sched/deadline: Fix sched_yield() behavior

yield_task_dl() is broken:

 o it forces current to be throttled setting its runtime to zero;
 o it sets current's dl_se->dl_new to one, expecting that dl_task_timer()
   will queue it back with proper parameters at replenish time.

Unfortunately, dl_task_timer() has this check at the very beginning:

	if (!dl_task(p) || dl_se->dl_new)
		goto unlock;

So, it just bails out and the task is never replenished. It actually
yielded forever.

To fix this, introduce a new flag indicating that the task properly yielded
the CPU before its current runtime expired. While this is a little overdoing
at the moment, the flag would be useful in the future to discriminate between
"good" jobs (of which remaining runtime could be reclaimed, i.e. recycled)
and "bad" jobs (for which dl_throttled task has been set) that needed to be
stopped.

Reported-by: yjay.kim <yjay.kim@lge.com>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140429103953.e68eba1b2ac3309214e3dc5a@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 4 insertions, 2 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9fe2190..e62c65a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3124,6 +3124,7 @@ __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
 	dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
 	dl_se->dl_throttled = 0;
 	dl_se->dl_new = 1;
+	dl_se->dl_yielded = 0;
 }
 
 static void __setscheduler_params(struct task_struct *p,
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index b080957..800e99b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -528,6 +528,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	sched_clock_tick();
 	update_rq_clock(rq);
 	dl_se->dl_throttled = 0;
+	dl_se->dl_yielded = 0;
 	if (p->on_rq) {
 		enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
 		if (task_has_dl_policy(rq->curr))
@@ -893,10 +894,10 @@ static void yield_task_dl(struct rq *rq)
 	 * We make the task go to sleep until its current deadline by
 	 * forcing its runtime to zero. This way, update_curr_dl() stops
 	 * it and the bandwidth timer will wake it up and will give it
-	 * new scheduling parameters (thanks to dl_new=1).
+	 * new scheduling parameters (thanks to dl_yielded=1).
 	 */
 	if (p->dl.runtime > 0) {
-		rq->curr->dl.dl_new = 1;
+		rq->curr->dl.dl_yielded = 1;
 		p->dl.runtime = 0;
 	}
 	update_curr_dl(rq);