Merge tag 'sched-urgent-2026-02-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull scheduler fix from Ingo Molnar:
 "Fix a regression in the deferrable dl_server code that can cause the
  dl_server to be stuck"

* tag 'sched-urgent-2026-02-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/deadline: Fix 'stuck' dl_server

Author: torvalds

kernel/sched/deadline.c

@@ -1034,6 +1034,12 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
 			return;
 		}
 
+		/*
+		 * When [4] D->A is followed by [1] A->B, dl_defer_running
+		 * needs to be cleared, otherwise it will fail to properly
+		 * start the zero-laxity timer.
+		 */
+		dl_se->dl_defer_running = 0;
 		replenish_dl_new_period(dl_se, rq);
 	} else if (dl_server(dl_se) && dl_se->dl_defer) {
 		/*
@@ -1655,6 +1661,12 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
  *   dl_server_active = 1;
  *   enqueue_dl_entity()
  *     update_dl_entity(WAKEUP)
+ *       if (dl_time_before() || dl_entity_overflow())
+ *         dl_defer_running = 0;
+ *         replenish_dl_new_period();
+ *           // fwd period
+ *           dl_throttled = 1;
+ *           dl_defer_armed = 1;
  *       if (!dl_defer_running)
  *         dl_defer_armed = 1;
  *         dl_throttled = 1;