Re: [FEATURE REQUEST] Specify a sqe won't generate a cqe

["Carter Li 李通洲" <carter.li@eoitek.com>]

> 2020年2月14日 下午6:34，Pavel Begunkov <asml.silence@gmail.com> 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.

> 
> 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