[PATCH V3 0/2] vhost_net polling optimization

Hi:

This series tries to optimize vhost_net polling at two points:

- Stop rx polling for reduicng the unnecessary wakeups during
  handle_rx().
- Conditonally enable tx polling for reducing the unnecessary
  traversing and spinlock touching.

Test shows about 17% improvement on rx pps.

Please review

Changes from V2:
- Don't enable rx vq if we meet an err or rx vq is empty
Changes from V1:
- use vhost_net_disable_vq()/vhost_net_enable_vq() instead of open
  coding.
- Add a new patch for conditionally enable tx polling.

Jason Wang (2):
  vhost_net: stop polling socket during rx processing
  vhost_net: conditionally enable tx polling

 drivers/vhost/net.c | 67 ++++++++++++++++++++++++++++-------------------------
 1 file changed, 36 insertions(+), 31 deletions(-)

-- 
1.8.3.1

We don't stop rx polling socket during rx processing, this will lead
unnecessary wakeups from under layer net devices (E.g
sock_def_readable() form tun). Rx will be slowed down in this
way. This patch avoids this by stop polling socket during rx
processing. A small drawback is that this introduces some overheads in
light load case because of the extra start/stop polling, but single
netperf TCP_RR does not notice any change. In a super heavy load case,
e.g using pktgen to inject packet to guest, we get about ~8.8%
improvement on pps:

before: ~1240000 pkt/s
after:  ~1350000 pkt/s

Signed-off-by: Jason Wang <redacted>
---
 drivers/vhost/net.c | 64 +++++++++++++++++++++++++++--------------------------
 1 file changed, 33 insertions(+), 31 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index f744eeb..1d3e45f 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -301,6 +301,32 @@ static bool vhost_can_busy_poll(struct vhost_dev *dev,
 	       !vhost_has_work(dev);
 }
 
+static void vhost_net_disable_vq(struct vhost_net *n,
+				 struct vhost_virtqueue *vq)
+{
+	struct vhost_net_virtqueue *nvq =
+		container_of(vq, struct vhost_net_virtqueue, vq);
+	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
+	if (!vq->private_data)
+		return;
+	vhost_poll_stop(poll);
+}
+
+static int vhost_net_enable_vq(struct vhost_net *n,
+				struct vhost_virtqueue *vq)
+{
+	struct vhost_net_virtqueue *nvq =
+		container_of(vq, struct vhost_net_virtqueue, vq);
+	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
+	struct socket *sock;
+
+	sock = vq->private_data;
+	if (!sock)
+		return 0;
+
+	return vhost_poll_start(poll, sock->file);
+}
+
 static int vhost_net_tx_get_vq_desc(struct vhost_net *net,
 				    struct vhost_virtqueue *vq,
 				    struct iovec iov[], unsigned int iov_size,

@@ -613,6 +639,7 @@ static void handle_rx(struct vhost_net *net)
 	if (!sock)
 		goto out;
 	vhost_disable_notify(&net->dev, vq);
+	vhost_net_disable_vq(net, vq);
 
 	vhost_hlen = nvq->vhost_hlen;
 	sock_hlen = nvq->sock_hlen;

@@ -629,7 +656,7 @@ static void handle_rx(struct vhost_net *net)
 					likely(mergeable) ? UIO_MAXIOV : 1);
 		/* On error, stop handling until the next kick. */
 		if (unlikely(headcount < 0))
-			break;
+			goto out;
 		/* On overrun, truncate and discard */
 		if (unlikely(headcount > UIO_MAXIOV)) {
 			iov_iter_init(&msg.msg_iter, READ, vq->iov, 1, 1);

@@ -648,7 +675,7 @@ static void handle_rx(struct vhost_net *net)
 			}
 			/* Nothing new?  Wait for eventfd to tell us
 			 * they refilled. */
-			break;
+			goto out;
 		}
 		/* We don't need to be notified again. */
 		iov_iter_init(&msg.msg_iter, READ, vq->iov, in, vhost_len);

