Re: [RFC PATCH v2] virtio/vsock: allocate multiple skbuffs on tx

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On Sun, Mar 19, 2023 at 09:46:10PM +0300, Arseniy Krasnov wrote:
This adds small optimization for tx path: instead of allocating single
skbuff on every call to transport, allocate multiple skbuff's until
credit space allows, thus trying to send as much as possible data without
return to af_vsock.c.

Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
---
Link to v1:
https://lore.kernel.org/netdev/2c52aa26-8181-d37a-bccd-a86bd3cbc6e1@xxxxxxxxxxxxxx/

Changelog:
v1 -> v2:
- If sent something, return number of bytes sent (even in
  case of error). Return error only if failed to sent first
  skbuff.

net/vmw_vsock/virtio_transport_common.c | 53 ++++++++++++++++++-------
1 file changed, 39 insertions(+), 14 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 6564192e7f20..3fdf1433ec28 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -196,7 +196,8 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
	const struct virtio_transport *t_ops;
	struct virtio_vsock_sock *vvs;
	u32 pkt_len = info->pkt_len;
-	struct sk_buff *skb;
+	u32 rest_len;
+	int ret;

	info->type = virtio_transport_get_type(sk_vsock(vsk));

@@ -216,10 +217,6 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,

	vvs = vsk->trans;

-	/* we can send less than pkt_len bytes */
-	if (pkt_len > VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
-		pkt_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
-
	/* virtio_transport_get_credit might return less than pkt_len credit */
	pkt_len = virtio_transport_get_credit(vvs, pkt_len);

@@ -227,17 +224,45 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
	if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
		return pkt_len;

-	skb = virtio_transport_alloc_skb(info, pkt_len,
-					 src_cid, src_port,
-					 dst_cid, dst_port);
-	if (!skb) {
-		virtio_transport_put_credit(vvs, pkt_len);
-		return -ENOMEM;
-	}
+	ret = 0;
+	rest_len = pkt_len;
+
+	do {
+		struct sk_buff *skb;
+		size_t skb_len;
+
+		skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE, rest_len);
+
+		skb = virtio_transport_alloc_skb(info, skb_len,
+						 src_cid, src_port,
+						 dst_cid, dst_port);
+		if (!skb) {
+			ret = -ENOMEM;
+			break;
+		}
+
+		virtio_transport_inc_tx_pkt(vvs, skb);
+
+		ret = t_ops->send_pkt(skb);
+
+		if (ret < 0)
+			break;

-	virtio_transport_inc_tx_pkt(vvs, skb);
+		rest_len -= skb_len;

t_ops->send_pkt() is returning the number of bytes sent. Current
implementations always return `skb_len`, so there should be no problem,
but it would be better to put a comment here, or we should handle the
case where ret != skb_len to avoid future issues.

+	} while (rest_len);

-	return t_ops->send_pkt(skb);
+	/* Don't call this function with zero as argument:
+	 * it tries to acquire spinlock and such argument
+	 * makes this call useless.

Good point, can we do the same also for virtio_transport_get_credit()?
(Maybe in a separate patch)

I'm thinking if may be better to do it directly inside the functions,
but I don't have a strong opinion on that since we only call them here.

Thanks,
Stefano

+	 */
+	if (rest_len)
+		virtio_transport_put_credit(vvs, rest_len);
+
+	/* Return number of bytes, if any data has been sent. */
+	if (rest_len != pkt_len)
+		ret = pkt_len - rest_len;
+
+	return ret;
}

static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
--
2.25.1





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