linux-kernel - Re: [PATCH v2 1/4] PM: EM: convert power field to micro-Watts precision and align drivers

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Date:   Thu, 7 Jul 2022 11:53:50 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        daniel.lezcano@...aro.org, amitk@...nel.org, rui.zhang@...el.com,
        rafael@...nel.org, dietmar.eggemann@....com, nm@...com,
        sboyd@...nel.org, sudeep.holla@....com, cristian.marussi@....com,
        matthias.bgg@...il.com, linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/4] PM: EM: convert power field to micro-Watts
 precision and align drivers



On 7/7/22 11:53, Viresh Kumar wrote:
> On 07-07-22, 08:15, Lukasz Luba wrote:
>> The milli-Watts precision causes rounding errors while calculating
>> efficiency cost for each OPP. This is especially visible in the 'simple'
>> Energy Model (EM), where the power for each OPP is provided from OPP
>> framework. This can cause some OPPs to be marked inefficient, while
>> using micro-Watts precision that might not happen.
>>
>> Update all EM users which access 'power' field and assume the value is
>> in milli-Watts.
>>
>> Solve also an issue with potential overflow in calculation of energy
>> estimation on 32bit machine. It's needed now since the power value
>> (thus the 'cost' as well) are higher.
>>
>> Example calculation which shows the rounding error and impact:
>>
>> power = 'dyn-power-coeff' * volt_mV * volt_mV * freq_MHz
>>
>> power_a_uW = (100 * 600mW * 600mW * 500MHz) / 10^6 = 18000
>> power_a_mW = (100 * 600mW * 600mW * 500MHz) / 10^9 = 18
>>
>> power_b_uW = (100 * 605mW * 605mW * 600MHz) / 10^6 = 21961
>> power_b_mW = (100 * 605mW * 605mW * 600MHz) / 10^9 = 21
>>
>> max_freq = 2000MHz
>>
>> cost_a_mW = 18 * 2000MHz/500MHz = 72
>> cost_a_uW = 18000 * 2000MHz/500MHz = 72000
>>
>> cost_b_mW = 21 * 2000MHz/600MHz = 70 // <- artificially better
>> cost_b_uW = 21961 * 2000MHz/600MHz = 73203
>>
>> The 'cost_b_mW' (which is based on old milli-Watts) is misleadingly
>> better that the 'cost_b_uW' (this patch uses micro-Watts) and such
>> would have impact on the 'inefficient OPPs' information in the Cpufreq
>> framework. This patch set removes the rounding issue.
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>> ---
>>   drivers/cpufreq/mediatek-cpufreq-hw.c |  7 ++--
>>   drivers/cpufreq/scmi-cpufreq.c        |  6 +++
>>   drivers/opp/of.c                      | 15 ++++----
>>   drivers/powercap/dtpm_cpu.c           |  5 +--
>>   drivers/thermal/cpufreq_cooling.c     | 13 ++++++-
>>   drivers/thermal/devfreq_cooling.c     | 19 ++++++++--
>>   include/linux/energy_model.h          | 54 +++++++++++++++++++--------
>>   kernel/power/energy_model.c           | 24 ++++++++----
>>   8 files changed, 100 insertions(+), 43 deletions(-)
> 
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> 

Thank you Viresh for the ACKs!