[PATCH] Fix smpnice high priority task hopping problem

[Peter Williams]

Suresh B. Siddha has reported:

"on a lightly loaded system, this will result in higher priority job 
hopping around from one processor to another processor.. This is because 
of the code in find_busiest_group() which assumes that SCHED_LOAD_SCALE 
represents a unit process load and with nice_to_bias calculations this 
is no longer true (in the presence of non nice-0 tasks)"

Analysis of this problem as revealed that the smpnice code results in 
the weighted load being larger than 1 and this triggers the active load 
balancing code.  However, in active_load_balance(), the migration thread 
fails to take into account itself when deciding if there are any tasks 
to be migrated from its run queue. I.e. even if there is only one other 
task on the run queue other than itself it will still migrate that other 
task.

The attached patch fixes that anomaly.

Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>

Peter
--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
Index: MM-2.6.X/kernel/sched.c
===================================================================
--- MM-2.6.X.orig/kernel/sched.c	2006-02-16 10:51:52.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-02-16 11:02:45.000000000 +1100
@@ -2406,7 +2406,7 @@ static void active_load_balance(runqueue
 	runqueue_t *target_rq;
 	int target_cpu = busiest_rq->push_cpu;
 
-	if (busiest_rq->nr_running <= 1)
+	if (busiest_rq->nr_running <= 2)
 		/* no task to move */
 		return;