On 2/14/2020 2:27 PM, Carter Li 李通洲 wrote: > >> 2020年2月14日 下午6:34，Pavel Begunkov <asml.silence@xxxxxxxxx> 写道： >> >> On 2/14/2020 11:29 AM, Carter Li 李通洲 wrote: >>> To implement io_uring_wait_cqe_timeout, we introduce a magic number >>> called `LIBURING_UDATA_TIMEOUT`. The problem is that not only we >>> must make sure that users should never set sqe->user_data to >>> LIBURING_UDATA_TIMEOUT, but also introduce extra complexity to >>> filter out TIMEOUT cqes. >>> >>> Former discussion: https://github.com/axboe/liburing/issues/53 >>> >>> I’m suggesting introducing a new SQE flag called IOSQE_IGNORE_CQE >>> to solve this problem. >>> >>> For a sqe tagged with IOSQE_IGNORE_CQE flag, it won’t generate a cqe >>> on completion. So that IORING_OP_TIMEOUT can be filtered on kernel >>> side. >>> >>> In addition, `IOSQE_IGNORE_CQE` can be used to save cq size. >>> >>> For example `POLL_ADD(POLLIN)->READ/RECV` link chain, people usually >>> don’t care the result of `POLL_ADD` is ( since it will always be >>> POLLIN ), `IOSQE_IGNORE_CQE` can be set on `POLL_ADD` to save lots >>> of cq size. >>> >>> Besides POLL_ADD, people usually don’t care the result of POLL_REMOVE >>> /TIMEOUT_REMOVE/ASYNC_CANCEL/CLOSE. These operations can also be tagged >>> with IOSQE_IGNORE_CQE. >>> >>> Thoughts? >>> >> >> I like the idea! And that's one of my TODOs for the eBPF plans. >> Let me list my use cases, so we can think how to extend it a bit. >> >> 1. In case of link fail, we need to reap all -ECANCELLED, analise it and >> resubmit the rest. It's quite inconvenient. We may want to have CQE only >> for not cancelled requests. >> >> 2. When chain succeeded, you in the most cases already know the result >> of all intermediate CQEs, but you still need to reap and match them. >> I'd prefer to have only 1 CQE per link, that is either for the first >> failed or for the last request in the chain. >> >> These 2 may shed much processing overhead from the userspace. > > I couldn't agree more! > > Another problem is that io_uring_enter will be awaked for completion of > every operation in a link, which results in unnecessary context switch. > When awaked, users have nothing to do but issue another io_uring_enter > syscall to wait for completion of the entire link chain. Good point. Sounds like I have one more thing to do :) Would the behaviour as in the (2) cover all your needs? There is a nuisance with linked timeouts, but I think it's reasonable for REQ->LINKED_TIMEOUT, where it didn't fired, notify only for REQ >> >> 3. If we generate requests by eBPF even the notion of per-request event >> may broke. >> - eBPF creating new requests would also need to specify user-data, and >> this may be problematic from the user perspective. >> - may want to not generate CQEs automatically, but let eBPF do it. >> >> -- >> Pavel Begunkov > -- Pavel Begunkov
