Re: [Qemu-devel] [PATCH v6 00/22] block: Asynchronous request cancellation

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Sep 11, 2014 at 01:41:06PM +0800, Fam Zheng wrote:
> v6: Drop bdrv_em_aiocb_info.cancel in patch 5. (Paolo)
> 
> v5: Fix IDE callback. (Paolo)
>     Fix blkdebug. (Paolo)
>     Drop the DMA fix which is independent of this series. (Paolo)
>     Incorperate Yuan's patch on quorum_aio_cancel. (Benoît)
>     Commit message wording fix. (Benoît)
>     Rename qemu_aio_release to qemu_aio_unref. (Benoît)
> 
> v4: Drop AIOCBInfo.cancel.
> 
> This series adds a new block layer API:
> 
>   void bdrv_aio_cancel_async(BlockDriverAIOCB *acb);
> 
> And use it to emulate bdrv_aio_cancel.
> 
> The function is similar to bdrv_aio_cancel in that it cancels an AIO request,
> but different that it doesn't block until the request is completely cancelled
> or done.
> 
> More importantly, the completion callback, BlockDriverAIOCB.cb, is guaranteed
> to be called, so that the cb can take care of resource releasing and status
> reporting to guest, etc.
> 
> In the following work, scsi emulation code will be shifted to use the async
> cancelling.
> 
> One major benefit would be that when guest tries to cancel a request, where the
> request cannot be cancelled easily, (due to throttled BlockDriverState, a lost
> connection, or a large request queue), we don't need to block the whole vm with
> a busy loop, which is how bdrv_aio_cancel is implemented now.
> 
> A test case that is easy to reproduce is, throttle a scsi-disk to a very low
> limit, for example 50 bps, then stress the guest block device with dd or fio.
> 
> Currently, the vm will quickly hang when it loses patience and send a tmf
> command to cancel the request, at which point we will busy wait in
> bdrv_aio_cancel, until the request is slowly spit out from throttled_reqs.
> 
> Later, we will change scsi device code to make this asynchronous, on top of
> bdrv_aio_cancel_async.
> 
> Fam
> 
> 
> Fam Zheng (21):
>   ide/ahci: Check for -ECANCELED in aio callbacks
>   block: Add refcnt in BlockDriverAIOCB
>   block: Add bdrv_aio_cancel_async
>   block: Drop bdrv_em_co_aiocb_info.cancel
>   block: Drop bdrv_em_aiocb_info.cancel
>   thread-pool: Convert thread_pool_aiocb_info.cancel to cancel_async
>   linux-aio: Convert laio_aiocb_info.cancel to .cancel_async
>   dma: Convert dma_aiocb_info.cancel to .cancel_async
>   iscsi: Convert iscsi_aiocb_info.cancel to .cancel_async
>   archipelago: Drop archipelago_aiocb_info.cancel
>   blkdebug: Drop blkdebug_aiocb_info.cancel
>   blkverify: Drop blkverify_aiocb_info.cancel
>   curl: Drop curl_aiocb_info.cancel
>   qed: Drop qed_aiocb_info.cancel
>   quorum: Convert quorum_aiocb_info.cancel to .cancel_async
>   rbd: Drop rbd_aiocb_info.cancel
>   sheepdog: Convert sd_aiocb_info.cancel to .cancel_async
>   win32-aio: Drop win32_aiocb_info.cancel
>   ide: Convert trim_aiocb_info.cancel to .cancel_async
>   block: Drop AIOCBInfo.cancel
>   block: Rename qemu_aio_release -> qemu_aio_unref
> 
> Liu Yuan (1):
>   quorum: fix quorum_aio_cancel()
> 
>  block.c                  | 72 ++++++++++++++++++++++++------------------------
>  block/archipelago.c      | 19 ++-----------
>  block/blkdebug.c         | 17 ++----------
>  block/blkverify.c        | 21 +-------------
>  block/curl.c             | 16 ++++-------
>  block/iscsi.c            | 23 ++++------------
>  block/linux-aio.c        | 34 +++++++----------------
>  block/qed.c              | 23 +---------------
>  block/quorum.c           | 11 ++++----
>  block/rbd.c              | 25 ++---------------
>  block/sheepdog.c         | 54 ++++++++++++++++--------------------
>  block/win32-aio.c        | 18 ++----------
>  dma-helpers.c            | 20 +++-----------
>  hw/ide/ahci.c            |  3 ++
>  hw/ide/core.c            | 26 +++++++++++------
>  include/block/aio.h      |  7 +++--
>  include/block/block.h    |  1 +
>  tests/test-thread-pool.c | 34 +++++++++++++++++------
>  thread-pool.c            | 36 +++++++++++-------------
>  19 files changed, 167 insertions(+), 293 deletions(-)

Nice series, it ends up being a good clean up while at the same time
improving the situation with synchronous cancellation!

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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