Re: [RFC PATCH v3] virtio/vsock: allocate multiple skbuffs on tx

[Stefano Garzarella]

On Tue, Mar 21, 2023 at 12:31:48AM +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/
> Link to v2:
> https://lore.kernel.org/netdev/ea5725eb-6cb5-cf15-2938-34e335a442fa@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.
>
> v2 -> v3:
> - Handle case when transport callback returns unexpected value which
>   is not equal to 'skb->len'. Break loop.
> - Don't check for zero value of 'rest_len' before calling
>   'virtio_transport_put_credit()'. Decided to add this check directly
>   to 'virtio_transport_put_credit()' in separate patch.
>
> net/vmw_vsock/virtio_transport_common.c | 59 +++++++++++++++++++------
> 1 file changed, 45 insertions(+), 14 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 6564192e7f20..e0b2c6ecbe22 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 +224,51 @@ 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);
>
> -	virtio_transport_inc_tx_pkt(vvs, skb);
> +		ret = t_ops->send_pkt(skb);
>
> -	return t_ops->send_pkt(skb);
> +		if (ret < 0)
> +			break;
> +
> +		/* Both virtio and vhost 'send_pkt()' returns 'skb_len',
> +		 * but for reliability use 'ret' instead of 'skb_len'.
> +		 * Also if partial send happens (e.g. 'ret' != 'skb_len')
> +		 * somehow, we break this loop, but account such returned
> +		 * value in 'virtio_transport_put_credit()'.
> +		 */
> +		rest_len -= ret;
> +
> +		if (ret != skb_len) {
> +			ret = -EFAULT;

Okay, but `ret` will be overwritten by the check we have before the
return ...

> +			break;
> +		}
> +	} while (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;

... here.

Since we don't expect this condition for now, perhaps we can avoid
setting ret with -EFAULT, but we can add a WARN_ONCE (interrupting the
loop as you did here).

This way we return the partial length as we did before.

Thanks,
Stefano

> +
> +	return ret;
> }
>
> static bool virtio_transport_inc_rx_pkt(struct virtio_vsock_sock *vvs,
> --
> 2.25.1