Messages in this thread | | | Date | Thu, 3 Feb 2022 15:44:21 +0000 | Subject | Re: [External] Re: [RFC] io_uring: avoid ring quiesce while registering/unregistering eventfd | From | Pavel Begunkov <> |
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On 2/3/22 15:14, Usama Arif wrote: > On 02/02/2022 19:18, Jens Axboe wrote: >> On 2/2/22 9:57 AM, Jens Axboe wrote: >>> On 2/2/22 8:59 AM, Usama Arif wrote: >>>> Acquire completion_lock at the start of __io_uring_register before >>>> registering/unregistering eventfd and release it at the end. Hence >>>> all calls to io_cqring_ev_posted which adds to the eventfd counter >>>> will finish before acquiring the spin_lock in io_uring_register, and >>>> all new calls will wait till the eventfd is registered. This avoids >>>> ring quiesce which is much more expensive than acquiring the >>>> spin_lock. >>>> >>>> On the system tested with this patch, io_uring_reigster with >>>> IORING_REGISTER_EVENTFD takes less than 1ms, compared to 15ms before. >>> >>> This seems like optimizing for the wrong thing, so I've got a few >>> questions. Are you doing a lot of eventfd registrations (and >>> unregister) in your workload? Or is it just the initial pain of >>> registering one? In talking to Pavel, he suggested that RCU might be a >>> good use case here, and I think so too. That would still remove the >>> need to quiesce, and the posted side just needs a fairly cheap rcu >>> read lock/unlock around it. >> >> Totally untested, but perhaps can serve as a starting point or >> inspiration. >> > > Hi, > > Thank you for the replies and comments. My usecase registers only one eventfd at the start.
Then it's overkill. Update io_register_op_must_quiesce(), set ->cq_ev_fd on registration with WRITE_ONCE(), read it in io_cqring_ev_posted* with READ_ONCE() and you're set.
There is a caveat, ->cq_ev_fd won't be immediately visible to already inflight requests, but we can say it's the responsibility of the userspace to wait for a grace period, i.e. for all inflight requests submitted before registration io_cqring_ev_posted* might or might not see updated ->cq_ev_fd, which works perfectly if there was no requests in the first place. Of course it changes the behaviour so will need a new register opcode.
-- Pavel Begunkov
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