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

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On Mon, Mar 20, 2023 at 09:02:19PM +0300, Arseniy Krasnov wrote:

On 20.03.2023 17:29, Stefano Garzarella wrote:
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:

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

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.

Hello, thanks for review!

I see. I think i'll handle such partial sends (ret != skb_len) as error, as
it is the only thing to do - we remove 'skb_len' from user's buffer, but
'send_pkt()' returns another value, so it will be strange for me to continue
this tx loop as everything is ok. Something like this:
+ if (ret < 0)
+    break;
+ if (ret != skb_len) {
+    ret = -EFAULT;//or may be -EIO
+    break;
+ }

Good for me.

+    } 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.

I think in this patch i can call 'virtio_transport_put_credit()' without if, but
i'll prepare separate patch which adds zero argument check to this function.

Yep, I agree.

As i see, the only function suitable for such 'if' condition is

Why not even for virtio_transport_get_credit() ?

When we send packets without payload (e.g. VIRTIO_VSOCK_OP_REQUEST,
VIRTIO_VSOCK_OP_SHUTDOWN) we call virtio_transport_get_credit()
with `credit` parameter equal to 0, then we acquire the spinlock but
in the end we do nothing.

Anyway - for future use this check won't be bad.

Yep, these are minor improvements ;-)


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