On 16/03/2017 17:02, Peter Lieven wrote:
> commit 3c80ca15 fixed a deadlock scenarion with nested aio_poll invocations.
>
> However, the rescheduling of the completion BH introcuded unnecessary spinning
> in the main-loop. On very fast file backends this can even lead to the
> "WARNING: I/O thread spun for 1000 iterations" message popping up.
>
> Callgrind reports about 3-4% less instructions with this patch running
> qemu-img bench on a ramdisk based VMDK file.
>
> Fixes: 3c80ca158c96ff902a30883a8933e755988948b1
> Cc: qemu-sta...@nongnu.org
> Signed-off-by: Peter Lieven <p...@kamp.de>
> ---
> util/thread-pool.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/util/thread-pool.c b/util/thread-pool.c
> index ce6cd30..610646d 100644
> --- a/util/thread-pool.c
> +++ b/util/thread-pool.c
> @@ -188,6 +188,13 @@ restart:
> aio_context_release(pool->ctx);
> elem->common.cb(elem->common.opaque, elem->ret);
> aio_context_acquire(pool->ctx);
> +
> + /* We can safely cancel the completion_bh here regardless of
> someone
> + * else having scheduled it meanwhile because we reenter the
> + * completion function anyway (goto restart).
> + */
> + qemu_bh_cancel(pool->completion_bh);
> +
> qemu_aio_unref(elem);
> goto restart;
> } else {
>
Right, this is the same thing that block/linux-aio.c does.
Paolo
