Re: [v6.12] WARNING: at kernel/sched/deadline.c:1995 enqueue_dl_entity (task blocked for more than 28262 seconds)

[Ilya Maximets <i.maximets@ovn.org>]

On 12/9/24 13:29, Vineeth Remanan Pillai wrote:
> On Mon, Dec 9, 2024 at 5:55 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> On Fri, Dec 06, 2024 at 11:57:30AM -0500, Vineeth Remanan Pillai wrote:
>>
>>> I was able to reproduce this WARN_ON couple of days back with
>>> syzkaller. dlserver's dl_se gets enqueued during a update_curr while
>>> the dlserver is stopped. And subsequent dlserver start will cause a
>>> double enqueue.
>>
>> Right, I spotted that hole late last week. There is this thread:
>>
>>   https://lore.kernel.org/all/20241209094941.GF21636@noisy.programming.kicks-ass.net/T/#u
>>
>> Where I just added this thunk:
>>
>>   @@ -1674,6 +1679,12 @@ void dl_server_start(struct sched_dl_entity *dl_se)
>>
>>  void dl_server_stop(struct sched_dl_entity *dl_se)
>>  {
>> +       if (current->dl_server == dl_se) {
>> +               struct rq *rq = rq_of_dl_se(dl_se);
>> +               trace_printk("stop fair server %d\n", cpu_of(rq));
>> +               current->dl_server = NULL;
>> +       }
>> +
>>         if (!dl_se->dl_runtime)
>>                 return;
>>
>> Which was my attempt at plugging said hole. But since I do not have
>> means of reproduction, I'm not at all sure it is sufficient :/
>>
> I think I was able to get to the root cause last week. So the issue
> seems to be that dlserver is stopped in the pick_task path of dlserver
> itself when the sched_delayed is set:
> __pick_next_task
> => pick_task_dl -> server_pick_task
>      => pick_task_fair
>           => pick_next_entity (if (sched_delayed))
>                => dequeue_entities
>                      => dl_server_stop
> 
> Now server_pick_task returns NULL and then we set dl_yielded and call
> update_curr_dl_se. But dl_se is already dequeued and now the code is
> confused and it does all sorts of things including setting a timer to
> enqueue it back. This ultimately leads to double enqueue when dlserver
> is started next time(based on timing of dl_server_start)
> 
> I think we should not call update_curr_dl_se when the dlserver is
> dequeued. Based on this I have a small patch and it seems to solve the
> issue:
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index d9d5a702f1a6..a9f3f020e421 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2419,12 +2419,18 @@ static struct task_struct *__pick_task_dl(struct rq *rq)
> 
>         if (dl_server(dl_se)) {
>                 p = dl_se->server_pick_task(dl_se);
> -               if (!p) {
> +               if (p) {
> +                       rq->dl_server = dl_se;
> +               } else if (WARN_ON_ONCE(on_dl_rq(dl_se))) {
> +                       /*
> +                        * If server_pick_task returns NULL and dlserver is
> +                        * enqueued, we have a problem. Lets yield and do a
> +                        * pick again.
> +                        */
>                         dl_se->dl_yielded = 1;
>                         update_curr_dl_se(rq, dl_se, 0);
>                         goto again;
>                 }
> -               rq->dl_server = dl_se;
>         } else {
>                 p = dl_task_of(dl_se);
>         }
> 
> I can send a formal patch if this looks okay to you all..

Thanks!

I can try this out on my setup today over the night (it takes a long time
to reproduce the issue on my setup).

Best regards, Ilya Maximets.