Re: [Qemu-devel] [PATCH 00/16] aio_context_acquire/release pushdown, part 2

[Stefan Hajnoczi]

On Fri, Jan 13, 2017 at 02:17:15PM +0100, Paolo Bonzini wrote:
> This series pushes down aio_context_acquire/release to the point
> where we can actually reason on using different fine-grained mutexes.
> 
> The main infrastructure is introduced in patch 1.  The new API aio_co_wake
> starts a coroutine with aio_context_acquire/release protection, which
> requires tracking each coroutine's "home" AioContext.  aio_co_schedule
> instead takes care of moving a sleeping coroutine to a different
> AioContext, also ensuring that it runs under aio_context_acquire/release.
> This is useful to implement bdrv_set_aio_context, as a simpler alternative
> to bottom halves.  Even though one-shot BHs are already simpler than
> what we had before, after this patch aio_co_wake and aio_co_schedule
> save you from having to do aio_context_acquire/release explicitly.
> 
> After patch 2 and 3, which are just small preparatory changes, patches
> 4 to 7 provide an example of how to use the new API.  In particular patch
> 4 to 6 implement a new organization of coroutines in the NBD client,
> which allows not blocking on partial reply header reads.
> 
> Patch 8 introduces helpers for AioContext locking in QED, which is
> the most complex AIO-based driver left.  Then the actual meat of the
> series runs from patch 9 to patch 13, followed by small optimizations
> in patches 14 and 15.
> 
> The patches do some back and forth in adding/removing
> aio_context_acquire/release calls in block/*.c but ultimately a small
> number of aio_context_acquire/release pairs are added after the pushdown.
> These are mostly in drivers that use external libraries (where they
> actually could already be replaced by QemuMutex) and in device models.
> 
> Notably, coroutines need not care about aio_context_acquire/release.
> The device models ensure that the first creation of the coroutine has
> the AioContext, while aio_co_wake/aio_co_schedule do the same after
> they yield.  Therefore, most of the files only need to use those two
> functions instead of, respectively, qemu_coroutine_enter and
> aio_bh_schedule_oneshot.
> 
> However, this is only an intermediate step which is needed because the
> block layer and qemu-coroutine locks are thread-unsafe.  So the next
> part will add separate locking, independent of AioContext, to block.c and
> mostly block/io.c---this includes making CoMutex thread-safe.  Patch 16
> therefore already documents the current locking policies block.h to
> prepare for the next series.
> 
> Paolo
> 
> Paolo Bonzini (16):
>   aio: introduce aio_co_schedule and aio_co_wake
>   block-backend: allow blk_prw from coroutine context
>   test-thread-pool: use generic AioContext infrastructure
>   io: add methods to set I/O handlers on AioContext
>   io: make qio_channel_yield aware of AioContexts
>   nbd: do not block on partial reply header reads
>   coroutine-lock: reschedule coroutine on the AioContext it was running
>     on
>   qed: introduce qed_aio_start_io and qed_aio_next_io_cb
>   aio: push aio_context_acquire/release down to dispatching
>   block: explicitly acquire aiocontext in timers that need it
>   block: explicitly acquire aiocontext in callbacks that need it
>   block: explicitly acquire aiocontext in bottom halves that need it
>   block: explicitly acquire aiocontext in aio callbacks that need it
>   aio-posix: partially inline aio_dispatch into aio_poll
>   async: remove unnecessary inc/dec pairs
>   block: document fields protected by AioContext lock
> 
>  aio-posix.c                    |  60 +++---------
>  aio-win32.c                    |  30 ++----
>  async.c                        |  81 ++++++++++++++--
>  block/blkdebug.c               |   9 +-
>  block/blkreplay.c              |   2 +-
>  block/block-backend.c          |  13 ++-
>  block/curl.c                   |  44 ++++++---
>  block/gluster.c                |   9 +-
>  block/io.c                     |   4 +-
>  block/iscsi.c                  |  15 ++-
>  block/linux-aio.c              |  10 +-
>  block/mirror.c                 |  12 ++-
>  block/nbd-client.c             | 108 ++++++++-------------
>  block/nbd-client.h             |   2 +-
>  block/nfs.c                    |   9 +-
>  block/qed-cluster.c            |   2 +
>  block/qed-table.c              |  12 ++-
>  block/qed.c                    |  58 +++++++----
>  block/qed.h                    |   3 +
>  block/sheepdog.c               |  29 +++---
>  block/ssh.c                    |  29 ++----
>  block/throttle-groups.c        |   2 +
>  block/win32-aio.c              |   9 +-
>  dma-helpers.c                  |   2 +
>  hw/block/virtio-blk.c          |  19 +++-
>  hw/scsi/scsi-bus.c             |   2 +
>  hw/scsi/scsi-disk.c            |  15 +++
>  hw/scsi/scsi-generic.c         |  20 +++-
>  hw/scsi/virtio-scsi.c          |   6 ++
>  include/block/aio.h            |  38 +++++++-
>  include/block/block_int.h      |  64 ++++++++-----
>  include/io/channel.h           |  59 +++++++++++-
>  include/qemu/coroutine_int.h   |  10 +-
>  include/sysemu/block-backend.h |  14 ++-
>  io/channel-command.c           |  13 +++
>  io/channel-file.c              |  11 +++
>  io/channel-socket.c            |  16 +++-
>  io/channel-tls.c               |  12 +++
>  io/channel-watch.c             |   6 ++
>  io/channel.c                   |  97 +++++++++++++++----
>  nbd/client.c                   |   2 +-
>  nbd/common.c                   |   9 +-
>  nbd/server.c                   |   4 +
>  tests/Makefile.include         |  15 ++-
>  tests/iothread.c               |  91 ++++++++++++++++++
>  tests/iothread.h               |  25 +++++
>  tests/test-aio-multithread.c   | 213 
> +++++++++++++++++++++++++++++++++++++++++
>  tests/test-thread-pool.c       |  12 +--
>  tests/test-vmstate.c           |  11 ---
>  thread-pool.c                  |   6 +-
>  trace-events                   |   4 +
>  util/qemu-coroutine-lock.c     |   5 +-
>  util/qemu-coroutine-sleep.c    |   2 +-
>  util/qemu-coroutine.c          |   8 ++
>  util/trace-events              |   1 -
>  55 files changed, 1012 insertions(+), 352 deletions(-)
>  create mode 100644 tests/iothread.c
>  create mode 100644 tests/iothread.h
>  create mode 100644 tests/test-aio-multithread.c

This is a big and somewhat risky change.  Have you run any performance
benchmarks?

Stefan

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