Subject: Re: [PATCH net-next 8/8] vhost-net: reduce vq polling on tx zerocopy

From: Vlad Yasevich <[email-masked]>
Date: Tue, 30 Oct 2012 11:47:45 -0400

On 10/29/2012 11:49 AM, Michael S. Tsirkin wrote:
> It seems that to avoid deadlocks it is enough to poll vq before
>   we are going to use the last buffer.  This should be faster than
> c70aa540c7a9f67add11ad3161096fb95233aa2e.
>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>   drivers/vhost/net.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 8e9de79..3967f82 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -197,8 +197,16 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, int status)
>   {
>   	struct vhost_ubuf_ref *ubufs = ubuf->ctx;
>   	struct vhost_virtqueue *vq = ubufs->vq;
> -
> -	vhost_poll_queue(&vq->poll);
> +	int cnt = atomic_read(&ubufs->kref.refcount);
> +
> +	/*
> +	 * Trigger polling thread if guest stopped submitting new buffers:
> +	 * in this case, the refcount after decrement will eventually reach 1
> +	 * so here it is 2.
> +	 * We also trigger polling periodically after each 16 packets.
> +	 */
> +	if (cnt <= 2 || !(cnt % 16))

Why 16?  Does it make sense to make it configurable?

-vlad

> +		vhost_poll_queue(&vq->poll);
>   	/* set len to mark this desc buffers done DMA */
>   	vq->heads[ubuf->desc].len = status ?
>   		VHOST_DMA_FAILED_LEN : VHOST_DMA_DONE_LEN;
>