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

[Geert Uytterhoeven]

Hi Kuyo,

On Mon, 16 Jun 2025 at 14:39, Kuyo Chang <kuyo.chang at mediatek.com> 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>

Thanks, this fixes the issue I was seeing!

Closes: https://lore.kernel.org/CAMuHMdXn4z1pioTtBGMfQM0jsLviqS2jwysaWXpoLxWYoGa82w@mail.gmail.com
Tested-by: Geert Uytterhoeven <geert at linux-m68k.org>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds