sched/deadline: Fix hrtick for a non-leftmost task

After update_curr_dl() the current task might not be the leftmost task
anymore. In that case do not start a new hrtick for it.

In this case NEED_RESCHED will be set and the next schedule will start
the hrtick for the new task if and when appropriate.

Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Acked-by: Juri Lelli <juri.lelli@arm.com>
[ Rewrote the changelog and comment. ]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Kirill Tkhai <ktkhai@parallels.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1416962647-76792-2-git-send-email-wanpeng.li@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/deadline.c

@@ -1073,7 +1073,13 @@ static void task_tick_dl(struct rq *rq, struct task_struct *p, int queued)
 {
 	update_curr_dl(rq);
 
-	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0)
+	/*
+	 * Even when we have runtime, update_curr_dl() might have resulted in us
+	 * not being the leftmost task anymore. In that case NEED_RESCHED will
+	 * be set and schedule() will start a new hrtick for the next task.
+	 */
+	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0 &&
+	    is_leftmost(p, &rq->dl))
 		start_hrtick_dl(rq, p);
 }