sched/deadline: Fix server stopping with runnable tasks

The deadline server can currently stop due to idle although fair tasks
are runnable. This happens essentially when:

 * the server is set to idle, a task wakes up, the server stops
 * a task wakes up, the server sets itself to idle and stops right away

Address both cases by clearing the server idle flag whenever a fair task
wakes up and accounting also for pending tasks in the definition of idle.

Fixes: f5a538c ("sched/deadline: Fix dl_server stop condition")
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260113085159.114226-3-gmonaco@redhat.com

Author: glemco

kernel/sched/deadline.c

@@ -1420,7 +1420,7 @@ update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se, int
 
 static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 delta_exec)
 {
-	bool idle = rq->curr == rq->idle;
+	bool idle = idle_rq(rq);
 	s64 scaled_delta_exec;
 
 	if (unlikely(delta_exec <= 0)) {
@@ -1603,8 +1603,8 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  * | 8 |       B:zero_laxity-wait       |     |    |
  * |   |                                | <---+    |
  * |   +--------------------------------+          |
- * |     |              ^     ^           2        |
- * |     | 7            | 2   +--------------------+
+ * |     |              ^         ^       2        |
+ * |     | 7            | 2, 1    +----------------+
  * |     v              |
  * |   +-------------+  |
  * +-- | C:idle-wait | -+
@@ -1649,8 +1649,11 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_defer_idle = 0
  *
  *
- * [1] A->B, A->D
+ * [1] A->B, A->D, C->B
  * dl_server_start()
+ *   dl_defer_idle = 0;
+ *   if (dl_server_active)
+ *     return; // [B]
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
@@ -1759,6 +1762,7 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   "B:zero_laxity-wait" -> "C:idle-wait"        [label="7:dl_server_update_idle"]
  *   "B:zero_laxity-wait" -> "D:running"          [label="3:dl_server_timer"]
  *   "C:idle-wait" -> "A:init"                    [label="8:dl_server_timer"]
+ *   "C:idle-wait" -> "B:zero_laxity-wait"        [label="1:dl_server_start"]
  *   "C:idle-wait" -> "B:zero_laxity-wait"        [label="2:dl_server_update"]
  *   "C:idle-wait" -> "C:idle-wait"               [label="7:dl_server_update_idle"]
  *   "D:running" -> "A:init"                      [label="4:pick_task_dl"]
@@ -1784,6 +1788,7 @@ void dl_server_start(struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = dl_se->rq;
 
+	dl_se->dl_defer_idle = 0;
 	if (!dl_server(dl_se) || dl_se->dl_server_active)
 		return;