[Libguestfs] [libnbd] Simultaneous read and write

[Richard W.M. Jones]

On Fri, May 31, 2019 at 04:47:52PM -0500, Eric Blake wrote:
> On 5/31/19 11:01 AM, Richard W.M. Jones wrote:
> > Problem (a) : Only one thread of control can hold the libnbd handle
> > lock (h->lock), and since there can only be one thread of control
> > running in each handle at any time, that thread can only be reading or
> > writing.
> 
> Indeed, when the writer thread calls nbd_aio_pread(), it is contending
> on the same lock as the nbd_aio_notify_read() in the reader thread, so
> we'd have to split up to several finer-grained locks (maybe keep all
> locking APIs with a grab on h->lock at the beginning, but while holding
> that lock we then grab the h->rstate or h->wstate lock for the rest of
> the call, and drop the main h->lock at that point even though the API
> doesn't return until the state machine blocks again).

I disagree there's any need to split h->lock, but to try and put what
you're saying more clearly (for me :-):

nbd_aio_pread and nbd_aio_notify_read return quickly without
blocking.  [I wonder if we can instrument libnbd in some way to make
sure this is always true?  Perhaps we could have an instrumented
libnbd which times all AIO calls and ensure they never block, or is
there another way to determine if a process blocked between two places
in the code?  Google turns up nothing]

Even if nbd_aio_pread ends up send(2)-ing to the socket or
nbd_aio_notify_read recv(2)'s, neither call should block.  At most
there is a copy of a small amount of data between the kernel and
userspace.

That isn't where the contention is happening.  If we go back to the
diagram:

   write                      write           write
   |===========|______________|=======|_______|===|  --> rq to server
   _____________|============|_________|=====|_____  <-- rp from server
                read                   read

During those times where I put that "write" xor "read" is happening,
we're actually waiting on poll(2).

The problem is the state machine is in (eg)
ISSUE_COMMAND.SEND_WRITE_PAYLOAD, poll is monitoring the fd for
POLLIN|POLLOUT because:

  https://github.com/libguestfs/libnbd/blob/e63a11736930c381a79a8cc2d03844cfff5db3ef/generator/generator#L667_L668

and if poll returns revents == POLLIN then we move to
ISSUE_COMMAND.PAUSE_WRITE_PAYLOAD and then straight to REPLY.START and
then we read the whole reply (even the payload) without ever checking
for POLLOUT, see:

  https://github.com/libguestfs/libnbd/blob/e63a11736930c381a79a8cc2d03844cfff5db3ef/generator/generator#L687-L800

(This is not a criticism of your earlier patches which nicely solved
what was a much worse problem with deadlocking.)

My other email attempts to solve this by having a second thread which
has already called recv() and buffered up the data, so we don't spend
any time in the receive states, although it's possible that we only
spend less time in the receive states.

> With my initial use of libnbd, the division of labor for which thread
> writes a packet falls into three classes:
> - if the state machine is ready, the libnbd fd is polling only on
> POLLIN; from there:
>   - if the request is small, the entire request is written by the nbdkit
> thread during nbd_aio_pwrite (in this case, our write to the
> pipe-to-self is actually wasted work, because the reader loop thread
> will spuriously wake up, check the directions, and see that it still
> only needs POLLIN for the next server response)
>   - if the request is large, the first half of the request is written
> from the nbdkit during nbd_aio_pwrite, and the second half of the
> request is written from the reader thread during nbd_aio_notify_write
> (here, our write to the pipe-to-self is essential, because we HAD to
> break the reader loop out of its POLLIN poll() in order to learn that it
> now needs to poll for POLLIN|POLLOUT)
> - if the state machine is not ready, the libnbd fd is either in the
> middle of processing a reply (POLLIN) or a request (POLLIN|POLLOUT,
> since we allow replies to interrupt the processing of a request).
> nbd_aio_pwrite queues the new request and returns immediately, without
> write()ing anything from the nbdkit thread, and the entire request gets
> written by the reader thread during subsequent
> nbd_aio_notify_{read,write} (in this case, our write to the pipe-to-self
> is useful if libnbd was in the middle of processing a reply, but wasted
> work if it was in the middle of processing a previous request)

I think this reiterates what I said above, if I'm understanding it
correctly.

