sched/fair: Remove SIS_AVG_CPU

SIS_AVG_CPU was introduced as a means of avoiding a search when the
average search cost indicated that the search would likely fail. It was
a blunt instrument and disabled by commit 4c77b18c ("sched/fair: Make
select_idle_cpu() more aggressive") and later replaced with a proportional
search depth by commit 1ad3aaf3 ("sched/core: Implement new approach
to scale select_idle_cpu()").

While there are corner cases where SIS_AVG_CPU is better, it has now been
disabled for almost three years. As the intent of SIS_PROP is to reduce
the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
on SIS_PROP as a throttling mechanism.
Signed-off-by: NMel Gorman <mgorman@techsingularity.net>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NVincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210125085909.4600-2-mgorman@techsingularity.net

kernel/sched/fair.c

@@ -6158,7 +6158,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 {
 	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
 	struct sched_domain *this_sd;
-	u64 avg_cost, avg_idle;
 	u64 time;
 	int this = smp_processor_id();
 	int cpu, nr = INT_MAX;
@@ -6167,18 +6166,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	if (!this_sd)
 		return -1;
 
-	/*
-	 * Due to large variance we need a large fuzz factor; hackbench in
-	 * particularly is sensitive here.
-	 */
-	avg_idle = this_rq()->avg_idle / 512;
-	avg_cost = this_sd->avg_scan_cost + 1;
+	if (sched_feat(SIS_PROP)) {
+		u64 avg_cost, avg_idle, span_avg;
 
-	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
-		return -1;
+		/*
+		 * Due to large variance we need a large fuzz factor;
+		 * hackbench in particularly is sensitive here.
+		 */
+		avg_idle = this_rq()->avg_idle / 512;
+		avg_cost = this_sd->avg_scan_cost + 1;
 
-	if (sched_feat(SIS_PROP)) {
-		u64 span_avg = sd->span_weight * avg_idle;
+		span_avg = sd->span_weight * avg_idle;
 		if (span_avg > 4*avg_cost)
 			nr = div_u64(span_avg, avg_cost);
 		else

kernel/sched/features.h

@@ -54,7 +54,6 @@ SCHED_FEAT(TTWU_QUEUE, true)
 /*
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
  */
-SCHED_FEAT(SIS_AVG_CPU, false)
 SCHED_FEAT(SIS_PROP, true)
 
 /*