sched/deadline: Fix RT task potential starvation when expiry time passed

JIRA: https://issues.redhat.com/browse/RHEL-110301

commit 421fc59
Author: kuyo chang <kuyo.chang@mediatek.com>
Date:   Sun Jun 15 21:10:56 2025 +0800

    sched/deadline: Fix RT task potential starvation when expiry time passed

    [Symptom]
    The fair server mechanism, which is intended to prevent fair starvation
    when higher-priority tasks monopolize the CPU.
    Specifically, RT tasks on the runqueue may not be scheduled as expected.

    [Analysis]
    The log "sched: DL replenish lagged too much" triggered.

    By memory dump of dl_server:
        curr = 0xFFFFFF80D6A0AC00 (
          dl_server = 0xFFFFFF83CD5B1470(
            dl_runtime = 0x02FAF080,
            dl_deadline = 0x3B9ACA00,
            dl_period = 0x3B9ACA00,
            dl_bw = 0xCCCC,
            dl_density = 0xCCCC,
            runtime = 0x02FAF080,
            deadline = 0x0000082031EB0E80,
            flags = 0x0,
            dl_throttled = 0x0,
            dl_yielded = 0x0,
            dl_non_contending = 0x0,
            dl_overrun = 0x0,
            dl_server = 0x1,
            dl_server_active = 0x1,
            dl_defer = 0x1,
            dl_defer_armed = 0x0,
            dl_defer_running = 0x1,
            dl_timer = (
              node = (
                expires = 0x000008199756E700),
              _softexpires = 0x000008199756E700,
              function = 0xFFFFFFDB9AF44D30 = dl_task_timer,
              base = 0xFFFFFF83CD5A12C0,
              state = 0x0,
              is_rel = 0x0,
              is_soft = 0x0,
        clock_update_flags = 0x4,
        clock = 0x000008204A496900,

     - The timer expiration time (rq->curr->dl_server->dl_timer->expires)
       is already in the past, indicating the timer has expired.
     - The timer state (rq->curr->dl_server->dl_timer->state) is 0.

    [Suspected Root Cause]
    The relevant code flow in the throttle path of
    update_curr_dl_se() as follows:

      dequeue_dl_entity(dl_se, 0);                // the DL entity is dequeued

      if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(dl_se))) {
          if (dl_server(dl_se))                   // timer registration fails
              enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);//enqueue immediately
          ...
      }

    The failure of `start_dl_timer` is caused by attempting to register a
    timer with an expiration time that is already in the past. When this
    situation persists, the code repeatedly re-enqueues the DL entity
    without properly replenishing or restarting the timer, resulting in RT
    task may not be scheduled as expected.

    [Proposed Solution]:
    Instead of immediately re-enqueuing the DL entity on timer registration
    failure, this change ensures the DL entity is properly replenished and
    the timer is restarted, preventing RT potential starvation.

    Fixes: 63ba842 ("sched/deadline: Introduce deadline servers")
    Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Closes: https://lore.kernel.org/CAMuHMdXn4z1pioTtBGMfQM0jsLviqS2jwysaWXpoLxWYoGa82w@mail.gmail.com
    Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
    Tested-by: Jiri Slaby <jirislaby@kernel.org>
    Tested-by: Diederik de Haas <didi.debian@cknow.org>
    Link: https://lkml.kernel.org/r/20250615131129.954975-1-kuyo.chang@mediatek.com

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

kernel/sched/deadline.c

@@ -1496,10 +1496,12 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
 		}
 
 		if (unlikely(is_dl_boosted(dl_se) || !start_dl_timer(dl_se))) {
-			if (dl_server(dl_se))
-				enqueue_dl_entity(dl_se, ENQUEUE_REPLENISH);
-			else
+			if (dl_server(dl_se)) {
+				replenish_dl_new_period(dl_se, rq);
+				start_dl_timer(dl_se);
+			} else {
 				enqueue_task_dl(rq, dl_task_of(dl_se), ENQUEUE_REPLENISH);
+			}
 		}
 
 		if (!is_leftmost(dl_se, &rq->dl))