sched: fix idle load balancing in softirqd context

Periodic load balancing in recent kernels happen in the softirq.  In
certain -rt configurations, these softirqs are handled in softirqd context.
 And hence the check for idle processor was always returning busy (as
nr_running > 1).

This patch captures the idle information at the tick and passes this info
to softirq context through an element 'idle_at_tick' in rq.

[kernel@kolivas.org: Fix reverse idle at tick logic]
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sched.c

@@ -223,6 +223,7 @@ struct rq {
 	unsigned long raw_weighted_load;
 #ifdef CONFIG_SMP
 	unsigned long cpu_load[3];
+	unsigned char idle_at_tick;
 #endif
 	unsigned long long nr_switches;
 
@@ -2943,12 +2944,7 @@ static void run_rebalance_domains(struct softirq_action *h)
 	struct rq *this_rq = cpu_rq(this_cpu);
 	unsigned long interval;
 	struct sched_domain *sd;
-	/*
-	 * We are idle if there are no processes running. This
-	 * is valid even if we are the idle process (SMT).
-	 */
-	enum idle_type idle = !this_rq->nr_running ?
-				SCHED_IDLE : NOT_IDLE;
+	enum idle_type idle = this_rq->idle_at_tick ? SCHED_IDLE : NOT_IDLE;
 	/* Earliest time when we have to call run_rebalance_domains again */
 	unsigned long next_balance = jiffies + 60*HZ;
 
@@ -3218,14 +3214,16 @@ void scheduler_tick(void)
 	unsigned long long now = sched_clock();
 	struct task_struct *p = current;
 	int cpu = smp_processor_id();
+	int idle_at_tick = idle_cpu(cpu);
 	struct rq *rq = cpu_rq(cpu);
 
 	update_cpu_clock(p, rq, now);
 
-	if (p != rq->idle)
+	if (!idle_at_tick)
 		task_running_tick(rq, p);
 #ifdef CONFIG_SMP
 	update_load(rq);
+	rq->idle_at_tick = idle_at_tick;
 	if (time_after_eq(jiffies, rq->next_balance))
 		raise_softirq(SCHED_SOFTIRQ);
 #endif