linux-kernel - Re: [PATCH v5 0/4] io_uring: remove ring quiesce in io_uring

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Date:   Fri, 4 Feb 2022 00:24:16 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Usama Arif <usama.arif@...edance.com>,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com
Subject: Re: [PATCH v5 0/4] io_uring: remove ring quiesce in io_uring_register

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