[patch] smpnice: don't consider sched groups which are lightly loaded for balancing

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

with smpnice, sched groups with highest priority tasks can mask the 
imbalance between the other sched groups with in the same domain. 
This patch fixes some of the listed down scenarios by not considering 
the sched groups which are lightly loaded. 

a) on a simple 4-way MP system, if we have one high priority and 4 normal
priority tasks, with smpnice we would like to see the high priority task
scheduled on one cpu, two other cpus getting one normal task each and the
fourth cpu getting the remaining two normal tasks. but with current smpnice
extra normal priority task keeps jumping from one cpu to another cpu having
the normal priority task.  This is because of the busiest_has_loaded_cpus, 
nr_loaded_cpus logic.. We are not including the cpu with high priority 
task in max_load calculations but including that in total and avg_load 
calcuations.. leading to max_load < avg_load and load balance between 
cpus running normal priority tasks(2 Vs 1) will always show imbalanace 
as one normal priority and the extra normal priority task will keep moving 
from one cpu to another cpu having normal priority task..

b) 4-way system with HT (8 logical processors). Package-P0 T0 has a highest 
priority task, T1 is idle. Package-P1 Both T0 and T1 have 1 normal priority 
task each..  P2 and P3 are idle.  With this patch, one of the normal priority 
tasks on P1 will be moved to P2 or P3..

c) With the current weighted smp nice calculations, it doesn't always make 
sense to look at the highest weighted runqueue in the busy group..
Consider a load balance scenario on a DP with HT system, with Package-0 
containing one high priority and one low priority, Package-1 containing 
one low priority(with other thread being idle)..  Package-1 thinks that it 
need to take the low priority thread from Package-0. And find_busiest_queue() 
returns the cpu thread with highest priority task.. And ultimately(with help 
of active load balance) we move high priority task to Package-1. And same 
continues with Package-0 now, moving high priority task from package-1 to 
package-0..  Even without the presence of active load balance, load balance 
will fail to balance the above scenario..  Fix find_busiest_queue to use 
"imbalance" when it is lightly loaded.

To fix all the scenarios, we probably need another look at the smpnice 
balancing patches..

This patch doesn't fix this issue for example:
4-way simple MP system. P0 containing two high priority tasks, P1 containing
one high priority and two normal priority tasks, one high priotity task
each on P2, P3. Current load balance doesn't detect/fix the
imbalance by moving one of the normal priority task running on P1 to P2 or P3.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>

--- linux-2.6.17-rc1/kernel/sched.c	2006-04-13 11:38:16.897333112 -0700
+++ linux~/kernel/sched.c	2006-04-19 16:19:01.294816528 -0700
@@ -2145,7 +2145,6 @@ find_busiest_group(struct sched_domain *
 	unsigned long max_pull;
 	unsigned long busiest_load_per_task, busiest_nr_running;
 	unsigned long this_load_per_task, this_nr_running;
-	unsigned int busiest_has_loaded_cpus = idle == NEWLY_IDLE;
 	int load_idx;
 
 	max_load = this_load = total_load = total_pwr = 0;
@@ -2200,15 +2199,8 @@ find_busiest_group(struct sched_domain *
 			this = group;
 			this_nr_running = sum_nr_running;
 			this_load_per_task = sum_weighted_load;
-		} else if (nr_loaded_cpus) {
-			if (avg_load > max_load || !busiest_has_loaded_cpus) {
-				max_load = avg_load;
-				busiest = group;
-				busiest_nr_running = sum_nr_running;
-				busiest_load_per_task = sum_weighted_load;
-				busiest_has_loaded_cpus = 1;
-			}
-		} else if (!busiest_has_loaded_cpus && avg_load > max_load) {
+		} else if (avg_load > max_load && 
+			   sum_nr_running > group->cpu_power / SCHED_LOAD_SCALE) {
 			max_load = avg_load;
 			busiest = group;
 			busiest_nr_running = sum_nr_running;
@@ -2241,6 +2233,16 @@ find_busiest_group(struct sched_domain *
 	if (max_load <= busiest_load_per_task)
 		goto out_balanced;
 
+	/*
+	 * In the presence of smp nice balancing, certain scenarios can have
+	 * max load less than avg load(as we skip the groups at or below
+	 * its cpu_power, while calculating max_load..) 
+	 */
+	if (max_load < avg_load) {
+		*imbalance = 0;
+		goto small_imbalance;
+	}
+
 	/* 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);
 
@@ -2255,6 +2257,7 @@ find_busiest_group(struct sched_domain *
 	 * a think about bumping its value to force at least one task to be
 	 * moved
 	 */
+small_imbalance:
 	if (*imbalance < busiest_load_per_task) {
 		unsigned long pwr_now = 0, pwr_move = 0;
 		unsigned long tmp;
@@ -2318,23 +2321,19 @@ out_balanced:
  * find_busiest_queue - find the busiest runqueue among the cpus in group.
  */
 static runqueue_t *find_busiest_queue(struct sched_group *group,
-	enum idle_type idle)
+	enum idle_type idle, unsigned long imbalance)
 {
 	unsigned long max_load = 0;
 	runqueue_t *busiest = NULL, *rqi;
-	unsigned int busiest_is_loaded = idle == NEWLY_IDLE;
 	int i;
 
 	for_each_cpu_mask(i, group->cpumask) {
 		rqi = cpu_rq(i);
 
-		if (rqi->nr_running > 1) {
-			if (rqi->raw_weighted_load > max_load || !busiest_is_loaded) {
-				max_load = rqi->raw_weighted_load;
-				busiest = rqi;
-				busiest_is_loaded = 1;
-			}
-		} else if (!busiest_is_loaded && rqi->raw_weighted_load > max_load) {
+		if (rqi->nr_running == 1 && rqi->raw_weighted_load > imbalance)
+			continue;
+
+		if (rqi->raw_weighted_load > max_load) {
 			max_load = rqi->raw_weighted_load;
 			busiest = rqi;
 		}
@@ -2377,7 +2376,7 @@ static int load_balance(int this_cpu, ru
 		goto out_balanced;
 	}
 
-	busiest = find_busiest_queue(group, idle);
+	busiest = find_busiest_queue(group, idle, imbalance);
 	if (!busiest) {
 		schedstat_inc(sd, lb_nobusyq[idle]);
 		goto out_balanced;
@@ -2501,7 +2500,7 @@ static int load_balance_newidle(int this
 		goto out_balanced;
 	}
 
-	busiest = find_busiest_queue(group, NEWLY_IDLE);
+	busiest = find_busiest_queue(group, NEWLY_IDLE, imbalance);
 	if (!busiest) {
 		schedstat_inc(sd, lb_nobusyq[NEWLY_IDLE]);
 		goto out_balanced;