Subject: Re: [PATCH V2 2/6] vhost_net: use vhost_add_used_and_signal_n() in vhost_zerocopy_signal_used()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 2 Sep 2013 08:50:48 +0300

On Fri, Aug 30, 2013 at 12:29:18PM +0800, Jason Wang wrote:
> We tend to batch the used adding and signaling in vhost_zerocopy_callback()
> which may result more than 100 used buffers to be updated in
> vhost_zerocopy_signal_used() in some cases. So wwitch to use

switch

> vhost_add_used_and_signal_n() to avoid multiple calls to
> vhost_add_used_and_signal(). Which means much more less times of used index
> updating and memory barriers.

pls put info on perf gain in commit log too

> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/vhost/net.c |   13 ++++++++-----
>  1 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 280ee66..8a6dd0d 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -281,7 +281,7 @@ static void vhost_zerocopy_signal_used(struct vhost_net *net,
>  {
>  	struct vhost_net_virtqueue *nvq =
>  		container_of(vq, struct vhost_net_virtqueue, vq);
> -	int i;
> +	int i, add;
>  	int j = 0;
>  
>  	for (i = nvq->done_idx; i != nvq->upend_idx; i = (i + 1) % UIO_MAXIOV) {
> @@ -289,14 +289,17 @@ static void vhost_zerocopy_signal_used(struct vhost_net *net,
>  			vhost_net_tx_err(net);
>  		if (VHOST_DMA_IS_DONE(vq->heads[i].len)) {
>  			vq->heads[i].len = VHOST_DMA_CLEAR_LEN;
> -			vhost_add_used_and_signal(vq->dev, vq,
> -						  vq->heads[i].id, 0);
>  			++j;
>  		} else
>  			break;
>  	}
> -	if (j)
> -		nvq->done_idx = i;
> +	while (j) {
> +		add = min(UIO_MAXIOV - nvq->done_idx, j);
> +		vhost_add_used_and_signal_n(vq->dev, vq,
> +					    &vq->heads[nvq->done_idx], add);
> +		nvq->done_idx = (nvq->done_idx + add) % UIO_MAXIOV;
> +		j -= add;
> +	}
>  }
>  
>  static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
> -- 
> 1.7.1