[PATCH] sched: protect calculation of max_pull from integer wrap

[Peter Williams]

Problem:

As the comment above the calculation of max_pull in the function states, 
there is a need to ensure that negative results of the subtractions do 
not wrap around to large numbers.  This has not been implemented for the 
(max_load - busiest_load_per_task) expression and the possible 
consequences are for undesirable movement of tasks from one group to 
another group. E.g. consider a numa system with two nodes, each
node containing four processors.  If there are two processes in node-0 
and with node-1 being completely idle,  one of those processes will be 
moved to node-1 whereas the desired behavior is to retain those two 
processes in node-0.

Fix:

Make sure that max_load is greater than busiest_load_per_task before 
making the calculation.  If it isn't max_pull will be zero and we skip 
directly to out_balanced.

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

--
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	2006-03-27 11:07:52.000000000 +1100
+++ MM-2.6.X/kernel/sched.c	2006-03-27 12:08:47.000000000 +1100
@@ -2179,6 +2179,8 @@ find_busiest_group(struct sched_domain *
 	 * by pulling tasks to us.  Be careful of negative numbers as they'll
 	 * appear as very large values with unsigned longs.
 	 */
+	if (max_load <= busiest_load_per_task)
+		goto out_balanced;
 
 	/* Don't want to pull so many tasks that a group would go idle */
 	max_pull = min(max_load - avg_load, max_load - busiest_load_per_task);