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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 11 Mar 2013 10:26:24 +0200

On Mon, Mar 11, 2013 at 03:33:16PM +0800, Jason Wang wrote:
> On 03/11/2013 03:09 PM, Jason Wang wrote:
> > On 03/11/2013 12:50 AM, Michael S. Tsirkin wrote:
> >> 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.
> > I think then it may damage the performance at least for tx. We
> > conditionally enable the tx polling in the past to reduce the
> > unnecessary wakeups of vhost thead when zerocopy or sndbuf is used. If
> > we don't stop the polling, after each packet were transmitted,
> > tap/macvtap will try to wakeup the vhost thread.
> 
> Or for zerocopy, looks like we can in fact just depends on
> ubuf->callback in stead of tx polling to wakeup tx thread to do tx
> completion, so we can even drop the num_pends and VHOST_MAX_PEND stuffs.

I think we still need to limit the number of outstanding buffers so
VHOST_MAX_PEND needs to stay.  I'm not against switching to callback
for this but let's make this a separate change.

> > Actually, I'm considering the reverse direction. Looks like we can
> > disable the rx polling like what we do for tx in handle_tx() to reduce
> > the uncessary wakeups.
> >> 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
> >> --
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