Actually have written all that, I wish there was a way we could
visualize the state machine, what we're polling for, and how long we
wait at each point.  I might try instrumenting the code with debug
statements to get a clearer picture.

> > Problem (b) : There is only one state machine per handle (h->state),
> > whereas to handle the write and read sides separately requires two
> > state machines.  In the IRC discussion we gave these the preliminary
> > names h->wstate and h->rstate.
> > 
> > ----------------------------------------------------------------------
> > 
> > It's worth also saying how the current API works, although we might
> > want to change it.
> > 
> > You grab the underlying file descriptor using nbd_aio_get_fd, which is
> > what you poll on.  You also have to call nbd_aio_get_direction which
> > returns READ, WRITE or BOTH (== READ|WRITE).  You then set up some
> > events mechanism (eg. poll, epoll, etc.), poll until the file
> > descriptor is ready, and call one of nbd_aio_notify_read or
> > nbd_aio_notify_write.
> > 
> > The direction can change any time the handle state changes, which
> > includes whenever you issue a command (eg. nbd_aio_pread), or whenever
> > you call nbd_aio_notify_*.  You therefore have to call
> > nbd_aio_get_direction frequently.
> > 
> > A typical loop using poll might look like:
> > 
> >   fd = nbd_aio_get_fd (nbd);
> >   for (;;) {
> >     /* <-- If you need to issue more commands, do that here. */
> >     dir = nbd_aio_get_direction (nbd);
> >     pollfd[0].fd = fd;
> >     pollfd[0].events = 0;
> >     if (dir & LIBNBD_AIO_DIRECTION_READ) pollfd[0].events |= POLLIN;
> >     if (dir & LIBNBD_AIO_DIRECTION_WRITE) pollfd[0].events |= POLLOUT;
> >     poll (pollfd, 1, -1);
> >     if (pollfd[0].revents & LIBNBD_AIO_DIRECTION_READ)
> 
> Rather, if (pollfd[0].revents & POLLIN && dir & LIBNBD_AIO_DIRECTION_READ)
> 
> >       nbd_aio_notify_read ();
> >     else if (pollfd[0].revents & LIBNBD_AIO_DIRECTION_WRITE)
> 
> and again
> 
> >       nbd_aio_notify_write ();
> >   }
> 
> or in the nbdkit case:
> 
> nbdkit thread
>   nbd_aio_pwrite ();
>   write (selfpipe[1], &c, 1);
> 
> reader thread
>   for (;;) {
>     dir = nbd_aio_get_direction (nbd);
>     pollfd[0].fd = fd;
>     pollfd[0].events = 0;
>     pollfd[1].fd = selfpipe[0];
>     pollfd[1].events = POLLIN;
>     if (dir & LIBNBD_AIO_DIRECTION_READ) pollfd[0].events |= POLLIN;
>     if (dir & LIBNBD_AIO_DIRECTION_WRITE) pollfd[0].events |= POLLOUT;
>     poll (pollfd, 2, -1);
>     if (pollfd[0].revents & POLLIN && dir & LIBNBD_AIO_DIRECTION_READ)
>        nbd_aio_notify_read ();
>     else if (pollfd[0].revents & POLLOUT && dir &
> LIBNBD_AIO_DIRECTION_WRITE)
>       nbd_aio_notify_write ();
>     if (pollfd[1].revents & POLLIN)
>       read (selfpipe[0], &c, 1);
>   }
> 
> > 
> > ----------------------------------------------------------------------
> > 
> > The above code is of course assuming a single thread.  But to
> > simultaneously read and write we will need at least two threads.
> 
> If we have two threads, an obvious split would be to state that requests
> must be done from one thread while replies are done from another, at
> which point the state machine would look something like:
> 
> nbdkit thread
>   id = nbd_aio_pwrite ();
>   while (!nbd_aio_command_inflight(id)) {
>     pollfd[0].fd = fd;
>     pollfd[0].events = POLLOUT;
>     poll (pollfd, 1, -1);
>     if (pollfd[0].revents & POLLOUT)
>       nbd_aio_notify_write ();
>   }
> 
> reader thread
>   while (!nbd_aio_is_rstate_ready()) {
>     pollfd[0].fd = fd;
>     pollfd[0].events = POLLIN;
>     poll (pollfd, 1, -1);
>     if (pollfd[0].revents & POLLIN)
>        nbd_aio_notify_read ();
>   }
> 
> where we could also add a new convenience API:
> 
> nbd_aio_pwrite_send()
> 
> that does the work mentioned for the nbdkit thread, and is documented to
> block the thread up until the point in time where the state machine has
> finished sending the request (even if it had to poll on POLLOUT in the
