Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller

Jan Kiszka
2025-04-09T06:41:47Z

On 12.10.23 17:07, Valentin Schneider wrote:
> Hi folks,
>
> We've had reports of stalls happening on our v6.0-ish frankenkernels, and while
> we haven't been able to come out with a reproducer (yet), I don't see anything
> upstream that would prevent them from happening.
>
> The setup involves eventpoll, CFS bandwidth controller and timer
> expiry, and the sequence looks as follows (time-ordered):
>
> p_read (on CPUn, CFS with bandwidth controller active)
> ======
>
> ep_poll_callback()
> read_lock_irqsave()
> ...
> try_to_wake_up() <- enqueue causes an update_curr() + sets need_resched
> due to having no more runtime
> preempt_enable()
> preempt_schedule() <- switch out due to p_read being now throttled
>
> p_write
> =======
>
> ep_poll()
> write_lock_irq() <- blocks due to having active readers (p_read)
>
> ktimers/n
> =========
>
> timerfd_tmrproc()
> `\
> ep_poll_callback()
> `\
> read_lock_irqsave() <- blocks due to having active writer (p_write)
>
>
> From this point we have a circular dependency:
>
> p_read -> ktimers/n (to replenish runtime of p_read)
> ktimers/n -> p_write (to let ktimers/n acquire the readlock)
> p_write -> p_read (to let p_write acquire the writelock)
>
> IIUC reverting
> 286deb7ec03d ("locking/rwbase: Mitigate indefinite writer starvation")
> should unblock this as the ktimers/n thread wouldn't block, but then we're back
> to having the indefinite starvation so I wouldn't necessarily call this a win.
>
> Two options I'm seeing:
> - Prevent p_read from being preempted when it's doing the wakeups under the
> readlock (icky)
> - Prevent ktimers / ksoftirqd (*) from running the wakeups that have
> ep_poll_callback() as a wait_queue_entry callback. Punting that to e.g. a
> kworker /should/ do.
>
> (*) It's not just timerfd, I've also seen it via net::sock_def_readable -
> it should be anything that's pollable.
>
> I'm still scratching my head on this, so any suggestions/comments welcome!
>

We are hunting for quite some time sporadic lock-ups or RT systems,
first only in the field (sigh), now finally also in the lab. Those have
a fairly high overlap with what was described here. Our baselines so
far: 6.1-rt, Debian and vanilla. We are currently preparing experiments
with latest mainline.

While this thread remained silent afterwards, we have found [1][2][3] as
apparently related. But this means we are still with this RT bug, even
in latest 6.15-rc1?

Jan

[1] https://lore.kernel.org/lkml/20231030145104.4107573-1-vschneid@redhat.com/
[2] https://lore.kernel.org/lkml/20240202080920.3337862-1-vschneid@redhat.com/
[3] https://lore.kernel.org/lkml/20250220093257.9380-1-kprateek.nayak@amd.com/

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