Re: [PATCH 1/3] io_uring: Add REQ_F_CQE_SKIP support for io_uring zerocopy

[Oliver Crumrine <ozlinuxc@gmail.com>]

Pavel Begunkov wrote:
> On 4/9/24 02:33, Oliver Crumrine wrote:
> > Pavel Begunkov wrote:
> >> On 4/7/24 20:14, Oliver Crumrine wrote:
> >>> Oliver Crumrine wrote:
> >>>> Pavel Begunkov wrote:
> >>>>> On 4/5/24 21:04, Oliver Crumrine wrote:
> >>>>>> Pavel Begunkov wrote:
> >>>>>>> On 4/4/24 23:17, Oliver Crumrine wrote:
> >>>>>>>> In his patch to enable zerocopy networking for io_uring, Pavel Begunkov
> >>>>>>>> specifically disabled REQ_F_CQE_SKIP, as (at least from my
> >>>>>>>> understanding) the userspace program wouldn't receive the
> >>>>>>>> IORING_CQE_F_MORE flag in the result value.
> >>>>>>>
> >>>>>>> No. IORING_CQE_F_MORE means there will be another CQE from this
> >>>>>>> request, so a single CQE without IORING_CQE_F_MORE is trivially
> >>>>>>> fine.
> >>>>>>>
> >>>>>>> The problem is the semantics, because by suppressing the first
> >>>>>>> CQE you're loosing the result value. You might rely on WAITALL
> >>>>>> That's already happening with io_send.
> >>>>>
> >>>>> Right, and it's still annoying and hard to use
> >>>> Another solution might be something where there is a counter that stores
> >>>> how many CQEs with REQ_F_CQE_SKIP have been processed. Before exiting,
> >>>> userspace could call a function like: io_wait_completions(int completions)
> >>>> which would wait until everything is done, and then userspace could peek
> >>>> the completion ring.
> >>>>>
> >>>>>>> as other sends and "fail" (in terms of io_uring) the request
> >>>>>>> in case of a partial send posting 2 CQEs, but that's not a great
> >>>>>>> way and it's getting userspace complicated pretty easily.
> >>>>>>>
> >>>>>>> In short, it was left out for later because there is a
> >>>>>>> better way to implement it, but it should be done carefully
> >>>>>> Maybe we could put the return values in the notifs? That would be a
> >>>>>> discrepancy between io_send and io_send_zc, though.
> >>>>>
> >>>>> Yes. And yes, having a custom flavour is not good. It'd only
> >>>>> be well usable if apart from returning the actual result
> >>>>> it also guarantees there will be one and only one CQE, then
> >>>>> the userspace doesn't have to do the dancing with counting
> >>>>> and checking F_MORE. In fact, I outlined before how a generic
> >>>>> solution may looks like:
> >>>>>
> >>>>> https://github.com/axboe/liburing/issues/824
> >>>>>
> >>>>> The only interesting part, IMHO, is to be able to merge the
> >>>>> main completion with its notification. Below is an old stash
> >>>>> rebased onto for-6.10. The only thing missing is relinking,
> >>>>> but maybe we don't even care about it. I need to cover it
> >>>>> well with tests.
> >>>> The patch looks pretty good. The only potential issue is that you store
> >>>> the res of the normal CQE into the notif CQE. This overwrites the
> >>>> IORING_CQE_F_NOTIF with IORING_CQE_F_MORE. This means that the notif would
> >>>> indicate to userspace that there will be another CQE, of which there
> >>>> won't.
> >>> I was wrong here; Mixed up flags and result value.
> >>
> >> Right, it's fine. And it's synchronised by the ubuf refcounting,
> >> though it might get more complicated if I'd try out some counting
> >> optimisations.
> >>
> >> FWIW, it shouldn't give any performance wins. The heavy stuff is
> >> notifications waking the task, which is still there. I can even
> >> imagine that having separate CQEs might be more flexible and would
> >> allow more efficient CQ batching.
> > I've actaully been working on this issue for a little while now. My current
> > idea is that an id is put into the optval section of the SQE, and then it
> > can be used to tag that req with a certain group. When a req has a flag
> > set on it, it can request for all of group's notifs to be "flushed" in one
> > notif that encompasses that entire group. If the id is zero, it won't be
> > associated with a group and will generate a notif. LMK if you see anything
> > in here that could overcomplicate userspace. I think it's pretty simple,
> > but you've had a crack at this before so I'd like to hear your opinion.
>
> You can take a look at early versions of the IORING_OP_SEND_ZC, e.g.
> patchset v1, probably even later ones. It was basically doing what
> you've described with minor uapi changes, like you had to declare groups
> (slots) in advance, i.e. register them.
My idea is that insead of allocating slots before making requests, "slots"
will be allocated as the group ids show up. Instead of an array of slots, a
linked list can be used so things can be kmalloc'ed on the fly to make
the uapi simpler.
>
> More flexible and so performant in some circumstances, but the overall
> feedback from people trying it is that it's complicated. The user should
> allocate group ids, track bound requests / buffers, do other management.
> The next question is how the user should decide what bind to what. There
> is some nastiness in using the same group for multiple sockets, and then
Then maybe we find a way to prevent multiple sockets on one group.
> what's the cut line to flush the previous notif? I probably forgot a
I'd make it the max for a u32 -- I'm (probably) going to use an atomic_t
to store the counter of how many reqs have been completed, so a u32 max
would make sense.
> couple more complaints.
>
> TL;DR;
>
> The performance is a bit of a longer story, problems are mostly coming
> from the async nature of io_uring, and it'd be nice to solve at least a
> part of it generically, not only for sendzc. The expensive stuff is
> waking up the task, it's not unique to notifications, recv will trigger
> it with polling as well as other opcodes. Then the key is completion
> batching.
Maybe the interface is made for sendzc first, and people could test it
there. Then if it is considered beneficial to other places, it could be
implemented there.
>
> What's interesting, take for example some tx only toy benchmark with
> DEFER_TASKRUN (recommended to use in any case). If you always wait for
> sends without notifications and add eventual *_get_events(), that would
> completely avoid the wake up overhead if there are enough buffers,
> and if it's not it can 1:1 replace tx polling.
Seems like an interesting way to eliminate waiting overhead.
>
> Try groups, see if numbers are good. And a heads up, I'm looking at
I will. Working hard to have the code done by Sunday.
> improving it a little bit for TCP because of a report, not changing
> uapi but might change performance math.
>
> --
> Pavel Begunkov