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

Peter Zijlstra
2012-09-24T16:55:04Z

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);