[PATCH v3 1/7] sched: limit cpu search in select_idle_cpu

[subhra mazumdar <subhra.mazumdar@oracle.com>]

Put upper and lower limit on cpu search of select_idle_cpu. The lower limit
is amount of cpus in a core while upper limit is twice that. This ensures
for any architecture we will usually search beyond a core. The upper limit
also helps in keeping the search cost low and constant.

Signed-off-by: subhra mazumdar <subhra.mazumdar@oracle.com>
---
 kernel/sched/fair.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f35930f..b58f08f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6188,7 +6188,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	u64 avg_cost, avg_idle;
 	u64 time, cost;
 	s64 delta;
-	int cpu, nr = INT_MAX;
+	int cpu, limit, floor, nr = INT_MAX;
 
 	this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
 	if (!this_sd)
@@ -6206,10 +6206,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
 	if (sched_feat(SIS_PROP)) {
 		u64 span_avg = sd->span_weight * avg_idle;
-		if (span_avg > 4*avg_cost)
+		floor = cpumask_weight(topology_sibling_cpumask(target));
+		if (floor < 2)
+			floor = 2;
+		limit = floor << 1;
+		if (span_avg > floor*avg_cost) {
 			nr = div_u64(span_avg, avg_cost);
-		else
-			nr = 4;
+			if (nr > limit)
+				nr = limit;
+		} else {
+			nr = floor;
+		}
 	}
 
 	time = local_clock();
-- 
2.9.3