[net-next,8/8] vhost-net: reduce vq polling on tx zerocopy

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 <mst@redhat.com>
---
 drivers/vhost/net.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

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 <mst@redhat.com>
> ---
>   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;
>

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On Tue, Oct 30, 2012 at 11:47:45AM -0400, Vlad Yasevich wrote:
> 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 <mst@redhat.com>
> >---
> >  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

Probably not but I'll add a comment explaining why.

> >+		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;
> >
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