[PATCH] sched: less locking

During periodic load balancing, don't hold this runqueue's lock while
scanning remote runqueues, which can take a non trivial amount of time
especially on very large systems.

Holding the runqueue lock will only help to stabilise ->nr_running, however
this doesn't do much to help because tasks being woken will simply get held
up on the runqueue lock, so ->nr_running would not provide a really
accurate picture of runqueue load in that case anyway.

What's more, ->nr_running (and possibly the cpu_load averages) of remote
runqueues won't be stable anyway, so load balancing is always an inexact
operation.

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>

kernel/sched.c

@@ -2075,7 +2075,6 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
 	int nr_moved, all_pinned = 0;
 	int active_balance = 0;
 
-	spin_lock(&this_rq->lock);
 	schedstat_inc(sd, lb_cnt[idle]);
 
 	group = find_busiest_group(sd, this_cpu, &imbalance, idle);
@@ -2102,18 +2101,16 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
 		 * still unbalanced. nr_moved simply stays zero, so it is
 		 * correctly treated as an imbalance.
 		 */
-		double_lock_balance(this_rq, busiest);
+		double_rq_lock(this_rq, busiest);
 		nr_moved = move_tasks(this_rq, this_cpu, busiest,
 					imbalance, sd, idle, &all_pinned);
-		spin_unlock(&busiest->lock);
+		double_rq_unlock(this_rq, busiest);
 
 		/* All tasks on this runqueue were pinned by CPU affinity */
 		if (unlikely(all_pinned))
 			goto out_balanced;
 	}
 
-	spin_unlock(&this_rq->lock);
-
 	if (!nr_moved) {
 		schedstat_inc(sd, lb_failed[idle]);
 		sd->nr_balance_failed++;
@@ -2156,8 +2153,6 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
 	return nr_moved;
 
 out_balanced:
-	spin_unlock(&this_rq->lock);
-
 	schedstat_inc(sd, lb_balanced[idle]);
 
 	sd->nr_balance_failed = 0;