linux-kernel - Re: [PATCH v2 3/3] thermal: gov_step_wise: Allow cooling level to be reduced earlier

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Message-ID: <57c54d50-9948-42be-985a-195b2027733a@arm.com>
Date: Thu, 4 Sep 2025 08:32:53 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] thermal: gov_step_wise: Allow cooling level to be
 reduced earlier



On 8/25/25 14:31, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The current behavior of the Step-wise thermal governor is to increase
> the cooling level one step at a time after trip point threshold passing
> by thermal zone temperature until the temperature stops to rise and then
> do nothing until it falls down below the (possibly new) trip point
> threshold, at which point the cooling level is reduced straight to the
> applicable minimum.

Quite long single sentence to describe these stuff...

> 
> While this generally works, it is not in agreement with the throttling
> logic description comment in step_wise_manage() any more after some
> relatively recent changes, and in the case of passive cooling, it may
> lead to undesirable performance oscillations between high and low
> levels.
> 
> For this reason, modify the governor's cooling device state selection
> function, get_target_state(), to reduce cooling by one level even if
> the temperature is still above the thermal zone threshold, but the
> temperature has started to fall down.  However, ensure that the cooling
> level will remain above the applicable minimum in that case to pull
> the zone temperature further down, possibly until it falls below the
> trip threshold (which may now be equal to the low temperature of the
> trip).
> 
> Doing so should help higher performance to be restored earlier in some
> cases which is desirable especially for passive trip points with
> relatively high hysteresis values.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>   drivers/thermal/gov_step_wise.c |   15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
> 
> --- a/drivers/thermal/gov_step_wise.c
> +++ b/drivers/thermal/gov_step_wise.c
> @@ -20,7 +20,9 @@
>    * If the temperature is higher than a trip point,
>    *    a. if the trend is THERMAL_TREND_RAISING, use higher cooling
>    *       state for this trip point
> - *    b. if the trend is THERMAL_TREND_DROPPING, do nothing
> + *    b. if the trend is THERMAL_TREND_DROPPING, use a lower cooling state
> + *       for this trip point, but keep the cooling state above the applicable
> + *       minimum
>    * If the temperature is lower than a trip point,
>    *    a. if the trend is THERMAL_TREND_RAISING, do nothing
>    *    b. if the trend is THERMAL_TREND_DROPPING, use the minimum applicable
> @@ -51,6 +53,17 @@
>   	if (throttle) {
>   		if (trend == THERMAL_TREND_RAISING)
>   			return clamp(cur_state + 1, instance->lower, instance->upper);
> +
> +		/*
> +		 * If the zone temperature is falling, the cooling level can
> +		 * be reduced, but it should still be above the lower state of
> +		 * the given thermal instance to pull the temperature further
> +		 * down.
> +		 */
> +		if (trend == THERMAL_TREND_DROPPING)
> +			return clamp(cur_state - 1,
> +				     min(instance->lower + 1, instance->upper),
> +				     instance->upper);
>   	} else if (trend == THERMAL_TREND_DROPPING) {
>   		if (cur_state <= instance->lower)
>   			return THERMAL_NO_TARGET;
> 
> 
> 

That make sense

Reviewed-by: Lukasz Luba <lukasz.luba@....com>