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Subject: Re: [PATCH net-next v2 3/5] virtio_ring: add packed ring support

On 2018/11/8 äå10:14, Michael S. Tsirkin wrote:
On Thu, Nov 08, 2018 at 04:18:25PM +0800, Jason Wang wrote:
On 2018/11/8 äå9:38, Tiwei Bie wrote:
+	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. 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.
I don't think this ever triggers to be frank. When would it?

I think it can happen e.g in the path of XDP transmission in __virtnet_xdp_xmit_one():

ÂÂÂÂÂÂÂ err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdpf, GFP_ATOMIC);
ÂÂÂÂÂÂÂ if (unlikely(err))
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -ENOSPC; /* Caller handle free/refcnt */

Consider the removal of lguest support recently,
maybe we can drop this for split ring as well?

If it's helpful, then for sure we can drop it for virtio 1.
Can you see any perf differences at all? With which device?

I don't test but consider the case of XDP_TX in guest plus vhost_net in host. Since vhost_net is half duplex, it's pretty easier to trigger this condition.


commit 44653eae1407f79dff6f52fcf594ae84cb165ec4
Author: Rusty Russell<rusty@rustcorp.com.au>
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
      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:
              network xmit 7859051 recv 236420
              network xmit 7858610 recv 118136
      Signed-off-by: Rusty Russell<rusty@rustcorp.com.au>

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);
   		return -ENOSPC;

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