sched/balancing: Reduce the rate of needless idle load balancing

The current no_hz idle load balancer do load balancing for *all* idle cpus,
even though the time due to load balance for a particular
idle cpu could be still a while in the future.  This introduces a much
higher load balancing rate than what is necessary.  The patch
changes the behavior by only doing idle load balancing on
behalf of an idle cpu only when it is due for load balancing.

On SGI's systems with over 3000 cores, the cpu responsible for idle balancing
got overwhelmed with idle balancing, and introduces a lot of OS noise
to workloads.  This patch fixes the issue.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Acked-by: Russ Anderson <rja@sgi.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Cc: Hedi Berriche <hedi@sgi.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: MichelLespinasse <walken@google.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1400621967.2970.280.camel@schen9-DESK
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -7193,12 +7193,17 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
 
 		rq = cpu_rq(balance_cpu);
 
-		raw_spin_lock_irq(&rq->lock);
-		update_rq_clock(rq);
-		update_idle_cpu_load(rq);
-		raw_spin_unlock_irq(&rq->lock);
-
-		rebalance_domains(rq, CPU_IDLE);
+		/*
+		 * If time for next balance is due,
+		 * do the balance.
+		 */
+		if (time_after_eq(jiffies, rq->next_balance)) {
+			raw_spin_lock_irq(&rq->lock);
+			update_rq_clock(rq);
+			update_idle_cpu_load(rq);
+			raw_spin_unlock_irq(&rq->lock);
+			rebalance_domains(rq, CPU_IDLE);
+		}
 
 		if (time_after(this_rq->next_balance, rq->next_balance))
 			this_rq->next_balance = rq->next_balance;