sched: Improve scalability via 'CPU buddies', which withstand random perturbations

Traversing an entire package is not only expensive, it also leads to tasks
bouncing all over a partially idle and possible quite large package.  Fix
that up by assigning a 'buddy' CPU to try to motivate.  Each buddy may try
to motivate that one other CPU, if it's busy, tough, it may then try its
SMT sibling, but that's all this optimization is allowed to cost.

Sibling cache buddies are cross-wired to prevent bouncing.

4 socket 40 core + SMT Westmere box, single 30 sec tbench runs, higher is better:

 clients     1       2       4        8       16       32       64      128
 ..........................................................................
 pre        30      41     118      645     3769     6214    12233    14312
 post      299     603    1211     2418     4697     6847    11606    14557

A nice increase in performance.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1339471112.7352.32.camel@marge.simpson.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 7 insertions, 21 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c099cc6eebe3..dd00aaf44fda 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2637,8 +2637,6 @@ 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
@@ -2655,29 +2653,17 @@ static int select_idle_sibling(struct task_struct *p, int target)
                 return prev_cpu;
 
         /*
-         * Otherwise, iterate the domains and find an elegible idle cpu.
+         * Otherwise, check assigned siblings to find an elegible idle cpu.
          */
         sd = rcu_dereference(per_cpu(sd_llc, target));
-        for_each_lower_domain(sd) {
-                sg = sd->groups;
-                do {
-                        if (!cpumask_intersects(sched_group_cpus(sg),
-                                                tsk_cpus_allowed(p)))
-                                goto next;
-
-                        for_each_cpu(i, sched_group_cpus(sg)) {
-                                if (!idle_cpu(i))
-                                        goto next;
-                        }
 
-                        target = cpumask_first_and(sched_group_cpus(sg),
-                                        tsk_cpus_allowed(p));
-                        goto done;
-next:
-                        sg = sg->next;
-                } while (sg != sd->groups);
+        for_each_lower_domain(sd) {
+                if (!cpumask_test_cpu(sd->idle_buddy, tsk_cpus_allowed(p)))
+                        continue;
+                if (idle_cpu(sd->idle_buddy))
+                        return sd->idle_buddy;
         }
-done:
+
         return target;
 }