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SubjectRe: [PATCH v4 2/5] vsock/virtio: reduce credit update messages
On Wed, Jul 17, 2019 at 01:30:27PM +0200, Stefano Garzarella wrote:
> In order to reduce the number of credit update messages,
> we send them only when the space available seen by the
> transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.
>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> include/linux/virtio_vsock.h | 1 +
> net/vmw_vsock/virtio_transport_common.c | 16 +++++++++++++---
> 2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index 7d973903f52e..49fc9d20bc43 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -41,6 +41,7 @@ struct virtio_vsock_sock {
>
> /* Protected by rx_lock */
> u32 fwd_cnt;
> + u32 last_fwd_cnt;
> u32 rx_bytes;
> struct list_head rx_queue;
> };
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 095221f94786..a85559d4d974 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -211,6 +211,7 @@ static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
> void virtio_transport_inc_tx_pkt(struct virtio_vsock_sock *vvs, struct virtio_vsock_pkt *pkt)
> {
> spin_lock_bh(&vvs->tx_lock);
> + vvs->last_fwd_cnt = vvs->fwd_cnt;
> pkt->hdr.fwd_cnt = cpu_to_le32(vvs->fwd_cnt);
> pkt->hdr.buf_alloc = cpu_to_le32(vvs->buf_alloc);
> spin_unlock_bh(&vvs->tx_lock);
> @@ -261,6 +262,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> struct virtio_vsock_sock *vvs = vsk->trans;
> struct virtio_vsock_pkt *pkt;
> size_t bytes, total = 0;
> + u32 free_space;
> int err = -EFAULT;
>
> spin_lock_bh(&vvs->rx_lock);
> @@ -291,11 +293,19 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> virtio_transport_free_pkt(pkt);
> }
> }
> +
> + free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> +
> spin_unlock_bh(&vvs->rx_lock);
>
> - /* Send a credit pkt to peer */
> - virtio_transport_send_credit_update(vsk, VIRTIO_VSOCK_TYPE_STREAM,
> - NULL);
> + /* We send a credit update only when the space available seen
> + * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE

This is just repeating what code does though.
Please include the *reason* for the condition.
E.g. here's a better comment:

/* To reduce number of credit update messages,
* don't update credits as long as lots of space is available.
* Note: the limit chosen here is arbitrary. Setting the limit
* 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.
*/


> + */
> + if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) {
> + virtio_transport_send_credit_update(vsk,
> + VIRTIO_VSOCK_TYPE_STREAM,
> + NULL);
> + }
>
> return total;
>
> --
> 2.20.1

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