sched/rt: Minimize rq->lock contention in do_sched_rt_period_timer()

With CONFIG_RT_GROUP_SCHED=y, do_sched_rt_period_timer() sequentially
takes each CPU's rq->lock. On a large, busy system, the cumulative time it
takes to acquire each lock can be excessive, even triggering a watchdog
timeout.

If rt_rq->rt_time and rt_rq->rt_nr_running are both zero, this function does
nothing while holding the lock, so don't bother taking it at all.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/a767637b-df85-912f-ba69-c90ee00a3fb6@oracle.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/rt.c

@@ -840,6 +840,17 @@ static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun)
 		int enqueue = 0;
 		struct rt_rq *rt_rq = sched_rt_period_rt_rq(rt_b, i);
 		struct rq *rq = rq_of_rt_rq(rt_rq);
+		int skip;
+
+		/*
+		 * When span == cpu_online_mask, taking each rq->lock
+		 * can be time-consuming. Try to avoid it when possible.
+		 */
+		raw_spin_lock(&rt_rq->rt_runtime_lock);
+		skip = !rt_rq->rt_time && !rt_rq->rt_nr_running;
+		raw_spin_unlock(&rt_rq->rt_runtime_lock);
+		if (skip)
+			continue;
 
 		raw_spin_lock(&rq->lock);
 		if (rt_rq->rt_time) {