linux-kernel - Re: [PATCH v1 13/16] thermal: gov_fair_share: Eliminate unnecessary integer divisions

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Date: Fri, 19 Apr 2024 19:46:01 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v1 13/16] thermal: gov_fair_share: Eliminate unnecessary
 integer divisions



On 4/10/24 18:00, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The computations carried out by fair_share_throttle() for each trip
> point include at least one redundant integer division which introduces
> superfluous rounding errors.  Also the multiplications by 100 in it are
> not really necessary and can be eliminated.
> 
> Rearrange fair_share_throttle() to carry out only one integer division per
> trip and only as many integer multiplications as necessary and rename one
> variable in it (while at it).
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>   drivers/thermal/gov_fair_share.c |   32 +++++++++++++++-----------------
>   1 file changed, 15 insertions(+), 17 deletions(-)
> 
> Index: linux-pm/drivers/thermal/gov_fair_share.c
> ===================================================================
> --- linux-pm.orig/drivers/thermal/gov_fair_share.c
> +++ linux-pm/drivers/thermal/gov_fair_share.c
> @@ -41,12 +41,6 @@ static int get_trip_level(struct thermal
>   	return trip_level;
>   }
>   
> -static long get_target_state(struct thermal_zone_device *tz,
> -		struct thermal_cooling_device *cdev, int percentage, int level)
> -{
> -	return (long)(percentage * level * cdev->max_state) / (100 * tz->num_trips);
> -}
> -
>   /**
>    * fair_share_throttle - throttles devices associated with the given zone
>    * @tz: thermal_zone_device
> @@ -58,7 +52,7 @@ static long get_target_state(struct ther
>    *
>    * Parameters used for Throttling:
>    * P1. max_state: Maximum throttle state exposed by the cooling device.
> - * P2. percentage[i]/100:
> + * P2. weight[i]/total_weight:
>    *	How 'effective' the 'i'th device is, in cooling the given zone.
>    * P3. trip_level/max_no_of_trips:
>    *	This describes the extent to which the devices should be throttled.
> @@ -72,30 +66,34 @@ static void fair_share_throttle(struct t
>   {
>   	struct thermal_instance *instance;
>   	int total_weight = 0;
> -	int total_instance = 0;
> +	int nr_instances = 0;
>   
>   	list_for_each_entry(instance, &tz->thermal_instances, tz_node) {
>   		if (instance->trip != trip)
>   			continue;
>   
>   		total_weight += instance->weight;
> -		total_instance++;
> +		nr_instances++;
>   	}
>   
>   	list_for_each_entry(instance, &tz->thermal_instances, tz_node) {
> -		int percentage;
>   		struct thermal_cooling_device *cdev = instance->cdev;
> +		u64 dividend;
> +		u32 divisor;
>   
>   		if (instance->trip != trip)
>   			continue;
>   
> -		if (!total_weight)
> -			percentage = 100 / total_instance;
> -		else
> -			percentage = (instance->weight * 100) / total_weight;
> -
> -		instance->target = get_target_state(tz, cdev, percentage,
> -						    trip_level);
> +		dividend = trip_level;
> +		dividend *= cdev->max_state;
> +		divisor = tz->num_trips;
> +		if (total_weight) {
> +			dividend *= instance->weight;
> +			divisor *= total_weight;
> +		} else {
> +			divisor *= nr_instances;
> +		}
> +		instance->target = div_u64(dividend, divisor);
>   
>   		mutex_lock(&cdev->lock);
>   		__thermal_cdev_update(cdev);
> 
> 
> 


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