[PATCH v3] sched/deadline: Change the time to replenish runtime for sleep tasks

[Byungchul Park]

Let's consider the following example.

timeline : o...................o.........o.......o..o
           ^                   ^         ^       ^  ^
           |                   |         |       |  |
       start                   |         |       |  |
                original runtime         |       |  |
                     sleep with (-)runtime       |  |
                                 original deadline  |
                                              wake up

When this task is woken up, a negative runtime should be considered,
which means that the task should get penalized when assigning runtime,
becasue it already spent more than expected. Current code handles this
by replenishing a runtime in hrtimer callback for deadline. But this
approach has room for improvement:

   It will be replenished twice unnecessarily if the task sleeps for
   long time so that the deadline, assigned in the hrtimer callback,
   also passed. In other words, one happens in the callback and the
   other happens in update_dl_entiry() when waking it up.

So force to replenish it for sleep tasks when waking it up.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/sched/deadline.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 27737f3..cb43ce9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -498,8 +498,9 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
 	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
 	struct rq *rq = rq_of_dl_rq(dl_rq);
 
-	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
-	    dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
+	if (dl_time_before(dl_se->deadline, rq_clock(rq)))
+		replenish_dl_entity(dl_se, pi_se);
+	else if (dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
 		dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
 		dl_se->runtime = pi_se->dl_runtime;
 	}
@@ -621,13 +622,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	 *         __dequeue_task_dl()
 	 *     prev->on_rq = 0;
 	 *
-	 * We can be both throttled and !queued. Replenish the counter
-	 * but do not enqueue -- wait for our wakeup to do that.
+	 * We can be both throttled and !queued. Wait for our wakeup to
+	 * replenish runtime and enqueue p.
 	 */
-	if (!task_on_rq_queued(p)) {
-		replenish_dl_entity(dl_se, dl_se);
+	if (!task_on_rq_queued(p))
 		goto unlock;
-	}
 
 #ifdef CONFIG_SMP
 	if (unlikely(!rq->online)) {
-- 
1.9.1