linux-kernel - Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans the old data

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Message-ID: <5eb8899b-8846-4ad1-a37f-38fb0280cde2@arm.com>
Date:   Thu, 12 Oct 2023 14:16:39 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>, Wei Wang <wvw@...gle.com>
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, mhiramat@...nel.org,
        qyousef@...alina.io
Subject: Re: [PATCH v4 10/18] PM: EM: Add RCU mechanism which safely cleans
 the old data



On 10/11/23 17:07, Rafael J. Wysocki wrote:
> On Wed, Oct 11, 2023 at 6:03 PM Wei Wang <wvw@...gle.com> wrote:
>>
>> On Fri, Oct 6, 2023 at 1:45 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>>
>>> Hi Rafael,
>>>
>>> A change of direction here, regarding your comment below.
>>>
>>> On 10/2/23 14:44, Lukasz Luba wrote:
>>>>
>>>>
>>>> On 9/29/23 13:59, Rafael J. Wysocki wrote:
>>>>> On Fri, Sep 29, 2023 at 11:36 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>>>
>>>> [snip]
>>>>
>>>
>>> [snip]
>>>
>>>>>>> Apparently, some frameworks are only going to use the default table
>>>>>>> while the runtime-updatable table will be used somewhere else at the
>>>>>>> same time.
>>>>>>>
>>>>>>> I'm not really sure if this is a good idea.
>>>>>>
>>>>>> Runtime table is only for driving the task placement in the EAS.
>>>>>>
>>>>>> The thermal gov IPA won't make better decisions because it already
>>>>>> has the mechanism to accumulate the error that it made.
>>>>>>
>>>>>> The same applies to DTPM, which works in a more 'configurable' way,
>>>>>> rather that hard optimization mechanism (like EAS).
>>>>>
>>>>> My understanding of the above is that the other EM users don't really
>>>>> care that much so they can get away with using the default table all
>>>>> the time, but EAS needs more accuracy, so the table used by it needs
>>>>> to be adjusted in certain situations.
>>>>
>>>> Yes
>>>>
>>>>>
>>>>> Fair enough, I'm assuming that you've done some research around it.
>>>>> Still, this is rather confusing.
>>>>
>>>> Yes, I have presented those ~2y ago in Android Gerrit world
>>>> (got feedback from a few vendors) and in a few Linux conferences.
>>>>
>>>> For now we don't plan to have this feature for the thermal
>>>> governor or something similar.
>>>>
>>>
>>> I have discussed with one of our partners your comment about 2 tables.
>>> They would like to have this runtime modified EM in other places
>>> as well: DTPM and thermal governor. So you had good gut feeling.
>>>
>>> In the past in our IPA (thermal gov ~2016 and kernel v4.14) we
>>> had two callbacks:
>>> - get_static_power() [1]
>>> - get_dynamic_power() [2]
>>>
>>> Later ~2017/2018 v4.16 the static power mechanism was removed
>>> completely by this commit 84fe2cab48590e4373978e4e.
>>> The way how it was design, implemented and used justified that
>>> decision. We later used EM in the cpu cooling which also only
>>> had dynamic power information.
>>>
>>> The PID mechanism in IPA tries to compensate that
>>> missing information (about changed static power in time or a chip
>>> binning) and adjusts the 'error'. How good and fast that is in all
>>> situations - it's a different story (out of this scope).
>>> So, IPA should not be worse with the runtime table.
>>>
>>> The static power was on the chips and probably will be still.
>>> You might remember my slide 13 from OSPM2024 showing two power
>>> usage plots for the same Big CPU and 1.4GHz fixed (50% of fmax):
>>> - w/ GPU working in the background using 1-1.5W
>>> - w/o GPU in the background
>>>
>>> The same workload run on Big, but power bigger is ~15% higher
>>> after ~1min.
>>>
>>> The static power (leakage) is the issue that this patch tries
>>> to address for EAS. Although, there is not only the leakage.
>>> It's about the whole 'profile', which can be different than what
>>> could be built during boot default information.
>>>
>>> So we would want to go for one single table in EM, which
>>> is runtime modifiable.
>>>
>>> That is something that you might be more confident and we would
>>> have less diversity (2 tables) in the kernel.
>>>
>>> Regards,
>>> Lukasz
>>>
>>>
>>
>> Indeed, we had a conversation about this with Lukasz recently. The key
>> idea is that there is no compelling reason to introduce diversity in
>> the mathematics involved. If we have confidence in the superior
>> accuracy of our model, it should be universally implemented. While the
>> governors are designed with some error tolerance, they can benefit
>> from enhanced accuracy in their operation.
> 
> I agree, thanks!
> 

Thank you Wei and Rafael. I'm working on that implementation and
will be in v5 soon.