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

From: Jason Wang <[email-masked]>
Date: Mon, 11 Mar 2013 15:33:16 +0800

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.
> 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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