Re: [PATCH V4 net-next 1/3] vhost: better detection of available buffers

On 2017年01月07日 03:55, Michael S. Tsirkin wrote:

On Fri, Jan 06, 2017 at 10:13:15AM +0800, Jason Wang wrote:

quoted

This patch tries to do several tweaks on vhost_vq_avail_empty() for a
better performance:

- check cached avail index first which could avoid userspace memory access.
- using unlikely() for the failure of userspace access
- check vq->last_avail_idx instead of cached avail index as the last
   step.

This patch is need for batching supports which needs to peek whether
or not there's still available buffers in the ring.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <redacted>
---
  drivers/vhost/vhost.c | 8 ++++++--
  1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d643260..9f11838 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -2241,11 +2241,15 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
  	__virtio16 avail_idx;
  	int r;
  
+	if (vq->avail_idx != vq->last_avail_idx)
+		return false;
+
  	r = vhost_get_user(vq, avail_idx, &vq->avail->idx);
-	if (r)
+	if (unlikely(r))
  		return false;
+	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
  
-	return vhost16_to_cpu(vq, avail_idx) == vq->avail_idx;
+	return vq->avail_idx == vq->last_avail_idx;
  }
  EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);

So again, this did not address the issue I pointed out in v1:
if we have 1 buffer in RX queue and
that is not enough to store the whole packet,
vhost_vq_avail_empty returns false, then we re-read
the descriptors again and again.

You have saved a single index access but not the more expensive
descriptor access.

Looks not, if I understand the code correctly, in this case, 
get_rx_bufs() will return zero, and we will try to enable rx kick and 
exit the loop.

Thanks

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