Re: [Qemu-devel] [PATCH 1/2] block: add the support for draining the throttled request queue

[Stefan Hajnoczi]

On Mon, Feb 20, 2012 at 12:50:30PM +0800, address@hidden wrote:
> From: Zhi Yong Wu <address@hidden>
> 
> If one guest has multiple disks with enabling I/O throttling function 
> separately, when draining activities are done, some requests maybe are in the 
> throttled queue; So we need to restart them at first.
> 
> Moreover, when only one disk need to be drained such as hotplug out, if 
> another disk still has some requests in its throttled queue, these request 
> should not be effected.
> 
> Signed-off-by: Zhi Yong Wu <address@hidden>
> ---
>  block.c     |   29 ++++++++++++++++++++++-------
>  block_int.h |    1 +
>  2 files changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/block.c b/block.c
> index ae297bb..f78df78 100644
> --- a/block.c
> +++ b/block.c
> @@ -853,25 +853,40 @@ void bdrv_close_all(void)
>      }
>  }
>  
> -/*
> - * Wait for pending requests to complete across all BlockDriverStates
> - *
> - * This function does not flush data to disk, use bdrv_flush_all() for that
> - * after calling this function.
> - */
> -void bdrv_drain_all(void)
> +void bdrv_drain_request(BlockDriverState *throttled_bs)
>  {
>      BlockDriverState *bs;
>  
> +    QTAILQ_FOREACH(bs, &bdrv_states, list) {
> +        if (throttled_bs && throttled_bs != bs) {
> +            continue;
> +        }
> +        qemu_co_queue_restart_all(&bs->throttled_reqs);
> +    }
> +
>      qemu_aio_flush();

Since I/O throttling is still enabled, the restarted requests could
enqueue again if they exceed the limit.  We could still hit the assert.

If the semantics of bdrv_drain_request() are that no requests are
pending when it returns then we need a loop here.

BTW bdrv_drain() would be a shorter name for this function.

Stefan