[Qemu-devel] [PATCH v3 0/8] block: Asynchronous request cancellation

[Fam Zheng]

v3: Drop "RFC".
    Improvements according to Paolo's comments:
    05: Just use THREAD_DONE and ret = -ECANCELED in thread-pool.c
    06: Don't check dbs->cancelled for twice.
        Don't set dbs->acb to NULL.

v2: Drop the unfinished scsi part, which was broken in v1. (Paolo)
    Add refcnt in BlockDriverAIOCB to maintain invariant of acb availability in
    bdrv_aio_cancel_async. (Paolo)
    Drop blkdebug change. (Stefan)

This series adds a new block layer API:

  void bdrv_aio_cancel_async(BlockDriverAIOCB *acb);

which is similar to existing 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 (8):
  block: Add refcnt in BlockDriverAIOCB
  block: Add bdrv_aio_cancel_async
  tests: Add testing code for bdrv_aio_cancel_async
  linux-aio: Implement .cancel_async
  thread-pool: Implement .cancel_async
  dma: Implement .cancel_async
  block: Implement bdrv_em_co_aiocb_info.cancel_async
  iscsi: Implement .cancel_async in acb info

 block.c                  | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 block/iscsi.c            | 18 +++++++++++++++---
 block/linux-aio.c        | 20 ++++++++++++++++++--
 dma-helpers.c            | 24 ++++++++++++++++++++++++
 include/block/aio.h      |  3 +++
 include/block/block.h    |  1 +
 tests/test-thread-pool.c | 39 +++++++++++++++++++++++++++++++--------
 thread-pool.c            | 44 +++++++++++++++++++++++++++++++++++---------
 8 files changed, 171 insertions(+), 23 deletions(-)

-- 
2.1.0