> middle), but before waiting for the server's reply; and while it can
> lock out other potential writers (that is, hold the h->wstate lock for
> the entire duration), but must not lock out readers (that is, it must
> drop h->lock as soon as we prove that we are okay sending the request).
> 
> There probably needs to be checks for POLLHUP or POLLERR in there, to
> ensure the loop quits if the server hangs up or if we encounter an error
> that prevents further request/reply sequences.
> 
> > 
> > It's hard for me to see a nice way to evolve the API to support
> > multiple threads, but I guess issues include:
> > 
> >  - If one thread is waiting in poll(2) and another thread issues a
> >    command, how do we get the first thread to return from poll and
> >    reevaluate direction?  (Eric suggests a pipe-to-self for this)
> 
> That is, if you liked my self-pipe solution, we could somehow fold that
> self-pipe into libnbd internals instead of making each caller
> re-implement it. One idea I had for that was, at least on Linux, to use
> a pollfd, where the user's poll loop is as simple as:
> 
>   while (!nbd_aio_is_rstate_ready()) {
>     pollfd[0].fd = fd;
>     pollfd[0].events = POLLIN;
>     poll (pollfd, 1, -1);
>     if (pollfd[0].revents & POLLIN)
>        nbd_aio_notify_read ();
>   }
> 
> because pollfd[0].fd is really a pollfd (and the libnbd, under the hood,
> actually calls epoll_wait() during nbd_aio_notify_read() to learn what
> REALLY happened, whether it was the wstate machine adding a request to
> the queue and tickling the self-pipe, or the rstate machine making
> progress). But in that model, all write()s are now done solely from the
> single reader loop, during nbd_aio_notify_read(); the other threads
> calling nbd_aio_pwrite() never call write directly, and we're back to
> the question of how can we read and write to the same socket
> simultaneously - so in addition to having two state machines, we might
> also have to make libnbd itself run two separate threads per handle.
> 
> It's a shame that Linux does not yet have true aio for network packets:
>  - glibc's implementation of POSIX aio_* creates a helper thread under
> the hood, the same as I suggested we might do above
>
>  - the Linux kernel's io_submit() still performs I/O in order rather
> than in parallel
> https://blog.cloudflare.com/io_submit-the-epoll-alternative-youve-never-heard-about/
>  - the Linux kernel's sendto(MSG_ZEROCOPY) can perform true async
> network traffic (you can regain control of your single thread prior to
> the full write going out the door, and thus can perform a read at the
> same time from a single thread), although it is only efficient once you
> reach the point of sending 10k or more at once
> 
> > 
> >  - If two threads are waiting in poll(2) and both are notified that
> >    the fd is ready, how do we get one of them to read and the other to
> >    write?  I think this implies that we don't have two threads doing
> >    poll(2), but if not how do we farm out the read/write work to two
> >    or more threads?
> 
> Either we document that when using two threads, the user runs one poll
> loop strictly on POLLIN and the other loop strictly on POLLOUT (and
> doesn't have to bother with direction changes) - but only after the nbd
> handle has reached is_ready() for the first time (the handshake phase is
> very much lock-step, where a single thread checking direction all the
> time really is the best solution). Or we tell the user to run a single
> loop, manage a second thread under the hood, and the user only has to
> poll on POLLIN (where we do the coordination with our helper thread
> under the hood).

I'm uneasy about creating threads behind the scenes in a library.  If
we need another thread, and it seems obvious we do, I think the caller
should always create it.

Similarly, relying on Linux-specific APIs like epoll.  Let's get the
libnbd API right.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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