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!
Cheers,
Valentin
