[PATCH v2] sched/rt: fix call to cpufreq_update_util

[Vincent Guittot]

With commit 8f111bc357aa ("cpufreq/schedutil: Rewrite CPUFREQ_RT support")
schedutil governor uses rq->rt.rt_nr_running to detect whether a RT task is
currently running on the CPU and to set frequency to max if necessary.
cpufreq_update_util() is called in enqueue/dequeue_top_rt_rq() but
rq->rt.rt_nr_running as not been updated yet when dequeue_top_rt_rq() is
called so schedutil still considers that a RT task is running when the
last task is dequeued. The update of rq->rt.rt_nr_running happens later
in dequeue_rt_stack()

Fixes: 8f111bc357aa ('cpufreq/schedutil: Rewrite CPUFREQ_RT support')
Cc: <stable@xxxxxxxxxxxxxxx> # v4.16+
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
- v2:
  - remove blank line

 kernel/sched/rt.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 7aef6b4..a9f1119 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1000,9 +1000,6 @@ dequeue_top_rt_rq(struct rt_rq *rt_rq)
 
 	sub_nr_running(rq, rt_rq->rt_nr_running);
 	rt_rq->rt_queued = 0;
-
-	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
-	cpufreq_update_util(rq, 0);
 }
 
 static void
@@ -1288,6 +1285,9 @@ static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)
 		if (on_rt_rq(rt_se))
 			__dequeue_rt_entity(rt_se, flags);
 	}
+
+	/* Kick cpufreq (see the comment in kernel/sched/sched.h). */
+	cpufreq_update_util(rq_of_rt_rq(rt_rq_of_se(back)), 0);
 }
 
 static void enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
-- 
2.7.4