com>,
"open list:THERMAL" <linux-pm@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Viresh Kumar <viresh.kumar@linaro.org>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
segall@google.com, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
CCj.Yeh@mediatek.com
Subject: Re: [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy
Date: Wed, 7 Jul 2021 08:49:46 +0100 [thread overview]
Message-ID: <a6a49480-7d5d-fd0e-3940-0b6baac5acc0@arm.com> (raw)
In-Reply-To: <CAKfTPtAV9GjQaXc2FV0OuEzTGQw9hFiKpwMfAxP-JQ_QFCUC3w@mail.gmail.com>
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.
next prev parent reply other threads:[~2021-07-07 7:49 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-25 15:26 [PATCH 0/3] Improve EAS energy estimation and increase precision Lukasz Luba
2021-06-25 15:26 ` [PATCH 1/3] sched/fair: Prepare variables for increased precision of EAS estimated energy Lukasz Luba
2021-06-30 17:01 ` Rafael J. Wysocki
2021-06-30 17:28 ` Lukasz Luba
2021-07-02 19:07 ` Lukasz Luba
2021-07-07 7:07 ` Vincent Guittot
2021-07-07 7:49 ` Lukasz Luba [this message]
2021-07-07 8:00 ` Vincent Guittot
2021-07-07 8:23 ` Lukasz Luba
2021-07-07 9:37 ` Vincent Guittot
2021-07-07 9:48 ` Lukasz Luba
2021-07-07 9:56 ` Vincent Guittot
2021-07-07 10:06 ` Lukasz Luba
2021-07-07 10:11 ` Vincent Guittot
2021-07-07 10:29 ` Lukasz Luba
2021-07-07 10:32 ` Vincent Guittot
2021-07-07 10:41 ` Lukasz Luba
2021-07-07 10:50 ` Vincent Guittot
2021-07-07 11:02 ` Lukasz Luba
2021-07-07 13:53 ` Vincent Guittot
2021-07-07 14:25 ` Lukasz Luba
2021-07-07 9:45 ` Dietmar Eggemann
2021-07-07 9:54 ` Lukasz Luba
2021-06-25 15:26 ` [PATCH 2/3] PM: EM: Make em_cpu_energy() able to return bigger values Lukasz Luba
2021-07-05 12:44 ` Dietmar Eggemann
2021-07-06 19:44 ` Lukasz Luba
2021-07-07 7:07 ` Peter Zijlstra
2021-07-07 8:09 ` Lukasz Luba
2021-07-07 10:01 ` Peter Zijlstra
2021-07-07 10:23 ` Lukasz Luba
2021-06-25 15:26 ` [PATCH 3/3] PM: EM: Increase energy calculation precision Lukasz Luba
2021-07-05 12:45 ` Dietmar Eggemann
2021-07-06 19:51 ` Lukasz Luba
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