linux-kernel - Re: [RFC v3 3/7] vsock: batch buffers in tx

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Date:   Wed, 7 Sep 2022 12:27:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Guo Zhi <qtxuning1999@...u.edu.cn>, eperezma@...hat.com,
        sgarzare@...hat.com, mst@...hat.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v3 3/7] vsock: batch buffers in tx


在 2022/9/1 13:54, Guo Zhi 写道:
> Vsock uses buffers in order, and for tx driver doesn't have to
> know the length of the buffer. So we can do a batch for vsock if
> in order negotiated, only write one used ring for a batch of buffers
>
> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> ---
>   drivers/vhost/vsock.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index 368330417bde..e08fbbb5439e 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -497,7 +497,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
>   	struct vhost_vsock *vsock = container_of(vq->dev, struct vhost_vsock,
>   						 dev);
>   	struct virtio_vsock_pkt *pkt;
> -	int head, pkts = 0, total_len = 0;
> +	int head, pkts = 0, total_len = 0, add = 0;
>   	unsigned int out, in;
>   	bool added = false;
>   
> @@ -551,10 +551,18 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
>   		else
>   			virtio_transport_free_pkt(pkt);
>   
> -		vhost_add_used(vq, head, 0);
> +		if (!vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
> +			vhost_add_used(vq, head, 0);


I'd do this step by step.

1) switch to use vhost_add_used_n() for vsock, less copy_to_user() 
better performance
2) do in-order on top


> +		} else {
> +			vq->heads[add].id = head;
> +			vq->heads[add++].len = 0;


How can we guarantee that we are in the boundary of the heads array?

Btw in the case of in-order we don't need to record the heads, instead 
we just need to know the head of the last buffer, it reduces the stress 
on the cache.

Thanks


> +		}
>   		added = true;
>   	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));
>   
> +	/* If in order feature negotiaged, we can do a batch to increase performance */
> +	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER) && added)
> +		vhost_add_used_n(vq, vq->heads, add);
>   no_more_replies:
>   	if (added)
>   		vhost_signal(&vsock->dev, vq);