[PATCH] sched,rt,nohz: stop scheduler tick if running realtime task

[Rik van Riel <riel@redhat.com>]

If the CPU is running a realtime task that does not round-robin with
another realtime task of equal priority, there is no point in keeping
the scheduler tick going. After all, whenever the scheduler tick runs,
the kernel will just decide not to reschedule.

Extend sched_can_stop_tick to recognize these situations, and inform
the rest of the kernel that the scheduler tick can be stopped.

Signed-off-by: Rik van Riel <riel@redhat.com>
Tested-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 kernel/sched/core.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ade2958a9197..ad985a632c4d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -745,6 +745,22 @@ static inline bool got_nohz_idle_kick(void)
 bool sched_can_stop_tick(void)
 {
 	/*
+	 * FIFO realtime policy runs the highest priority task. Other runnable
+	 * tasks are of a lower priority. The scheduler tick does nothing.
+	 */
+	if (current->policy == SCHED_FIFO)
+		return true;
+
+	/*
+	 * Round-robin realtime tasks time slice with other tasks at the same
+	 * realtime priority. Is this task the only one at this priority?
+	 */
+	if (current->policy == SCHED_RR) {
+		struct sched_rt_entity *rt_se = &current->rt;
+		return rt_se->run_list.prev == rt_se->run_list.next;
+	}
+
+	/*
 	 * More than one running task need preemption.
 	 * nr_running update is assumed to be visible
 	 * after IPI is sent from wakers.