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

[Christian Loehle]

On 7/30/25 11:06, Geert Uytterhoeven wrote:
> 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>
> 

FWIW the reported issue is also present on an arm64 rk3399 and
$SUBJECT fixes that.