Messages in this thread | | | Subject | Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd | From | Usama Arif <> | Date | Thu, 3 Feb 2022 19:54:49 +0000 |
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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:
" 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.
Thanks, Usama
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