Messages in this thread | | | Date | Thu, 3 Feb 2022 21:47:21 +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 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.
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.
fwiw, it alters userpace visible behaviour in either case, shouldn't be as important here but there is always a chance to break userspace
-- Pavel Begunkov
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