Re: [PATCH v5 0/4] io_uring: remove ring quiesce in io_uring_register

[Pavel Begunkov]

On 2/4/22 00:15, Jens Axboe wrote:
> On 2/3/22 5:02 PM, Pavel Begunkov wrote:
> > On 2/3/22 23:34, Usama Arif wrote:
> > > For opcodes relating to registering/unregistering eventfds, this is done by
> > > creating a new RCU data structure (io_ev_fd) as part of io_ring_ctx that
> > > holds the eventfd_ctx, with reads to the structure protected by
> > > rcu_read_lock and writes (register/unregister calls) protected by a mutex.
> > >
> > > With the above approach ring quiesce can be avoided which is much more
> > > expensive then using RCU lock. On the system tested, io_uring_reigster with
> > > IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
> > > before with ring quiesce.
> > >
> > > The second patch creates the RCU protected data structure and removes ring
> > > quiesce for IORING_REGISTER_EVENTFD and IORING_UNREGISTER_EVENTFD.
> > >
> > > The third patch builds on top of the second patch and removes ring quiesce
> > > for IORING_REGISTER_EVENTFD_ASYNC.
> > >
> > > The fourth patch completely removes ring quiesce from io_uring_register,
> > > as IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS dont need
> > > them.
> >
> > Let me leave it just for history: I strongly dislike it considering
> > there is no one who uses or going to use it.
>
> Are you referring to the 4th patch? Or the patchset as a whole? Not clear
> to me, because eventfd registration is most certainly used by folks
> today.

I refer to optimising eventfd unregister with no users of it, which
lead to the RCU approach.

1/4 is good, taking ENABLE_RINGS and RESTRICTIONS out of quiesce is
also great. 4/4 per se is not a problem, even if I'd need to revert
it later.

> > Even more, I can't find a single user of io_uring_unregister_eventfd()
> > in liburing tests, so most probably the paths are not tested at all.
>
> That's definitely a general issue, not related to this patchset.
> Something that most certainly should get added! Ring exit will use the
> same unregister path for eventfd, however, so it does get exercised from
> there with existing tests too.

io_ring_ctx_free()
 -> io_eventfd_unregister()

It's called after full quiesce in io_ring_exit_work() + even more
extra sync, so not completely

> But for this change, we definitely need a test that exercises both
> register and unregister, trying to trigger something funky there.

--
Pavel Begunkov