linux-kernel - Re: [PATCH V4] thermal/core/power_allocator: reset thermal governor when trip point is changed

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Message-ID: <6faf93fe-f8ef-32ea-47ad-08531d630dd8@arm.com>
Date:   Mon, 26 Jun 2023 08:45:40 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Di Shen <cindygm567@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>, daniel.lezcano@...aro.org,
        rui.zhang@...el.com, Di Shen <di.shen@...soc.com>,
        amitk@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, xuewen.yan@...soc.com,
        jeson.gao@...soc.com, zhanglyra@...il.com, orsonzhai@...il.com
Subject: Re: [PATCH V4] thermal/core/power_allocator: reset thermal governor
 when trip point is changed



On 6/25/23 09:39, Di Shen wrote:
> On Fri, Jun 23, 2023 at 4:10 PM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>>
>>
>> On 6/22/23 19:27, Rafael J. Wysocki wrote:

[snip]

>>>
>>> So there seems to be a claim that IPA is the only governor needing the
>>> ->reset() callback, but I have not seen any solid analysis confirming
>>> that.  It very well may be the case, but then the changelog should
>>> clearly explain why this is the case IMO.
>>
>> I agree, the patch header doesn't explain that properly. Here is the
>> explanation for this Intelligent Power Allocator (IPA):
>>
>> The IPA controls temperature using PID mechanism. It's a closed
>> feedback loop. That algorithm can 'learn' from the 'observed'
>> in the past reaction for it's control decisions and accumulates that
>> information in the part called 'error integral'. Those accumulated
>> 'error' gaps are the differences between the set target value and the
>> actually achieved value. In our case the target value is the target
>> temperature which is coming from the trip point. That part is then used
>> with the 'I' (of PID) component, so we can compensate for those
>> 'learned' mistakes.
>> Now, when you change the target temperature value - all your previous
>> learned errors won't help you. That's why Intelligent Power Allocator
>> should reset previously accumulated history.
>>
> 
> Yes, THAT's the point!
> Maybe I need to write the commit message in more detail.
> 

Yes, please extend that description.

Regards,
Lukasz