Message-ID: <6aedceec-797d-9f33-d409-050eb2ac8d85@arm.com>
Date:   Mon, 3 Jul 2023 12:08:57 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, Pierre.Gondois@....com,
        ionela.voinescu@....com, rostedt@...dmis.org, mhiramat@...nel.org
Subject: Re: [PATCH v2 00/17] Introduce runtime modifiable Energy Model

Hi Rafael,


On 5/24/23 18:25, Rafael J. Wysocki wrote:
> Hi Lukasz,
> 
> On Fri, May 12, 2023 at 11:58 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> Hi all,
>>
>> This patch set adds a new feature which allows to modify Energy Model (EM)
>> power values at runtime. It will allow to better reflect power model of
>> a recent SoCs and silicon. Different characteristics of the power usage
>> can be leveraged and thus better decisions made during task placement in EAS.
>>
>> It's part of feature set know as Dynamic Energy Model. It has been presented
>> and discussed recently at OSPM2023 [3]. This patch set implements the 1st
>> improvement for the EM.
>>
>> The concepts:
>> 1. The CPU power usage can vary due to the workload that it's running or due
>> to the temperature of the SoC. The same workload can use more power when the
>> temperature of the silicon has increased (e.g. due to hot GPU or ISP).
>> In such situation or EM can be adjusted and reflect the fact of increased
>> power usage. That power increase is due to a factor called static power
>> (sometimes called simply: leakage). The CPUs in recent SoCs are different.
>> We have heterogeneous SoCs with 3 (or even 4) different microarchitectures.
>> They are also built differently with High Performance (HP) cells or
>> Low Power (LP) cells. They are affected by the temperature increase
>> differently: HP cells have bigger leakage. The SW model can leverage that
>> knowledge.
>> 2. It is also possible to change the EM to better reflect the currently
>> running workload. Usually the EM is derived from some average power values
>> taken from experiments with benchmark (e.g. Dhrystone). The model derived
>> from such scenario might not represent properly the workloads usually running
>> on the device. Therefore, runtime modification of the EM allows to switch to
>> a different model, when there is a need.
>> 3. The EM can be adjusted after boot, when all the modules are loaded and
>> more information about the SoC is available e.g. chip binning. This would help
>> to better reflect the silicon characteristics. Thus, this EM modification
>> API allows it now. It wasn't possible in the past and the EM had to be
>> 'set in stone'.
>>
>> Some design details:
>> The internal mechanisms for the memory allocation are handled internally in the
>> EM. Kernel modules can just call the new API to update the EM data and the
>> new memory would be provided and owned by the EM. The EM memory is used by
>> EAS, which impacts those design decisions. The EM writers are protected by
>> a mutex. This new runtime modified EM table is protected using RCU mechanism,
>> which fits the current EAS hot path (which already uses RCU read lock).
>> The unregister API handles only non-CPU (e.g. GPU, ISP) devices and uses the
>> same mutex as EM modifiers to make sure the memory is safely freed.
>>
>> More detailed explanation and background can be found in presentations
>> during LPC2022 [1][2] or in the documentation patches.
>>
>> Changelog:
>> v2:
>> - solved build warning of unused variable in patch 13/17 when EM is
>>    not compiled in, e.g. on Intel platform for this cpufreq_cooling
>> - re-based on top of v6.4-rc1
>> v1:
>> - implementation can be found here [4]
>>
>> [1] https://lpc.events/event/16/contributions/1341/attachments/955/1873/Dynamic_Energy_Model_to_handle_leakage_power.pdf
>> [2] https://lpc.events/event/16/contributions/1194/attachments/1114/2139/LPC2022_Energy_model_accuracy.pdf
>> [3] https://www.youtube.com/watch?v=2C-5uikSbtM&list=PL0fKordpLTjKsBOUcZqnzlHShri4YBL1H
>> [4] https://lore.kernel.org/lkml/20230314103357.26010-1-lukasz.luba@arm.com/
>>
>> Lukasz Luba (17):
>>    PM: EM: Refactor em_cpufreq_update_efficiencies() arguments
>>    PM: EM: Find first CPU online while updating OPP efficiency
>>    PM: EM: Refactor em_pd_get_efficient_state() to be more flexible
>>    PM: EM: Create a new function em_compute_costs()
>>    trace: energy_model: Add trace event for EM runtime modifications
>>    PM: EM: Add update_power() callback for runtime modifications
>>    PM: EM: Check if the get_cost() callback is present in
>>      em_compute_costs()
>>    PM: EM: Introduce runtime modifiable table
>>    PM: EM: Add RCU mechanism which safely cleans the old data
>>    PM: EM: Add runtime update interface to modify EM power
>>    PM: EM: Use runtime modified EM for CPUs energy estimation in EAS
>>    PM: EM: Add argument to get_cost() for runtime modification
>>    PM: EM: Refactor struct em_perf_domain and add default_table
>>    Documentation: EM: Add a new section about the design
>>    Documentation: EM: Add a runtime modifiable EM design description
>>    Documentation: EM: Add example with driver modifying the EM
>>    Documentation: EM: Describe the API of runtime modifications
> 
> I haven't seen any responses from anyone having a vested interest in
> the Energy Model code.
> 
> I'm not sure what this means, but I surely can't do much about it
> myself without any input from the potentially interested parties.

My apologies for the delay. Correct, it has been missing attention,
but now Dietmar is reviewing the stuff. He commented a few things
and I'm going to respond and address them in v3.

Thanks for having a look into this thread!

Lukasz

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