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

[Mike Galbraith <bitbucket@online.de>]

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
 				goto next;
 
 			for_each_cpu(i, sched_group_cpus(sg)) {
-				if (!idle_cpu(i))
+				if (i == target || !idle_cpu(i))
 					goto next;
 			}