On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:

On 2018/11/8 上午9:38, Tiwei Bie wrote:

quoted

quoted

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+
+	if (vq->vq.num_free < descs_used) {
+		pr_debug("Can't add buf len %i - avail = %i\n",
+			 descs_used, vq->vq.num_free);
+		/* FIXME: for historical reasons, we force a notify here if
+		 * there are outgoing parts to the buffer.  Presumably the
+		 * host should service the ring ASAP. */

I don't think we have a reason to do this for packed ring.
No historical baggage there, right?

Based on the original commit log, it seems that the notify here
is just an "optimization". But I don't quite understand what does
the "the heuristics which KVM uses" refer to. If it's safe to drop
this in packed ring, I'd like to do it.

According to the commit log, it seems like a workaround of lguest networking
backend.

Do you know why removing this notify in Tx will break "the
heuristics which KVM uses"? Or what does "the heuristics
which KVM uses" refer to?

I agree to drop it, we should not have such burden.

But we should notice that, with this removed, the compare between packed vs
split is kind of unfair. Consider the removal of lguest support recently,
maybe we can drop this for split ring as well?

Thanks

quoted

commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
Author: Rusty Russell[off-list ref]
Date:   Fri Jul 25 12:06:04 2008 -0500

     virtio: don't always force a notification when ring is full
     We force notification when the ring is full, even if the host has
     indicated it doesn't want to know.  This seemed like a good idea at
     the time: if we fill the transmit ring, we should tell the host
     immediately.
     Unfortunately this logic also applies to the receiving ring, which is
     refilled constantly.  We should introduce real notification thesholds
     to replace this logic.  Meanwhile, removing the logic altogether breaks
     the heuristics which KVM uses, so we use a hack: only notify if there are
     outgoing parts of the new buffer.
     Here are the number of exits with lguest's crappy network implementation:
     Before:
             network xmit 7859051 recv 236420
     After:
             network xmit 7858610 recv 118136
     Signed-off-by: Rusty Russell[off-list ref]

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 72bf8bc09014..21d9a62767af 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -87,8 +87,11 @@ static int vring_add_buf(struct virtqueue *_vq,
  	if (vq->num_free < out + in) {
  		pr_debug("Can't add buf len %i - avail = %i\n",
  			 out + in, vq->num_free);
-		/* We notify*even if*  VRING_USED_F_NO_NOTIFY is set here. */
-		vq->notify(&vq->vq);
+		/* FIXME: for historical reasons, we force a notify here if
+		 * there are outgoing parts to the buffer.  Presumably the
+		 * host should service the ring ASAP. */
+		if (out)
+			vq->notify(&vq->vq);
  		END_USE(vq);
  		return -ENOSPC;
  	}