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>

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index c18b50094fab962b52d74547a7eb8fcf9e099ff2..581d5c7a52645c4fdedd53c310247fc8fad6421d 100644 (file)
--- a/kernel/sched/rt.c
+++ b/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) {