[PATCH] sched: less newidle locking

Similarly to the earlier change in load_balance, only lock the runqueue in
load_balance_newidle if the busiest queue found has a nr_running > 1.  This
will reduce frequency of expensive remote runqueue lock aquisitions in the
schedule() path on some workloads.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 10 insertions, 7 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index c61ee3451a04..930189540f3b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2104,8 +2104,7 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
                  */
                 double_lock_balance(this_rq, busiest);
                 nr_moved = move_tasks(this_rq, this_cpu, busiest,
-                                                imbalance, sd, idle,
-                                                &all_pinned);
+                                        imbalance, sd, idle, &all_pinned);
                 spin_unlock(&busiest->lock);
 
                 /* All tasks on this runqueue were pinned by CPU affinity */
@@ -2200,18 +2199,22 @@ static int load_balance_newidle(int this_cpu, runqueue_t *this_rq,
 
         BUG_ON(busiest == this_rq);
 
-        /* Attempt to move tasks */
-        double_lock_balance(this_rq, busiest);
-
         schedstat_add(sd, lb_imbalance[NEWLY_IDLE], imbalance);
-        nr_moved = move_tasks(this_rq, this_cpu, busiest,
+
+        nr_moved = 0;
+        if (busiest->nr_running > 1) {
+                /* Attempt to move tasks */
+                double_lock_balance(this_rq, busiest);
+                nr_moved = move_tasks(this_rq, this_cpu, busiest,
                                         imbalance, sd, NEWLY_IDLE, NULL);
+                spin_unlock(&busiest->lock);
+        }
+
         if (!nr_moved)
                 schedstat_inc(sd, lb_failed[NEWLY_IDLE]);
         else
                 sd->nr_balance_failed = 0;
 
-        spin_unlock(&busiest->lock);
         return nr_moved;
 
 out_balanced: