On 11/24/21 10:55 AM, Pavel Begunkov wrote: > On 11/10/21 16:47, Jens Axboe wrote: >> On 11/10/21 9:42 AM, Pavel Begunkov wrote: >>> On 11/10/21 16:14, Jens Axboe wrote: >>>> On 11/10/21 8:49 AM, Pavel Begunkov wrote: >>>>> It's expensive enough to post an CQE, and there are other >>>>> reasons to want to ignore them, e.g. for link handling and >>>>> it may just be more convenient for the userspace. >>>>> >>>>> Try to cover most of the use cases with one flag. The overhead >>>>> is one "if (cqe->flags & IOSQE_CQE_SKIP_SUCCESS)" check per >>>>> requests and a bit bloated req_set_fail(), should be bearable. >>>> >>>> I like the idea, one thing I'm struggling with is I think a normal use >>>> case of this would be fast IO where we still need to know if a >>>> completion event has happened, we just don't need to know the details of >>>> it since we already know what those details would be if it ends up in >>>> success. >>>> >>>> How about having a skip counter? That would supposedly also allow drain >>>> to work, and it could be mapped with the other cq parts to allow the app >>>> to see it as well. >>> >>> It doesn't go through expensive io_cqring_ev_posted(), so the >>> userspace can't really wait on it. It can do some linking tricks to >>> alleviate that, but I don't see any new capabilities from the current >>> approach. >> >> I'm not talking about waiting, just reading the cqring entry to see how >> many were skipped. If you ask for no cqe, by definition there would be >> nothing to wait on for you. Though it'd probably be better as an sqring >> entry, since we'd be accounting at that time. Only caveat there is then >> if the sqe errors and we do end up posting a cqe.. >> >>> Also the locking is a problem, I was thinking about it, mainly hoping >>> that I can adjust cq_extra and leave draining, but it didn't appear >>> great to me. AFAIK, it's either an atomic, beating the purpose of the >>> thing. >> >> If we do submission side, then the ring mutex would cover it. No need >> for any extra locking > > Jens, let's decide what we're going to do with this feature Only weird bit is the drain, but apart from that I think it looks sane. Are you going to send a documentation update to liburing as well? Should be detailed in terms of what it does and the usability of it. -- Jens Axboe
