Re: [PATCH V2 3/3] vhost_net: basic polling support
From: Yang Zhang
Date: Thu Jan 21 2016 - 01:40:06 EST
On 2016/1/21 13:13, Michael S. Tsirkin wrote:
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.
Indeed!
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
--
best regards
yang