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

Michael S. Tsirkin
2013-09-02T05:50:48Z

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 <jasowang@redhat.com>
> ---
> 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