linux-kernel - Re: [PATCH RFC 1/4] vsock/virtio: reduce credit update messages

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Date:   Thu, 4 Apr 2019 20:15:39 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH RFC 1/4] vsock/virtio: reduce credit update messages

On Thu, Apr 04, 2019 at 12:58:35PM +0200, Stefano Garzarella wrote:
> @@ -256,6 +257,7 @@ 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;
> +	s64 free_space;

Why s64?  buf_alloc, fwd_cnt, and last_fwd_cnt are all u32.  fwd_cnt -
last_fwd_cnt <= buf_alloc is always true.

>  	int err = -EFAULT;
>  
>  	spin_lock_bh(&vvs->rx_lock);
> @@ -288,9 +290,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>  	}
>  	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
> +	 */
> +	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);

Locking?  These fields should be accessed under tx_lock.

> +	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE) {
> +		virtio_transport_send_credit_update(vsk,
> +						    VIRTIO_VSOCK_TYPE_STREAM,
> +						    NULL);
> +	}

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