Re: [patch] sched: minimalist select_idle_sibling() bouncing cow syndrome fix

Mike Galbraith
2013-01-28T11:19:47Z

On Mon, 2013-01-28 at 11:53 +0100, Ingo Molnar wrote:
> * Mike Galbraith <bitbucket@online.de> wrote:
>
> > If the previous CPU is cache affine and idle, select it.
>
> No objections in principle - but would be nice to have a
> changelog with numbers, % of improvement included and so?

Well, that like was my changelog, guess it needs improvement.

Take 2.

sched: minimalist select_idle_sibling() bouncing cow syndrome fix

If the previous CPU is cache affine and idle, select it.

The current implementation simply traverses the sd_llc domain,
taking the first idle CPU encountered, which walks buddy pairs
hand in hand over the package, inflicting excruciating pain.

1 tbench pair (worst case) in a 10 core + SMT package:

pre 15.22 MB/sec 1 procs
post 252.01 MB/sec 1 procs

Signed-off-by: Mike Galbraith <bitbucket@online.de>
---
kernel/sched/fair.c | 21 +++++++--------------
1 file changed, 7 insertions(+), 14 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3270,25 +3270,18 @@ find_idlest_cpu(struct sched_group *grou
*/
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;
+ int i = task_cpu(p);

- /*
- * 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 (idle_cpu(target))
+ return target;

/*
- * 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 the prevous cpu is cache affine and idle, don't be stupid.
*/
- if (target == prev_cpu && idle_cpu(prev_cpu))
- return prev_cpu;
+ if (i != target && cpus_share_cache(i, target) && idle_cpu(i))
+ return i;

/*
* Otherwise, iterate the domains and find an elegible idle cpu.
@@ -3302,7 +3295,7 @@ static int select_idle_sibling(struct ta