On 2/4/22 7:51 AM, Usama Arif wrote: > Ring quiesce is currently used for registering/unregistering eventfds, > registering restrictions and enabling rings. > > For opcodes relating to registering/unregistering eventfds, ring quiesce > can be avoided by creating a new RCU data structure (io_ev_fd) as part > of io_ring_ctx that holds the eventfd_ctx, with reads to the structure > protected by rcu_read_lock and writes (register/unregister calls) > protected by a mutex. > > With the above approach ring quiesce can be avoided which is much more > expensive then using RCU lock. On the system tested, io_uring_reigster with > IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms > before with ring quiesce. > > IORING_SETUP_R_DISABLED prevents submitting requests and > so there will be no requests until IORING_REGISTER_ENABLE_RINGS > is called. And IORING_REGISTER_RESTRICTIONS works only before > IORING_REGISTER_ENABLE_RINGS is called. Hence ring quiesce is > not needed for these opcodes. I wrote a simple test case just verifying register+unregister, and also doing a loop to catch any issues around that. Here's the current kernel: [root@archlinux liburing]# time test/eventfd-reg real 0m7.980s user 0m0.004s sys 0m0.000s [root@archlinux liburing]# time test/eventfd-reg real 0m8.197s user 0m0.004s sys 0m0.000s which is around ~80ms for each register/unregister cycle, and here are the results with this patchset: [root@archlinux liburing]# time test/eventfd-reg real 0m0.002s user 0m0.001s sys 0m0.000s [root@archlinux liburing]# time test/eventfd-reg real 0m0.001s user 0m0.001s sys 0m0.000s which looks a lot more reasonable. I'll look over this one and see if I've got anything to complain about, just ran it first since I wrote the test anyway. Here's the test case, btw: https://git.kernel.dk/cgit/liburing/commit/?id=5bde26e4587168a439cabdbe73740454249e5204 -- Jens Axboe
