Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2012-09-24 at 09:33 -0700, Linus Torvalds wrote:
> Sure, the "scan bits" bitops will return ">= nr_cpu_ids" for the "I
> couldn't find a bit" thing, but that doesn't mean that everything else
> should. 

Fair enough..

---
 kernel/sched/fair.c | 42 +++++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 21 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b800a1..329f78d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2634,25 +2634,12 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
  */
 static int select_idle_sibling(struct task_struct *p, int target)
 {
-	int cpu = smp_processor_id();
-	int prev_cpu = task_cpu(p);
 	struct sched_domain *sd;
 	struct sched_group *sg;
 	int i;
 
-	/*
-	 * If the task is going to be woken-up on this cpu and if it is
-	 * already idle, then it is the right target.
-	 */
-	if (target == cpu && idle_cpu(cpu))
-		return cpu;
-
-	/*
-	 * If the task is going to be woken-up on the cpu where it previously
-	 * ran and if it is currently idle, then it the right target.
-	 */
-	if (target == prev_cpu && idle_cpu(prev_cpu))
-		return prev_cpu;
+	if (idle_cpu(target))
+		return target;
 
 	/*
 	 * Otherwise, iterate the domains and find an elegible idle cpu.
@@ -2661,18 +2648,31 @@ static int select_idle_sibling(struct task_struct *p, int target)
 	for_each_lower_domain(sd) {
 		sg = sd->groups;
 		do {
-			if (!cpumask_intersects(sched_group_cpus(sg),
-						tsk_cpus_allowed(p)))
-				goto next;
+			int candidate = -1;
 
+			/*
+			 * In the SMT case the groups are the SMT-siblings,
+			 * otherwise they're singleton groups.
+			 */
 			for_each_cpu(i, sched_group_cpus(sg)) {
+				if (!cpumask_test_cpu(i, tsk_cpus_allowed(p)))
+					continue;
+
+				/*
+				 * If any of the SMT-siblings are !idle, the
+				 * core isn't idle.
+				 */
 				if (!idle_cpu(i))
 					goto next;
+
+				if (candidate < 0)
+					candidate = i;
 			}
 
-			target = cpumask_first_and(sched_group_cpus(sg),
-					tsk_cpus_allowed(p));
-			goto done;
+			if (candidate >= 0) {
+				target = candidate;
+				goto done;
+			}
 next:
 			sg = sg->next;
 		} while (sg != sd->groups);