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

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Date:   Mon, 20 Mar 2023 15:29:59 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...rdevices.ru>
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>,
        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: [RFC PATCH v2] virtio/vsock: allocate multiple skbuffs on tx

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@...rdevices.ru>
>---
> Link to v1:
> https://lore.kernel.org/netdev/2c52aa26-8181-d37a-bccd-a86bd3cbc6e1@sberdevices.ru/
>
> 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.
>
> 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.

>+	} 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.

Thanks,
Stefano

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