Re: [BUG almost bisected] Splat in dequeue_rt_stack() and build error

Paul E. McKenney
2024-08-28T18:39:21Z

On Wed, Aug 28, 2024 at 08:17:06PM +0200, Valentin Schneider wrote:
> On 28/08/24 09:35, Paul E. McKenney wrote:
> > On Wed, Aug 28, 2024 at 04:32:41PM +0200, Valentin Schneider wrote:
> >> On 28/08/24 21:44, Chen Yu wrote:
> >> >
> >> > One question, although there is no DEQUEUE_DELAYED flag, it is possible
> >> > the delayed task could be dequeued from CFS tree. Because the dequeue in
> >> > set_schedule() does not have DEQUEUE_SLEEP. And in dequeue_entity():
> >> >
> >> > bool sleep = flags & DEQUEUE_SLEEP;
> >> >
> >> > if (flags & DEQUEUE_DELAYED) {
> >> >
> >> > } else {
> >> > bool delay = sleep;
> >> > if (sched_feat(DELAY_DEQUEUE) && delay && //false
> >> > !entity_eligible(cfs_rq, se) {
> >> > //do not dequeue
> >> > }
> >> > }
> >> >
> >> > //dequeue the task <---- we should reach here?
> >> >
> >>
> >> You're quite right, so really here the main missing bit would be the final
> >> __block_task() that a DEQUEUE_DELAYED dequeue_entities() would get us.
> >
> > 50*TREE03 passed, yay! Thank you both!!!
>
> Fantastic, I'll hammer this into a "proper" patch then. Thanks again for
> all the testing!
>
> > I started a 500*TREE03.
> >
> > Yes, the odds all 50 passing given the baseline 52% failure rate is
> > something like 10^-16, but software bugs are not necessarily constrained
> > by elementary statistics...
>
> :-)

The 500*TREE03 run had exactly one failure that was the dreaded
enqueue_dl_entity() failure, followed by RCU CPU stall warnings.

But a huge improvement over the prior state!

Plus, this failure is likely unrelated (see earlier discussions with
Peter). I just started a 5000*TREE03 run, just in case we can now
reproduce this thing.

Thanx, Paul