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

[Juri Lelli]

Hello,

On 15/06/25 21:10, Kuyo Chang wrote:
> From: kuyo chang <kuyo.chang at 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 at 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);

But, even today, enqueue_dl_entity() is called with ENQUEUE_REPLENISH
flag, so I don't get why you say 're-enqueues the DL entity without
properly replenishing'.

Also, why restarting the replenishing timer right after having
replenished the entity?

Thanks,
Juri