Re: Short sends returned in IORING

[Constantine Gavrilov <constantine.gavrilov@gmail.com>]

On Wed, May 4, 2022 at 6:55 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 5/4/22 9:28 AM, Jens Axboe wrote:
> > On 5/4/22 9:21 AM, Constantine Gavrilov wrote:
> >> On Wed, May 4, 2022 at 4:54 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>> On 5/3/22 5:05 PM, Constantine Gavrilov wrote:
> >>>> Jens:
> >>>>
> >>>> This is related to the previous thread "Fix MSG_WAITALL for
> >>>> IORING_OP_RECV/RECVMSG".
> >>>>
> >>>> We have a similar issue with TCP socket sends. I see short sends
> >>>> regarding of the method (I tried write, writev, send, and sendmsg
> >>>> opcodes, while using MSG_WAITALL for send and sendmsg). It does not
> >>>> make a difference.
> >>>>
> >>>> Most of the time, sends are not short, and I never saw short sends
> >>>> with loopback and my app. But on real network media, I see short
> >>>> sends.
> >>>>
> >>>> This is a real problem, since because of this it is not possible to
> >>>> implement queue size of > 1 on a TCP socket, which limits the benefit
> >>>> of IORING. When we have a short send, the next send in queue will
> >>>> "corrupt" the stream.
> >>>>
> >>>> Can we have complete send before it completes, unless the socket is
> >>>> disconnected?
> >>>
> >>> I'm guessing that this happens because we get a task_work item queued
> >>> after we've processed some of the send, but not all. What kernel are you
> >>> using?
> >>>
> >>> This:
> >>>
> >>> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.19/io_uring&id=4c3c09439c08b03d9503df0ca4c7619c5842892e
> >>>
> >>> is queued up for 5.19, would be worth trying.
> >>>
> >>> --
> >>> Jens Axboe
> >>>
> >>
> >> Jens:
> >>
> >> Thank you for your reply.
> >>
> >> The kernel is 5.17.4-200.fc35.x86_64. I have looked at the patch. With
> >> the solution in place, I am wondering whether it will be possible to
> >> use multiple uring send IOs on the same socket. I expect that Linux
> >> TCP will serialize multiple send operations on the same socket. I am
> >> not sure it happens with uring (meaning that socket is blocked for
> >> processing a new IO until the pending IO completes). Do I need
> >> IOSQE_IO_DRAIN / IOSQE_IO_LINK for this to work? Would not be optimal
> >> because of multiple different sockets in the same uring. While I
> >> already have a workaround in the form of a "software" queue for
> >> streaming data on TCP sockets, I would rather have kernel to do
> >> "native" queueing in sockets layer, and have exrtra CPU cycles
> >> available to the  application.
> >
> > The patch above will mess with ordering potentially. If the cause is as
> > I suspect, task_work causing it to think it's signaled, then the better
> > approach may indeed be to just flush that work and retry without
> > re-queueing the current one. I can try a patch against 5.18 if you are
> > willing and able to test?
>
> You can try something like this, if you run my for-5.19/io_uring branch.
> I'd be curious to know if this solves the short send issue for you.
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index f6b6db216478..b835e80be1fa 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5684,6 +5684,7 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
>         if (flags & MSG_WAITALL)
>                 min_ret = iov_iter_count(&kmsg->msg.msg_iter);
>
> +retry:
>         ret = __sys_sendmsg_sock(sock, &kmsg->msg, flags);
>
>         if (ret < min_ret) {
> @@ -5694,6 +5695,8 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
>                 if (ret > 0 && io_net_retry(sock, flags)) {
>                         sr->done_io += ret;
>                         req->flags |= REQ_F_PARTIAL_IO;
> +                       if (io_run_task_work())
> +                               goto retry;
>                         return io_setup_async_msg(req, kmsg);
>                 }
>                 req_set_fail(req);
> @@ -5744,6 +5747,7 @@ static int io_send(struct io_kiocb *req, unsigned int issue_flags)
>                 min_ret = iov_iter_count(&msg.msg_iter);
>
>         msg.msg_flags = flags;
> +retry:
>         ret = sock_sendmsg(sock, &msg);
>         if (ret < min_ret) {
>                 if (ret == -EAGAIN && (issue_flags & IO_URING_F_NONBLOCK))
> @@ -5755,6 +5759,8 @@ static int io_send(struct io_kiocb *req, unsigned int issue_flags)
>                         sr->buf += ret;
>                         sr->done_io += ret;
>                         req->flags |= REQ_F_PARTIAL_IO;
> +                       if (io_run_task_work())
> +                               goto retry;
>                         return -EAGAIN;
>                 }
>                 req_set_fail(req);
>
> --
> Jens Axboe
>

Jens:

I was able to test the first change on the top of Linus kernel git (5.18.0-rc6).

I do not get short sends anymore, but I get corruption in  sent
packets (corruption is detected by the receiver). It looks like short
sends handled by the patch intermix data from multiple send SQEs in
the stream, so ordering of multiple SQEs in URING becomes broken.

To test it, I had two implementations of the send functions:
1. Uses SEND opcode, asserts on short sends. No asserts but data corruption.
2. Uses TCP send queue implementation (internally uses SEND and
SENDMSG opcodes in URING, only one pending send at a time, and tail of
the short sends is resent until all data is sent). This always works.

I would like to test the second patch now. Is it on the top of the
first patch or by itself? Do I really need your tree for that? If yes,
can you send me the git pull info, please?

-- 
----------------------------------------
Constantine Gavrilov
Storage Architect
Master Inventor
Tel-Aviv IBM Storage Lab
1 Azrieli Center, Tel-Aviv
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