On 3/10/22 6:34 AM, Pavel Begunkov wrote: > On 3/10/22 03:00, Jens Axboe wrote: >> On 3/9/22 7:11 PM, Artyom Pavlov wrote: >>> 10.03.2022 04:36, Jens Axboe wrote: >>>> On 3/9/22 4:49 PM, Artyom Pavlov wrote: >>>>> Greetings! >>>>> >>>>> A common approach for multi-threaded servers is to have a number of >>>>> threads equal to a number of cores and launch a separate ring in each >>>>> one. AFAIK currently if we want to send an event to a different ring, >>>>> we have to write-lock this ring, create SQE, and update the index >>>>> ring. Alternatively, we could use some kind of user-space message >>>>> passing. >>>>> >>>>> Such approaches are somewhat inefficient and I think it can be solved >>>>> elegantly by updating the io_uring_sqe type to allow accepting fd of a >>>>> ring to which CQE must be sent by kernel. It can be done by >>>>> introducing an IOSQE_ flag and using one of currently unused padding >>>>> u64s. >>>>> >>>>> Such feature could be useful for load balancing and message passing >>>>> between threads which would ride on top of io-uring, i.e. you could >>>>> send NOP with user_data pointing to a message payload. >>>> >>>> So what you want is a NOP with 'fd' set to the fd of another ring, and >>>> that nop posts a CQE on that other ring? I don't think we'd need IOSQE >>>> flags for that, we just need a NOP that supports that. I see a few ways >>>> of going about that: >>>> >>>> 1) Add a new 'NOP' that takes an fd, and validates that that fd is an >>>> io_uring instance. It can then grab the completion lock on that ring >>>> and post an empty CQE. >>>> >>>> 2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where >>>> 'fd' is another ring. Posting CQE same as above. >>>> >>>> 3) We add a specific opcode for this. Basically the same as #2, but >>>> maybe with a more descriptive name than NOP. >>>> >>>> Might make sense to pair that with a CQE flag or something like that, as >>>> there's no specific user_data that could be used as it doesn't match an >>>> existing SQE that has been issued. IORING_CQE_F_WAKEUP for example. >>>> Would be applicable to all the above cases. >>>> >>>> I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require >>>> that sqe->fd point to a ring (could even be the ring itself, doesn't >>>> matter). And add IORING_CQE_F_WAKEUP as a specific flag for that. >>>> >>> >>> No, ideally I would like to be able to send any type of SQE to a >>> different ring. For example, if I see that the current ring is >>> overloaded, I can create exactly the same SQEs as during usual >>> operation, but with a changed recipient ring. >>> >>> Your approach with a new "sendable" NOP will allow to emulate it in >>> user-space, but it will involve unnecessary ring round-trip and will >>> be a bit less pleasant in user code, e.g. we would need to encode a >>> separate state "the task is being sent to a different ring" instead of >>> simply telling io-uring "read data and report CQE on this ring" >>> without any intermediate states. >> >> OK, so what you're asking is to be able to submit an sqe to ring1, but >> have the completion show up in ring2? With the idea being that the rings >> are setup so that you're basing this on which thread should ultimately >> process the request when it completes, which is why you want it to >> target another ring? >> >> It'd certainly be doable, but it's a bit of a strange beast. My main >> concern with that would be: >> >> 1) It's a fast path code addition to every request, we'd need to check >> some new field (sqe->completion_ring_fd) and then also grab a >> reference to that file for use at completion time. >> >> 2) Completions are protected by the completion lock, and it isn't >> trivial to nest these. What happens if ring1 submits an sqe with >> ring2 as the cqe target, and ring2 submits an sqe with ring1 as the >> cqe target? We can't safely nest these, as we could easily introduce >> deadlocks that way. >> >> My knee jerk reaction is that it'd be both simpler and cheaper to >> implement this in userspace... Unless there's an elegant solution to it, >> which I don't immediately see. > > Per request fd will be ugly and slow unfortunately. As people asked about > a similar thing before, the only thing I can suggest is to add a way > to pass another SQ. The execution will be slower, but at least can be > made zero overhead for the normal path. The MSG_RING command seems like a good fit for me, and it'll both cater to the "I just need to wakeup this ring and I don't want to use signals" crowd, and passing actual (limited) information like what is needed in this case. -- Jens Axboe
