sched: Fix select_idle_sibling() bouncing cow syndrome

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>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1359371965.5783.127.camel@marge.simpson.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8dbee9f..ed18c74 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3252,25 +3252,18 @@ 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;
+       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.
@@ -3284,7 +3277,7 @@ static int select_idle_sibling(struct task_struct *p, int target)
                                goto next;
 
                        for_each_cpu(i, sched_group_cpus(sg)) {
-                               if (!idle_cpu(i))
+                               if (i == target || !idle_cpu(i))
                                        goto next;
                        }