Messages in this thread | | | Date | Thu, 3 Feb 2022 15:55:09 +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 15:44, Pavel Begunkov wrote: > 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.
Actually needs smp_store_release/smp_load_acquire
> 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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