Subject: Re: [PATCH] vhost_net: remove tx polling state

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 10 Mar 2013 18:50:30 +0200

On Thu, Mar 07, 2013 at 12:31:56PM +0800, Jason Wang wrote:
> After commit 2b8b328b61c799957a456a5a8dab8cc7dea68575 (vhost_net: handle polling
> errors when setting backend), we in fact track the polling state through
> poll->wqh, so there's no need to duplicate the work with an extra
> vhost_net_polling_state. So this patch removes this and make the code simpler.
> 
> Signed-off-by: Jason Wang <[email-masked]>

I'd prefer a more radical approach, since I think it can be even
simpler: tap and macvtap backends both only send events when tx queue
overruns which should almost never happen.

So let's just start polling when VQ is enabled
drop all poll_start/stop calls on data path.

The optimization was written for packet socket backend but I know of no
one caring about performance of that one that much.
Needs a bit of perf testing to make sure I didn't miss anything though
but it's not 3.9 material anyway so there's no rush.

> ---
>  drivers/vhost/net.c   |   60 ++++++++----------------------------------------
>  drivers/vhost/vhost.c |    3 ++
>  2 files changed, 13 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 959b1cd..d1a03dd 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -64,20 +64,10 @@ enum {
>  	VHOST_NET_VQ_MAX = 2,
>  };
>  
> -enum vhost_net_poll_state {
> -	VHOST_NET_POLL_DISABLED = 0,
> -	VHOST_NET_POLL_STARTED = 1,
> -	VHOST_NET_POLL_STOPPED = 2,
> -};
> -
>  struct vhost_net {
>  	struct vhost_dev dev;
>  	struct vhost_virtqueue vqs[VHOST_NET_VQ_MAX];
>  	struct vhost_poll poll[VHOST_NET_VQ_MAX];
> -	/* Tells us whether we are polling a socket for TX.
> -	 * We only do this when socket buffer fills up.
> -	 * Protected by tx vq lock. */
> -	enum vhost_net_poll_state tx_poll_state;
>  	/* Number of TX recently submitted.
>  	 * Protected by tx vq lock. */
>  	unsigned tx_packets;
> @@ -155,28 +145,6 @@ static void copy_iovec_hdr(const struct iovec *from, struct iovec *to,
>  	}
>  }
>  
> -/* Caller must have TX VQ lock */
> -static void tx_poll_stop(struct vhost_net *net)
> -{
> -	if (likely(net->tx_poll_state != VHOST_NET_POLL_STARTED))
> -		return;
> -	vhost_poll_stop(net->poll + VHOST_NET_VQ_TX);
> -	net->tx_poll_state = VHOST_NET_POLL_STOPPED;
> -}
> -
> -/* Caller must have TX VQ lock */
> -static int tx_poll_start(struct vhost_net *net, struct socket *sock)
> -{
> -	int ret;
> -
> -	if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
> -		return 0;
> -	ret = vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
> -	if (!ret)
> -		net->tx_poll_state = VHOST_NET_POLL_STARTED;
> -	return ret;
> -}
> -
>  /* In case of DMA done not in order in lower device driver for some reason.
>   * upend_idx is used to track end of used idx, done_idx is used to track head
>   * of used idx. Once lower device DMA done contiguously, we will signal KVM
> @@ -231,6 +199,7 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>  static void handle_tx(struct vhost_net *net)
>  {
>  	struct vhost_virtqueue *vq = &net->dev.vqs[VHOST_NET_VQ_TX];
> +	struct vhost_poll *poll = net->poll + VHOST_NET_VQ_TX;
>  	unsigned out, in, s;
>  	int head;
>  	struct msghdr msg = {
> @@ -256,7 +225,7 @@ static void handle_tx(struct vhost_net *net)
>  	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>  	if (wmem >= sock->sk->sk_sndbuf) {
>  		mutex_lock(&vq->mutex);
> -		tx_poll_start(net, sock);
> +		vhost_poll_start(poll, sock->file);
>  		mutex_unlock(&vq->mutex);
>  		return;
>  	}
> @@ -265,7 +234,7 @@ static void handle_tx(struct vhost_net *net)
>  	vhost_disable_notify(&net->dev, vq);
>  
>  	if (wmem < sock->sk->sk_sndbuf / 2)
> -		tx_poll_stop(net);
> +		vhost_poll_stop(poll);
>  	hdr_size = vq->vhost_hlen;
>  	zcopy = vq->ubufs;
>  
> @@ -287,7 +256,7 @@ static void handle_tx(struct vhost_net *net)
>  
>  			wmem = atomic_read(&sock->sk->sk_wmem_alloc);
>  			if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
> -				tx_poll_start(net, sock);
> +				vhost_poll_start(poll, sock->file);
>  				set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
>  				break;
>  			}
> @@ -298,7 +267,7 @@ static void handle_tx(struct vhost_net *net)
>  				    (vq->upend_idx - vq->done_idx) :
>  				    (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
>  			if (unlikely(num_pends > VHOST_MAX_PEND)) {
> -				tx_poll_start(net, sock);
> +				vhost_poll_start(poll, sock->file);
>  				set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
>  				break;
>  			}
> @@ -364,7 +333,7 @@ static void handle_tx(struct vhost_net *net)
>  			}
>  			vhost_discard_vq_desc(vq, 1);
>  			if (err == -EAGAIN || err == -ENOBUFS)
> -				tx_poll_start(net, sock);
> +				vhost_poll_start(poll, sock->file);
>  			break;
>  		}
>  		if (err != len)
> @@ -627,7 +596,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  
>  	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
>  	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
> -	n->tx_poll_state = VHOST_NET_POLL_DISABLED;
>  
>  	f->private_data = n;
>  
> @@ -637,32 +605,24 @@ static int vhost_net_open(struct inode *inode, struct file *f)
>  static void vhost_net_disable_vq(struct vhost_net *n,
>  				 struct vhost_virtqueue *vq)
>  {
> +	struct vhost_poll *poll = n->poll + (vq - n->vqs);
>  	if (!vq->private_data)
>  		return;
> -	if (vq == n->vqs + VHOST_NET_VQ_TX) {
> -		tx_poll_stop(n);
> -		n->tx_poll_state = VHOST_NET_POLL_DISABLED;
> -	} else
> -		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
> +	vhost_poll_stop(poll);
>  }
>  
>  static int vhost_net_enable_vq(struct vhost_net *n,
>  				struct vhost_virtqueue *vq)
>  {
> +	struct vhost_poll *poll = n->poll + (vq - n->vqs);
>  	struct socket *sock;
> -	int ret;
>  
>  	sock = rcu_dereference_protected(vq->private_data,
>  					 lockdep_is_held(&vq->mutex));
>  	if (!sock)
>  		return 0;
> -	if (vq == n->vqs + VHOST_NET_VQ_TX) {
> -		n->tx_poll_state = VHOST_NET_POLL_STOPPED;
> -		ret = tx_poll_start(n, sock);
> -	} else
> -		ret = vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
>  
> -	return ret;
> +	return vhost_poll_start(poll, sock->file);
>  }
>  
>  static struct socket *vhost_net_stop_vq(struct vhost_net *n,
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 9759249..4eecdb8 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -89,6 +89,9 @@ int vhost_poll_start(struct vhost_poll *poll, struct file *file)
>  	unsigned long mask;
>  	int ret = 0;
>  
> +	if (poll->wqh)
> +		return 0;
> +
>  	mask = file->f_op->poll(file, &poll->table);
>  	if (mask)
>  		vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
> -- 
> 1.7.1