Date:   Fri, 8 Sep 2017 13:47:18 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Keerthy <j-keerthy@...com>, rui.zhang@...el.com,
        edubezval@...il.com
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        john.stultz@...aro.org, leo.yan@...aro.org
Subject: Re: [PATCH V2] thermal/drivers/step_wise: Fix temperature regulation
 misbehavior

On 08/09/2017 11:49, Keerthy wrote:
> 
> 
> On Friday 08 September 2017 02:35 PM, Daniel Lezcano wrote:
>> There is a particular situation when the cooling device is cpufreq and the heat
>> dissipation is not efficient enough where the temperature increases little by
>> little until reaching the critical threshold and leading to a SoC reset.
>>
>> The behavior is reproducible on a hikey6220 with bad heat dissipation (eg.
>> stacked with other boards).
>>
>> Running a simple C program doing while(1); for each CPU of the SoC makes the
>> temperature to reach the passive regulation trip point and ends up to the
>> maximum allowed temperature followed by a reset.
>>
>> This issue has been also reported by running the libhugetlbfs test suite.
>>
>> What is observed is a ping pong between two cpu frequencies, 1.2GHz and 900MHz
>> while the temperature continues to grow.
>>
>> It appears the step wise governor calls get_target_state() the first time with
>> the throttle set to true and the trend to 'raising'. The code selects logically
>> the next state, so the cpu frequency decreases from 1.2GHz to 900MHz, so far so
>> good. The temperature decreases immediately but still stays greater than the
>> trip point, then get_target_state() is called again, this time with the
>> throttle set to true *and* the trend to 'dropping'. From there the algorithm
>> assumes we have to step down the state and the cpu frequency jumps back to
>> 1.2GHz. But the temperature is still higher than the trip point, so
>> get_target_state() is called with throttle=1 and trend='raising' again, we jump
>> to 900MHz, then get_target_state() is called with throttle=1 and
>> trend='dropping', we jump to 1.2GHz, etc ... but the temperature does not
>> stabilizes and continues to increase.
>>
>> [  237.922654] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
>> [  237.922678] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
>> [  237.922690] thermal cooling_device0: cur_state=0
>> [  237.922701] thermal cooling_device0: old_target=0, target=1
>> [  238.026656] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
>> [  238.026680] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=1
>> [  238.026694] thermal cooling_device0: cur_state=1
>> [  238.026707] thermal cooling_device0: old_target=1, target=0
>> [  238.134647] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
>> [  238.134667] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
>> [  238.134679] thermal cooling_device0: cur_state=0
>> [  238.134690] thermal cooling_device0: old_target=0, target=1
>>
>> In this situation the temperature continues to increase while the trend is
>> oscillating between 'dropping' and 'raising'. We need to keep the current state
>> untouched if the throttle is set, so the temperature can decrease or a higher
>> state could be selected, thus prevening this oscillation.
>>
>> Keeping the next_target untouched when 'throttle' is true at 'dropping' time
>> fixes the issue.
>>
>> The following traces show the governor does not change the next state if
>> trend==2 (dropping) and throttle==1.
>>
>> [ 2306.127987] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
>> [ 2306.128009] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
>> [ 2306.128021] thermal cooling_device0: cur_state=0
>> [ 2306.128031] thermal cooling_device0: old_target=0, target=1
>> [ 2306.231991] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
>> [ 2306.232016] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=1
>> [ 2306.232030] thermal cooling_device0: cur_state=1
>> [ 2306.232042] thermal cooling_device0: old_target=1, target=1
>> [ 2306.335982] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
>> [ 2306.336006] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=1
>> [ 2306.336021] thermal cooling_device0: cur_state=1
>> [ 2306.336034] thermal cooling_device0: old_target=1, target=1
>> [ 2306.439984] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
>> [ 2306.440008] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=0
>> [ 2306.440022] thermal cooling_device0: cur_state=1
>> [ 2306.440034] thermal cooling_device0: old_target=1, target=0
>>
>> [ ... ]
>>
>> After a while, if the temperature continues to increase, the next state becomes
>> 2 which is 720MHz on the hikey. That results in the temperature stabilizing
>> around the trip point.
>>
>> [ 2455.831982] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
>> [ 2455.832006] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=0
>> [ 2455.832019] thermal cooling_device0: cur_state=1
>> [ 2455.832032] thermal cooling_device0: old_target=1, target=1
>> [ 2455.935985] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
>> [ 2455.936013] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=0
>> [ 2455.936027] thermal cooling_device0: cur_state=1
>> [ 2455.936040] thermal cooling_device0: old_target=1, target=1
>> [ 2456.043984] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=0,throttle=1
>> [ 2456.044009] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=0,throttle=0
>> [ 2456.044023] thermal cooling_device0: cur_state=1
>> [ 2456.044036] thermal cooling_device0: old_target=1, target=1
>> [ 2456.148001] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=1,throttle=1
>> [ 2456.148028] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=1,throttle=1
>> [ 2456.148042] thermal cooling_device0: cur_state=1
>> [ 2456.148055] thermal cooling_device0: old_target=1, target=2
>> [ 2456.252009] thermal thermal_zone0: Trip0[type=1,temp=65000]:trend=2,throttle=1
>> [ 2456.252041] thermal thermal_zone0: Trip1[type=1,temp=75000]:trend=2,throttle=0
>> [ 2456.252058] thermal cooling_device0: cur_state=2
>> [ 2456.252075] thermal cooling_device0: old_target=2, target=1
>>
>> IOW, this change is needed to keep the state for a cooling device if the
>> temperature trend is oscillating while the temperature increases slightly.
>>
>> Without this change, the situation above leads to a catastrophic crash by a
>> hardware reset on hikey.
> 
> Daniel,
> 
> By design this governor is throttling and un-throttling based on the
> computed trend.

