On 2016/1/20 22:35, Michael S. Tsirkin wrote:
On Tue, Dec 01, 2015 at 02:39:45PM +0800, Jason Wang wrote:
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 <jasow...@redhat.com>
---
  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.

+
+       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.

+{
+       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?


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

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

+
+               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.

--
best regards
yang

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