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block: avoid unnecessary bottom halves

Enterprise / Virtualization / QEMU - Paolo Bonzini [redhat.com] - 28 April 2015 08:36 UTC

bdrv_aio_* APIs can use coroutines to achieve asynchronicity. However, the coroutine may terminate without having yielded back to the caller (for example because of something that invokes a nested event loop, or because the coroutine is doing nothing at all). In this case, the bdrv_aio_* API must delay the completion to the next iteration of the main loop, because bdrv_aio_* will never invoke the callback before returning.

This can be done with a bottom half, and indeed bdrv_aio_* is always using one for simplicity. It is possible to gain some performance (~3%) by avoiding this in the common case. A new field in the BlockAIOCBCoroutine struct is set to true until the first time the corotine has yielded to its creator, and completion goes through a new function bdrv_co_complete. If the flag is false, bdrv_co_complete invokes the callback immediately. If it is true, the caller will notice that the coroutine has completed and schedule the bottom half itself.

0b5a244 block: avoid unnecessary bottom halves
block.c | 43 ++++++++++++++++++++++++++++++++++---------
1 file changed, 34 insertions(+), 9 deletions(-)

Upstream: git.qemu.org