Re: [PATCH net-next v6 3/4] virtio/vsock: fix logic which reduces credit update messages

[Stefano Garzarella]

On Tue, Dec 05, 2023 at 09:48:05AM +0300, Arseniy Krasnov wrote:
> Add one more condition for sending credit update during dequeue from
> stream socket: when number of bytes in the rx queue is smaller than
> SO_RCVLOWAT value of the socket. This is actual for non-default value
> of SO_RCVLOWAT (e.g. not 1) - idea is to "kick" peer to continue data
> transmission, because we need at least SO_RCVLOWAT bytes in our rx
> queue to wake up user for reading data (in corner case it is also
> possible to stuck both tx and rx sides, this is why 'Fixes' is used).
>
> Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
> Signed-off-by: Arseniy Krasnov <avkrasnov@xxxxxxxxxxxxxxxxx>
> ---
> net/vmw_vsock/virtio_transport_common.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index e137d740804e..461c89882142 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -558,6 +558,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	struct virtio_vsock_sock *vvs = vsk->trans;
> 	size_t bytes, total = 0;
> 	struct sk_buff *skb;
> +	bool low_rx_bytes;
> 	int err = -EFAULT;
> 	u32 free_space;
>
> @@ -602,6 +603,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	}
>
> 	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> +	low_rx_bytes = (vvs->rx_bytes <
> +			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));

As in the previous patch, should we avoid the update it if `fwd_cnt` and 
`last_fwd_cnt` are the same?

Now I'm thinking if it is better to add that check directly in 
virtio_transport_send_credit_update().

Stefano

> 	spin_unlock_bh(&vvs->rx_lock);
>
> @@ -611,9 +614,11 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 	 * too high causes extra messages. Too low causes transmitter
> 	 * stalls. As stalls are in theory more expensive than extra
> 	 * messages, we set the limit to a high value. TODO: experiment
> -	 * with different values.
> +	 * with different values. Also send credit update message when
> +	 * number of bytes in rx queue is not enough to wake up reader.
> 	 */
> -	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
> +	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE ||
> +	    low_rx_bytes)
> 		virtio_transport_send_credit_update(vsk);
>
> 	return total;
> --
> 2.25.1