On Thu, Jan 21, 2016 at 10:11:35AM +0800, Yang Zhang wrote:
> 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 <jasowang@xxxxxxxxxx>
> >>---
> >> 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.
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
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