Re: Short sends returned in IORING

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

On Wed, May 11, 2022 at 5:56 PM Constantine Gavrilov
<constantine.gavrilov@gmail.com> wrote:
>
> 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
> ----------------------------------------

Jens: for git branch, is it under
git://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git?

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