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From: Lukasz Luba <lukasz.luba@arm.com>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>,
	linux-kernel@vger.kernel.org
Cc: Chris.Redpath@arm.com, morten.rasmussen@arm.com,
	qperret@google.com, linux-pm@vger.kernel.org,
	peterz@infradead.org, rjw@rjwysocki.net, viresh.kumar@linaro.org,
	vincent.guittot@linaro.org, mingo@redhat.com,
	juri.lelli@redhat.com, rostedt@goodmis.org, segall@google.com,
	mgorman@suse.de, bristot@redhat.com, CCj.Yeh@mediatek.com
Subject: Re: [PATCH 3/3] PM: EM: Increase energy calculation precision
Date: Tue, 6 Jul 2021 20:51:00 +0100	[thread overview]
Message-ID: <fa2bac05-1992-9166-0b5f-2477af39bb55@arm.com> (raw)
In-Reply-To: <be567416-e7ac-e672-ddfe-f9175ba1c016@arm.com>



On 7/5/21 1:45 PM, Dietmar Eggemann wrote:
> On 25/06/2021 17:26, Lukasz Luba wrote:
>> The Energy Model (EM) provides useful information about device power in
>> each performance state to other subsystems like: Energy Aware Scheduler
>> (EAS). The energy calculation in EAS does arithmetic operation based on
>> the EM em_cpu_energy(). Current implementation of that function uses
>> em_perf_state::cost as a pre-computed cost coefficient equal to:
>> cost = power * max_frequency / frequency.
>> The 'power' is expressed in milli-Watts (or in abstract scale).
>>
>> There are corner cases then the EAS energy calculation for two Performance
>              ^^^^^^^^^^^^
> 
> Again, an easy to understand example to describe in which situation this
> change would bring a benefit would help.
> 
>> Domains (PDs) return the same value, e.g. 10mW. The EAS compares these
>> values to choose smaller one. It might happen that this values are equal
>> due to rounding error. In such scenario, we need better precision, e.g.
>> 10000 times better. To provide this possibility increase the precision on
>> the em_perf_state::cost.
>>
>> This patch allows to avoid the rounding to milli-Watt errors, which might
>> occur in EAS energy estimation for each Performance Domains (PD). The
>> rounding error is common for small tasks which have small utilization
>> values.
> 
> What's the influence of the CPU utilization 'cpu_util_next()' here?
> 
> compute_energy()
>      em_cpu_energy()
>              return ps->cost * sum_util / scale_cpu
>                                ^^^^^^^^

This is the place where the rounding error triggers. If sum_util is
small and scale_cpu is e.g. 1024, then we have a small fraction here.
It depends on the EM 'cost', but for most platforms we have small
power and cost values, so we suffer this rounding.
The example that I gave in my response in patch 2/3 shows this.

>> The rest of the EM code doesn't change, em_perf_state::power is still
>> expressed in milli-Watts (or in abstract scale). Thus, all existing
>> platforms don't have to change their reported power. The same applies to
> 
> Not only existing platforms since there are no changes. So why
> highlighting `existing` here.?

I just wanted to be clear that it doesn't affect existing platforms
at all. We don't require to report power in better resolution e.g.
micro-Watts.
Also, the clients in the kernel won't be affected, since they use
EM 'power' filed, not 'cost'.

      reply	other threads:[~2021-07-06 19:51 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
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 [this message]

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