Re: [PATCH V2 3/3] vhost_net: basic polling support

On Thu, Jan 21, 2016 at 10:11:35AM +0800, Yang Zhang wrote:

On 2016/1/20 22:35, Michael S. Tsirkin wrote:

quoted

On Tue, Dec 01, 2015 at 02:39:45PM +0800, Jason Wang wrote:

quoted

This patch tries to poll for new added tx buffer or socket receive
queue for a while at the end of tx/rx processing. The maximum time
spent on polling were specified through a new kind of vring ioctl.

Signed-off-by: Jason Wang <redacted>
---
 drivers/vhost/net.c        | 72 ++++++++++++++++++++++++++++++++++++++++++----
 drivers/vhost/vhost.c      | 15 ++++++++++
 drivers/vhost/vhost.h      |  1 +
 include/uapi/linux/vhost.h | 11 +++++++
 4 files changed, 94 insertions(+), 5 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 9eda69e..ce6da77 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -287,6 +287,41 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
 	rcu_read_unlock_bh();
 }

+static inline unsigned long busy_clock(void)
+{
+	return local_clock() >> 10;
+}
+
+static bool vhost_can_busy_poll(struct vhost_dev *dev,
+				unsigned long endtime)
+{
+	return likely(!need_resched()) &&
+	       likely(!time_after(busy_clock(), endtime)) &&
+	       likely(!signal_pending(current)) &&
+	       !vhost_has_work(dev) &&
+	       single_task_running();
+}
+
+static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
+				    struct vhost_virtqueue *vq,
+				    struct iovec iov[], unsigned int iov_size,
+				    unsigned int *out_num, unsigned int *in_num)
+{
+	unsigned long uninitialized_var(endtime);
+
+	if (vq->busyloop_timeout) {
+		preempt_disable();
+		endtime = busy_clock() + vq->busyloop_timeout;
+		while (vhost_can_busy_poll(vq->dev, endtime) &&
+		       !vhost_vq_more_avail(vq->dev, vq))
+			cpu_relax();
+		preempt_enable();
+	}

Isn't there a way to call all this after vhost_get_vq_desc?
First, this will reduce the good path overhead as you
won't have to play with timers and preemption.

Second, this will reduce the chance of a pagefault on avail ring read.

quoted

+
+	return vhost_get_vq_desc(vq, vq->iov, ARRAY_SIZE(vq->iov),
+				 out_num, in_num, NULL, NULL);
+}
+
 /* Expects to be always run from workqueue - which acts as
  * read-size critical section for our kind of RCU. */
 static void handle_tx(struct vhost_net *net)

@@ -331,10 +366,9 @@ static void handle_tx(struct vhost_net *net)
 			      % UIO_MAXIOV == nvq->done_idx))
 			break;

-		head = vhost_get_vq_desc(vq, vq->iov,
-					 ARRAY_SIZE(vq->iov),
-					 &out, &in,
-					 NULL, NULL);
+		head = vhost_net_tx_get_vq_desc(net, vq, vq->iov,
+						ARRAY_SIZE(vq->iov),
+						&out, &in);
 		/* On error, stop handling until the next kick. */
 		if (unlikely(head < 0))
 			break;

@@ -435,6 +469,34 @@ static int peek_head_len(struct sock *sk)
 	return len;
 }

+static int vhost_net_peek_head_len(struct vhost_net *net, struct sock *sk)

Need a hint that it's rx related in the name.

quoted

+{
+	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
+	struct vhost_virtqueue *vq = &nvq->vq;
+	unsigned long uninitialized_var(endtime);
+
+	if (vq->busyloop_timeout) {
+		mutex_lock(&vq->mutex);

This appears to be called under vq mutex in handle_rx.
So how does this work then?

quoted

+		vhost_disable_notify(&net->dev, vq);

This appears to be called after disable notify
in handle_rx - so why disable here again?

quoted

+
+		preempt_disable();
+		endtime = busy_clock() + vq->busyloop_timeout;
+
+		while (vhost_can_busy_poll(&net->dev, endtime) &&
+		       skb_queue_empty(&sk->sk_receive_queue) &&
+		       !vhost_vq_more_avail(&net->dev, vq))
+			cpu_relax();

This seems to mix in several items.
RX queue is normally not empty. I don't think
we need to poll for that.

I have seen the RX queue is easy to be empty under some extreme conditions
like lots of small packet. So maybe the check is useful here.

It's not useful *here*.
If you have an rx packet but no space in the ring,
this will exit immediately.

It might be useful elsewhere but I doubt it -
if rx ring is out of buffers, you are better off
backing out and giving guest some breathing space.

-- 
best regards
yang