Messages in this thread | | | Date | Thu, 3 Feb 2022 23:21:41 +0000 | Subject | Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd | From | Pavel Begunkov <> |
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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
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