linux-kernel - Re: [PATCH] drivers: thermal: step_wise: add support for hysteresis

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Date:   Thu, 21 Nov 2019 16:07:19 -0500
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Amit Kucheria <amit.kucheria@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Stephen Boyd <swboyd@...omium.org>,
        "J, KEERTHY" <j-keerthy@...com>, Zhang Rui <rui.zhang@...el.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Amit Kucheria <amit.kucheria@...durent.com>,
        Ram Chandrasekar <rkumbako@...eaurora.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] drivers: thermal: step_wise: add support for hysteresis

On 11/21/2019 09:38 AM, Amit Kucheria wrote:
> On Thu, Nov 21, 2019 at 7:40 PM Thara Gopinath
> <thara.gopinath@...aro.org> wrote:
>>
>> On 11/21/2019 12:50 AM, Amit Kucheria wrote:
>>> From: Ram Chandrasekar <rkumbako@...eaurora.org>
>>>
>>> Currently, step wise governor increases the mitigation when the
>>> temperature goes above a threshold and decreases the mitigation when the
>>> temperature goes below the threshold. If there is a case where the
>>> temperature is wavering around the threshold, the mitigation will be
>>> applied and removed every iteration, which is not very efficient.
>>>
>>> The use of hysteresis temperature could avoid this ping-pong of
>>> mitigation by relaxing the mitigation to happen only when the
>>> temperature goes below this lower hysteresis value.
>> Hi Amit,
>>
>> Can this not lead to ping-pong around the hysteresis temperature?
> 
> That isn't how hysteresis is supposed to work if there is a sufficient
> delta between your trip point and your hysteresis value.
> 
> e.g. if you have a trip at 80C and a hysteresis of 10C, it means that
> you will start throttling at 80C, but you won't STOP throttling until
> you cool down to 70C. At that point, you will wait again to get to 80C
> before throttling again.
> IOW, on the downward slope (80 -> 70) you still have throttling active
> and on the upward slope (70 -> 80), you have no[1] throttling, so
> different behaviour is the same temperature range depending on
> direction.
> 
> If your hysteresis value was very low .e.g. 1C, it would certainly be useless.

Thanks for the explanation. I think averaging can still give a smoother
experience/transition. But then it has to be implemented and tested
against this solution. Other reason for this solution is hysteresis can
be a higher value if needed where as averaging might not give that
flexibility. I have some other comments on the patch which I have posted
separately.
> 
>> If the idea is to minimize ping-pong isn't average a better method?
> 
> We shouldn't ping-pong with the asymmetric behaviour described above.
> 
> Regards,
> Amit
> [1] This is a simple example with a single trip point.
> 


-- 
Warm Regards
Thara