Hi Keerthy,

that was true until commit 3dbfff3d (Nov 2012) but now the function has:

[ ... ]

        case THERMAL_TREND_RAISING:
                if (throttle) {
                        next_target = cur_state < instance->upper ?
                                    (cur_state + 1) : instance->upper;
                        if (next_target < instance->lower)
                                next_target = instance->lower;
                }
                break;

  If "the trend is raising and we have to throttle" then state++

The change I'm proposing is an action which is the mirror of the one above.

  If "the trend is dropping and we don't have to throttle" then state--


> Why not add an intermediate trip point with the highest cooling enabled.
> Say High alert trip point that allows only the lowest OPP for cpufreq to
> operate.
> 
> For example: alert trip is at 100C (where cpufreq cooling kicks in)
>              critical trip is at 125C(shutdown temperature).
> 
> We have Something like below @110C which allows only the lowest
> frequency or lowest 2 frequencies based on experimentation:
> 
> +&cpu_trips {
> +	cpu_high_alert: cpu_high_alert {
> +		temperature = <110000>; /* millicelsius */
> +		hysteresis = <2000>; /* millicelsius */
> +		type = "passive";
> +	};
> +};
> +
> +&cpu_cooling_maps {
> +	map1: map1 {
> +		trip = <&cpu_high_alert>;
> +		cooling-device =
> +				<&cpu0 2 THERMAL_NO_LIMIT>;
> +	};
> +};
> +
> 
> I have seen this problem myself on dra7 platforms. The above will cool
> the device post 110C by keeping the cpu frequency at the lowest or lower
> values. That way when you are in between 100 - 110C you can still get
> highest performance depending on the trend computed and post 110C.

Yeah, so we can drop the graveyard legendary card at clash royal without
any lag and boil our hands ;)

Seriously, from my POV, that is a hack to workaround a governor which is
unable to stabilize the temperature in a specific situation.

This patch fixes this.

  -- Daniel

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