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

Jens Axboe
2022-02-04T00:16:02Z

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.

> 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.

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

--
Jens Axboe