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

Hillf Danton
2012-09-25T12:10:37Z

On Tue, Sep 25, 2012 at 12:54 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 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;

Any reason to determine candidate by scanning a non-idle core?
> }
>
> - 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);
>
> --