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

Michael S. Tsirkin
2012-10-30T15:52:06Z

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;
> >