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

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Date: Wed, 10 Apr 2024 19:00:10 +0200
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Lukasz Luba <lukasz.luba@....com>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject:
 [PATCH v1 13/16] thermal: gov_fair_share: Eliminate unnecessary integer
 divisions

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);