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

[Jens Axboe]

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@xxxxxxxxx/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...

> 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.

-- 
Jens Axboe