[RFC PATCH 1/1] sched/deadline: Fix RT task potential starvation when expiry time passed

[Kuyo Chang <kuyo.chang@mediatek.com>]

From: kuyo chang <kuyo.chang@mediatek.com>

[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.

Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
---
 kernel/sched/deadline.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ad45a8fea245..e50cb76c961b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1556,10 +1556,12 @@ 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))
-- 
2.45.2