[PATCH] sched: move enough load to balance average load per task

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Problem:

The current implementation of find_busiest_group() recognizes that approximately equal average loads per task for each group/queue are desirable (e.g. this condition will increase the probability that the top N highest priority tasks on an N CPU system will be on different CPUs) by being slightly more aggressive when *imbalance is small but the average load per task in "busiest" group is more than that in "this" group. Unfortunately, the amount moved from "busiest" to "this" is too small to reduce the average load per task on "busiest" (at best there will be no change and at worst it will get bigger).

Solution:

Increase the amount of load moved from "busiest" to "this" in these circumstances while making sure that the amount of load moved won't increase the (absolute) difference in the two groups' total weighted loads. A task with a weighted load greater than the average needs to be moved to cause the average to be reduced.

NB This makes no difference to load balancing for the case where all tasks have nice==0.

Signed-off-by: Peter Williams <[email protected]>

--
Peter Williams                                   [email protected]

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
Index: MM-2.6.17-rc1-mm2/kernel/sched.c
===================================================================
--- MM-2.6.17-rc1-mm2.orig/kernel/sched.c	2006-04-10 10:46:53.000000000 +1000
+++ MM-2.6.17-rc1-mm2/kernel/sched.c	2006-04-10 14:16:32.000000000 +1000
@@ -2258,16 +2258,20 @@ find_busiest_group(struct sched_domain *
 	if (*imbalance < busiest_load_per_task) {
 		unsigned long pwr_now = 0, pwr_move = 0;
 		unsigned long tmp;
-		unsigned int imbn = 2;
 
-		if (this_nr_running) {
+		if (this_nr_running)
 			this_load_per_task /= this_nr_running;
-			if (busiest_load_per_task > this_load_per_task)
-				imbn = 1;
-		} else
+		else
 			this_load_per_task = SCHED_LOAD_SCALE;
 
-		if (max_load - this_load >= busiest_load_per_task * imbn) {
+		if (busiest_load_per_task > this_load_per_task) {
+			unsigned long dld = max_load - this_load;
+
+			if (dld > busiest_load_per_task) {
+				*imbalance = (dld + busiest_load_per_task) / 2;
+				return busiest;
+			}
+		} else if (max_load - this_load >= busiest_load_per_task * 2) {
 			*imbalance = busiest_load_per_task;
 			return busiest;
 		}

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