[PATCH] sched: relax pinned balancing

The maximum rebalance interval allowed by the multiprocessor balancing
backoff is often not large enough to handle corner cases where there are
lots of tasks pinned on a CPU.  Suresh reported:

	I see system livelock's if for example I have 7000 processes
	pinned onto one cpu (this is on the fastest 8-way system I
	have access to).

After this patch, the machine is reported to go well above this number.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/sched.c

@@ -2030,6 +2030,12 @@ static runqueue_t *find_busiest_queue(struct sched_group *group)
 	return busiest;
 }
 
+/*
+ * Max backoff if we encounter pinned tasks. Pretty arbitrary value, but
+ * so long as it is large enough.
+ */
+#define MAX_PINNED_INTERVAL	512
+
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
@@ -2042,7 +2048,7 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
 	struct sched_group *group;
 	runqueue_t *busiest;
 	unsigned long imbalance;
-	int nr_moved, all_pinned;
+	int nr_moved, all_pinned = 0;
 	int active_balance = 0;
 
 	spin_lock(&this_rq->lock);
@@ -2133,7 +2139,8 @@ out_balanced:
 
 	sd->nr_balance_failed = 0;
 	/* tune up the balancing interval */
-	if (sd->balance_interval < sd->max_interval)
+	if ((all_pinned && sd->balance_interval < MAX_PINNED_INTERVAL) ||
+			(sd->balance_interval < sd->max_interval))
 		sd->balance_interval *= 2;
 
 	return 0;