Re: [Qemu-devel] [RFC 0/5] nbd: Adapt for dataplane

[Stefan Hajnoczi <stefanha@gmail.com>]

On Sat, May 31, 2014 at 08:43:07PM +0200, Max Reitz wrote:
> For the NBD server to work with dataplane, it needs to correctly access
> the exported BDS. It makes the most sense to run both in the same
> AioContext, therefore this series implements methods for tracking a
> BDS's AioContext and makes NBD make use of this for keeping the clients
> connected to that BDS in the same AioContext.
> 
> The reason this is an RFC and not a PATCH is my inexperience with AIO,
> coroutines and the like. Also, I'm not sure about what to do about the
> coroutines. The NBD server has up to two coroutines per client: One for
> receiving and one for sending. Theoretically, both have to be
> "transferred" to the new AioContext if it is changed; however, as far as
> I see it, coroutines are not really bound to an AioContext, they are
> simply run in the AioContext entering them. Therefore, I think a
> transfer is unnecessary. All coroutines are entered from nbd_read() and
> nbd_restart_write(), both of which are AIO routines registered via
> aio_set_fd_handler2().
> 
> As bs_aio_detach() unregisters all of these routines, the coroutines can
> no longer be entered, but only after bs_aio_attach() is called again.
> Then, when they are called, they will enter the coroutines in the new
> AioContext. Therefore, I think an explicit transfer unnecessary.
> 
> However, if bs_aio_detach() is called from a different thread than the
> old AioContext is running in, we may still have coroutines running for
> which we should wait before returning from bs_aio_detach().
> 
> But because of my inexperience with coroutines, I'm not sure. I now have
> these patches nearly unchanged here for about a week and I'm looking for
> ways of testing them, but so far I could only test whether the old use
> cases work, but not whether they will work for what they are intended to
> do: With BDS changing their AioContext.
> 
> So, because I'm not sure what else to do and because I don't know how to
> test multiple AIO threads (how do I move a BDS into another iothread?)
> I'm just sending this out as an RFC.
> 
> 
> Max Reitz (5):
>   nbd: Correct name comparison for export_set_name()
>   aio: Add io_read_poll() callback
>   nbd: Use aio_set_fd_handler2()
>   block: Add AIO followers
>   nbd: Follow the BDS' AIO context
> 
>  aio-posix.c               | 26 ++++++++++++++++-----
>  block.c                   | 55 +++++++++++++++++++++++++++++++++++++++++++
>  include/block/aio.h       | 12 ++++++++++
>  include/block/block_int.h | 40 ++++++++++++++++++++++++++++++++
>  include/qemu/main-loop.h  |  1 -
>  nbd.c                     | 59 ++++++++++++++++++++++++++++++++++++++---------
>  6 files changed, 175 insertions(+), 18 deletions(-)

Thanks for the RFC.  It looks like the concept will work correctly.  I
left comments, a lot of them are just ideas and you don't have to
implement them if you don't want to.

Stefan