Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Pavel Begunkov <asml.silence@gmail.com>]

On 2/3/22 22:16, Jens Axboe wrote:
> On 2/3/22 2:47 PM, Pavel Begunkov wrote:
>> On 2/3/22 19:54, Usama Arif wrote:
>>> On 03/02/2022 19:06, Jens Axboe wrote:
>>>> On 2/3/22 12:00 PM, Pavel Begunkov wrote:
>>>>> On 2/3/22 18:29, Jens Axboe wrote:
>>>>>> On 2/3/22 11:26 AM, Usama Arif wrote:
>>>>>>> Hmm, maybe i didn't understand you and Pavel correctly. Are you
>>>>>>> suggesting to do the below diff over patch 3? I dont think that would be
>>>>>>> correct, as it is possible that just after checking if ctx->io_ev_fd is
>>>>>>> present unregister can be called by another thread and set ctx->io_ev_fd
>>>>>>> to NULL that would cause a NULL pointer exception later? In the current
>>>>>>> patch, the check of whether ev_fd exists happens as the first thing
>>>>>>> after rcu_read_lock and the rcu_read_lock are extremely cheap i believe.
>>>>>>
>>>>>> They are cheap, but they are still noticeable at high requests/sec
>>>>>> rates. So would be best to avoid them.
>>>>>>
>>>>>> And yes it's obviously racy, there's the potential to miss an eventfd
>>>>>> notification if it races with registering an eventfd descriptor. But
>>>>>> that's not really a concern, as if you register with inflight IO
>>>>>> pending, then that always exists just depending on timing. The only
>>>>>> thing I care about here is that it's always _safe_. Hence something ala
>>>>>> what you did below is totally fine, as we're re-evaluating under rcu
>>>>>> protection.
>>>>>
>>>>> Indeed, the patch doesn't have any formal guarantees for propagation
>>>>> to already inflight requests, so this extra unsynchronised check
>>>>> doesn't change anything.
>>>>>
>>>>> I'm still more сurious why we need RCU and extra complexity when
>>>>> apparently there is no use case for that. If it's only about
>>>>> initial initialisation, then as I described there is a much
>>>>> simpler approach.
>>>>
>>>> Would be nice if we could get rid of the quiesce code in general, but I
>>>> haven't done a check to see what'd be missing after this...
>>>>
>>>
>>> I had checked! I had posted below in in reply to v1 (https://lore.kernel.org/io-uring/02fb0bc3-fc38-b8f0-3067-edd2a525ef29@gmail.com/T/#m5ac7867ac61d86fe62c099be793ffe5a9a334976), but i think it got missed! Copy-pasting here for reference:
>>
>> May have missed it then, apologies
>>
>>> "
>>> I see that if we remove ring quiesce from the the above 3 opcodes, then
>>> only IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS is
>>> left for ring quiesce. I just had a quick look at those, and from what i
>>> see we might not need to enter ring quiesce in
>>> IORING_REGISTER_ENABLE_RINGS as the ring is already disabled at that point?
>>> And for IORING_REGISTER_RESTRICTIONS if we do a similar approach to
>>> IORING_REGISTER_EVENTFD, i.e. wrap ctx->restrictions inside an RCU
>>> protected data structure, use spin_lock to prevent multiple
>>> io_register_restrictions calls at the same time, and use read_rcu_lock
>>> in io_check_restriction, then we can remove ring quiesce from
>>> io_uring_register altogether?
>>>
>>> My usecase only uses IORING_REGISTER_EVENTFD, but i think entering ring
>>> quiesce costs similar in other opcodes. If the above sounds reasonable,
>>> please let me know and i can send patches for removing ring quiesce for
>>> io_uring_register.
>>> "
>>>
>>> Let me know if above makes sense, i can add patches on top of the current patchset, or we can do it after they get merged.
>>>
>>> As for why, quiesce state is very expensive. its making io_uring_register the most expensive syscall in my usecase (~15ms) compared to ~0.1ms now with RCU, which is why i started investigating this. And this patchset avoids ring quiesce for 3 of the opcodes, so it would generally be quite helpful if someone does registers and unregisters eventfd multiple times.
>>
>> I agree that 15ms for initial setup is silly and it has to be
>> reduced. However, I'm trying weight the extra complexity against
>> potential benefits of _also_ optimising [de,re]-registration
>>
>> Considering that you only register it one time at the beginning,
>> we risk adding a yet another feature that nobody is going to ever
>> use. This doesn't give me a nice feeling, well, unless you do
>> have a use case.
> 
> It's not really a new feature, it's just making the existing one not
> suck quite as much...

Does it matter when nobody uses it? My point is that does not.


>> To emphasise, I'm comparing 15->0.1 improvement for only initial
>> registration (which is simpler) vs 15->0.1 for both registration
>> and unregistration.
> 
> reg+unreg should be way faster too, if done properly with the assignment
> tricks.
> 
>> fwiw, it alters userpace visible behaviour in either case, shouldn't
>> be as important here but there is always a chance to break userspace
> 
> It doesn't alter userspace behavior, if the registration works like I
> described with being able to assign a new one while the old one is being
> torn down.
> 
> Or do you mean wrt inflight IO? I don't think the risk is very high
> there, to be honest.

Right, if somebody tries such a trick it'll be pretty confusing to
get randomly firing eventfd, though it's rather a marginal case.

-- 
Pavel Begunkov