Re: [BUG: NULL pointer dereference] cgroups and RT scheduling interact badly.

["Daniel K." <dk@uw.no>]

Peter Zijlstra wrote:
> On Tue, 2008-06-17 at 21:48 +0000, Daniel K. wrote:
>> I had almost given up trying to break it, but then this happened.
>>
>> [...]
> 
> Ah, fun a race between dequeueing because of runtime quota and
> requeueing because of RR slice length.
> 
>> Yes, I realize I'm starting to sound like a broken record.
> 
> Ah, don't worry - I was just hoping there was an end to the amount of
> glaring bugs in my code :-/

:)

> Reproducing was a bit harder than for you, it took me a whole minute of
> runtime and setting the runtime limit above the RR slice length (and
> realizing you're running RR, not FIFO).
> 
> The below patch (on top of the other one) seems to not make it crash
> this case for at least 15 minutes.

I am happy to say that this nailed it squarely on the head. I no longer
see any of the Oops'es I could quite easily trigger before. I added my
Tested-by, please add it to the patch you sent yesterday as well.

I still have a few gripes with RR scheduling, but that is a topic for
another mail.

> ---
> Subject: sched: rt-group: fix RR buglet
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> In tick_task_rt() we first call update_curr_rt() which can dequeue a runqueue
> due to it running out of runtime, and then we try to requeue it, of it also 
> having exhausted its RR quota. Obviously requeueing something that is no longer
> on the runqueue will not have the expected result.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Daniel K. <dk@uw.no>

> ---
>  kernel/sched_rt.c |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/kernel/sched_rt.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_rt.c
> +++ linux-2.6/kernel/sched_rt.c
> @@ -549,8 +549,10 @@ static
>  void requeue_rt_entity(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se)
>  {
>  	struct rt_prio_array *array = &rt_rq->active;
> +	struct list_head *queue = array->queue + rt_se_prio(rt_se);
>  
> -	list_move_tail(&rt_se->run_list, array->queue + rt_se_prio(rt_se));
> +	if (on_rt_rq(rt_se))
> +		list_move_tail(&rt_se->run_list, queue);
>  }
>  
>  static void requeue_task_rt(struct rq *rq, struct task_struct *p)

Daniel K.