netdev - Re: [PATCH net-next v8 3/4] virtio/vsock: fix logic which reduces credit update messages

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Date: Tue, 12 Dec 2023 09:56:45 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Arseniy Krasnov <avkrasnov@...utedevices.com>
Cc: Stefan Hajnoczi <stefanha@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>, 
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel@...rdevices.ru, 
	oxffffaa@...il.com
Subject: Re: [PATCH net-next v8 3/4] virtio/vsock: fix logic which reduces
 credit update messages

On Tue, Dec 12, 2023 at 12:16:57AM +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@...utedevices.com>
>---
> Changelog:
> v6 -> v7:
>  * Handle wrap of 'fwd_cnt'.
>  * Do to send credit update when 'fwd_cnt' == 'last_fwd_cnt'.
> v7 -> v8:
>  * Remove unneeded/wrong handling of wrap for 'fwd_cnt'.
>
> net/vmw_vsock/virtio_transport_common.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

Thanks!
Stefano

>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index e137d740804e..8572f94bba88 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -558,6 +558,8 @@ 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;
>+	u32 fwd_cnt_delta;
>+	bool low_rx_bytes;
> 	int err = -EFAULT;
> 	u32 free_space;
>
>@@ -601,7 +603,10 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> 		}
> 	}
>
>-	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
>+	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>+	free_space = vvs->buf_alloc - fwd_cnt_delta;
>+	low_rx_bytes = (vvs->rx_bytes <
>+			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>
> 	spin_unlock_bh(&vvs->rx_lock);
>
>@@ -611,9 +616,11 @@ 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 (fwd_cnt_delta &&
>+	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
> 		virtio_transport_send_credit_update(vsk);
>
> 	return total;
>-- 
>2.25.1
>