Re: Short sends returned in IORING

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

On Wed, May 11, 2022 at 6:11 PM Constantine Gavrilov
<constantine.gavrilov@gmail.com> wrote:
>
> 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
> ----------------------------------------

Jens: checked out yout branch, the first patch is already in, applied
the second. Will build and test now.

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