sched/fair: Optimize find_idlest_group()

find_idlest_group() now reads CPU's load_avg in two different ways.

Consolidate the function to read and use load_avg only once and simplify
the algorithm to only look for the group with lowest load_avg.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Morten.Rasmussen@arm.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hdanton@sina.com
Cc: parth@linux.ibm.com
Cc: pauld@redhat.com
Cc: quentin.perret@arm.com
Cc: riel@surriel.com
Cc: srikar@linux.vnet.ibm.com
Cc: valentin.schneider@arm.com
Link: https://lkml.kernel.org/r/1571405198-27570-11-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -5550,16 +5550,14 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 {
 	struct sched_group *idlest = NULL, *group = sd->groups;
 	struct sched_group *most_spare_sg = NULL;
-	unsigned long min_runnable_load = ULONG_MAX;
-	unsigned long this_runnable_load = ULONG_MAX;
-	unsigned long min_avg_load = ULONG_MAX, this_avg_load = ULONG_MAX;
+	unsigned long min_load = ULONG_MAX, this_load = ULONG_MAX;
 	unsigned long most_spare = 0, this_spare = 0;
 	int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;
 	unsigned long imbalance = scale_load_down(NICE_0_LOAD) *
 				(sd->imbalance_pct-100) / 100;
 
 	do {
-		unsigned long load, avg_load, runnable_load;
+		unsigned long load;
 		unsigned long spare_cap, max_spare_cap;
 		int local_group;
 		int i;
@@ -5576,15 +5574,11 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		 * Tally up the load of all CPUs in the group and find
 		 * the group containing the CPU with most spare capacity.
 		 */
-		avg_load = 0;
-		runnable_load = 0;
+		load = 0;
 		max_spare_cap = 0;
 
 		for_each_cpu(i, sched_group_span(group)) {
-			load = cpu_load(cpu_rq(i));
-			runnable_load += load;
-
-			avg_load += cfs_rq_load_avg(&cpu_rq(i)->cfs);
+			load += cpu_load(cpu_rq(i));
 
 			spare_cap = capacity_spare_without(i, p);
 
@@ -5593,31 +5587,15 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
 		}
 
 		/* Adjust by relative CPU capacity of the group */
-		avg_load = (avg_load * SCHED_CAPACITY_SCALE) /
-					group->sgc->capacity;
-		runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
+		load = (load * SCHED_CAPACITY_SCALE) /
 					group->sgc->capacity;
 
 		if (local_group) {
-			this_runnable_load = runnable_load;
-			this_avg_load = avg_load;
+			this_load = load;
 			this_spare = max_spare_cap;
 		} else {
-			if (min_runnable_load > (runnable_load + imbalance)) {
-				/*
-				 * The runnable load is significantly smaller
-				 * so we can pick this new CPU:
-				 */
-				min_runnable_load = runnable_load;
-				min_avg_load = avg_load;
-				idlest = group;
-			} else if ((runnable_load < (min_runnable_load + imbalance)) &&
-				   (100*min_avg_load > imbalance_scale*avg_load)) {
-				/*
-				 * The runnable loads are close so take the
-				 * blocked load into account through avg_load:
-				 */
-				min_avg_load = avg_load;
+			if (load < min_load) {
+				min_load = load;
 				idlest = group;
 			}
 
@@ -5658,18 +5636,18 @@ skip_spare:
 	 * local domain to be very lightly loaded relative to the remote
 	 * domains but "imbalance" skews the comparison making remote CPUs
 	 * look much more favourable. When considering cross-domain, add
-	 * imbalance to the runnable load on the remote node and consider
-	 * staying local.
+	 * imbalance to the load on the remote node and consider staying
+	 * local.
 	 */
 	if ((sd->flags & SD_NUMA) &&
-	    min_runnable_load + imbalance >= this_runnable_load)
+	     min_load + imbalance >= this_load)
 		return NULL;
 
-	if (min_runnable_load > (this_runnable_load + imbalance))
+	if (min_load >= this_load + imbalance)
 		return NULL;
 
-	if ((this_runnable_load < (min_runnable_load + imbalance)) &&
-	     (100*this_avg_load < imbalance_scale*min_avg_load))
+	if ((this_load < (min_load + imbalance)) &&
+	    (100*this_load < imbalance_scale*min_load))
 		return NULL;
 
 	return idlest;