[PATCH] sched: Avoid unnecessarily moving highest priority task move_tasks()

Problem:

To help distribute high priority tasks evenly across the available CPUs
move_tasks() does not, under some circumstances, skip tasks whose load
weight is bigger than the designated amount.  Because the highest priority
task on the busiest queue may be on the expired array it may be moved as a
result of this mechanism.  Apart from not being the most desirable way to
redistribute the high priority tasks (we'd rather move the second highest
priority task), there is a risk that this could set up a loop with this
task bouncing backwards and forwards between the two queues.  (This latter
possibility can be demonstrated by running a nice==-20 CPU bound task on an
otherwise quiet 2 CPU system.)

Solution:

Modify the mechanism so that it does not override skip for the highest
priority task on the CPU.  Of course, if there are more than one tasks at
the highest priority then it will allow the override for one of them as
this is a desirable redistribution of high priority tasks.
Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -1941,6 +1941,7 @@ int can_migrate_task(task_t *p, runqueue_t *rq, int this_cpu,
 	return 1;
 }
 
+#define rq_best_prio(rq) min((rq)->curr->prio, (rq)->best_expired_prio)
 /*
  * move_tasks tries to move up to max_nr_move tasks and max_load_move weighted
  * load from busiest to this_rq, as part of a balancing operation within
@@ -1955,7 +1956,9 @@ static int move_tasks(runqueue_t *this_rq, int this_cpu, runqueue_t *busiest,
 {
 	prio_array_t *array, *dst_array;
 	struct list_head *head, *curr;
-	int idx, pulled = 0, pinned = 0, this_min_prio;
+	int idx, pulled = 0, pinned = 0, this_best_prio, busiest_best_prio;
+	int busiest_best_prio_seen;
+	int skip_for_load; /* skip the task based on weighted load issues */
 	long rem_load_move;
 	task_t *tmp;
 
@@ -1964,7 +1967,16 @@ static int move_tasks(runqueue_t *this_rq, int this_cpu, runqueue_t *busiest,
 
 	rem_load_move = max_load_move;
 	pinned = 1;
-	this_min_prio = this_rq->curr->prio;
+	this_best_prio = rq_best_prio(this_rq);
+	busiest_best_prio = rq_best_prio(busiest);
+	/*
+	 * Enable handling of the case where there is more than one task
+	 * with the best priority.   If the current running task is one
+	 * of those with prio==busiest_best_prio we know it won't be moved
+	 * and therefore it's safe to override the skip (based on load) of
+	 * any task we find with that prio.
+	 */
+	busiest_best_prio_seen = busiest_best_prio == busiest->curr->prio;
 
 	/*
 	 * We first consider expired tasks. Those will likely not be
@@ -2009,8 +2021,12 @@ static int move_tasks(runqueue_t *this_rq, int this_cpu, runqueue_t *busiest,
 	 * skip a task if it will be the highest priority task (i.e. smallest
 	 * prio value) on its new queue regardless of its load weight
 	 */
-	if ((idx >= this_min_prio && tmp->load_weight > rem_load_move) ||
+	skip_for_load = tmp->load_weight > rem_load_move;
+	if (skip_for_load && idx < this_best_prio)
+		skip_for_load = !busiest_best_prio_seen && idx == busiest_best_prio;
+	if (skip_for_load ||
 	    !can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
+		busiest_best_prio_seen |= idx == busiest_best_prio;
 		if (curr != head)
 			goto skip_queue;
 		idx++;
@@ -2031,8 +2047,8 @@ static int move_tasks(runqueue_t *this_rq, int this_cpu, runqueue_t *busiest,
 	 * and the prescribed amount of weighted load.
 	 */
 	if (pulled < max_nr_move && rem_load_move > 0) {
-		if (idx < this_min_prio)
-			this_min_prio = idx;
+		if (idx < this_best_prio)
+			this_best_prio = idx;
 		if (curr != head)
 			goto skip_queue;
 		idx++;