@@ -676,7 +703,7 @@ static void handle_rx(struct vhost_net *net)
 					 &fixup) != sizeof(hdr)) {
 				vq_err(vq, "Unable to write vnet_hdr "
 				       "at addr %p\n", vq->iov->iov_base);
-				break;
+				goto out;
 			}
 		} else {
 			/* Header came from socket; we'll need to patch

@@ -692,7 +719,7 @@ static void handle_rx(struct vhost_net *net)
 				 &fixup) != sizeof num_buffers) {
 			vq_err(vq, "Failed num_buffers write");
 			vhost_discard_vq_desc(vq, headcount);
-			break;
+			goto out;
 		}
 		vhost_add_used_and_signal_n(&net->dev, vq, vq->heads,
 					    headcount);

@@ -701,9 +728,10 @@ static void handle_rx(struct vhost_net *net)
 		total_len += vhost_len;
 		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
 			vhost_poll_queue(&vq->poll);
-			break;
+			goto out;
 		}
 	}
+	vhost_net_enable_vq(net, vq);
 out:
 	mutex_unlock(&vq->mutex);
 }

@@ -782,32 +810,6 @@ static int vhost_net_open(struct inode *inode, struct file *f)
 	return 0;
 }
 
-static void vhost_net_disable_vq(struct vhost_net *n,
-				 struct vhost_virtqueue *vq)
-{
-	struct vhost_net_virtqueue *nvq =
-		container_of(vq, struct vhost_net_virtqueue, vq);
-	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
-	if (!vq->private_data)
-		return;
-	vhost_poll_stop(poll);
-}
-
-static int vhost_net_enable_vq(struct vhost_net *n,
-				struct vhost_virtqueue *vq)
-{
-	struct vhost_net_virtqueue *nvq =
-		container_of(vq, struct vhost_net_virtqueue, vq);
-	struct vhost_poll *poll = n->poll + (nvq - n->vqs);
-	struct socket *sock;
-
-	sock = vq->private_data;
-	if (!sock)
-		return 0;
-
-	return vhost_poll_start(poll, sock->file);
-}
-
 static struct socket *vhost_net_stop_vq(struct vhost_net *n,
 					struct vhost_virtqueue *vq)
 {

-- 
1.8.3.1

We always poll tx for socket, this is sub optimal since:

- it will be only used when we exceed the sndbuf of the socket.
- since we use two independent polls for tx and vq, this will slightly
  increase the waitqueue traversing time and more important, vhost
  could not benefit from commit
  9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
  tun_do_read for better sleep/wakeup efficiency") even if we've
  stopped rx polling during handle_rx since tx poll were still left in
  the waitqueue.

Fix this by conditionally enable tx polling only when -EAGAIN were
met.

Test shows about 8% improvement on guest rx pps.

Before: ~1350000
After:  ~1460000

Signed-off-by: Jason Wang <redacted>
---
 drivers/vhost/net.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 1d3e45f..e75ffcc 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
 		goto out;
 
 	vhost_disable_notify(&net->dev, vq);
+	vhost_net_disable_vq(net, vq);
 
 	hdr_size = nvq->vhost_hlen;
 	zcopy = nvq->ubufs;

@@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
 					% UIO_MAXIOV;
 			}
 			vhost_discard_vq_desc(vq, 1);
+			if (err == -EAGAIN)
+				vhost_net_enable_vq(net, vq);
 			break;
 		}
 		if (err != len)

-- 
1.8.3.1

From: Jason Wang <redacted>
Date: Wed,  1 Jun 2016 01:56:32 -0400

This series tries to optimize vhost_net polling at two points:

- Stop rx polling for reduicng the unnecessary wakeups during
  handle_rx().
- Conditonally enable tx polling for reducing the unnecessary
  traversing and spinlock touching.

Test shows about 17% improvement on rx pps.

Please review

Changes from V2:
- Don't enable rx vq if we meet an err or rx vq is empty
Changes from V1:
- use vhost_net_disable_vq()/vhost_net_enable_vq() instead of open
  coding.
- Add a new patch for conditionally enable tx polling.

Michael, please review this patch series.

Thanks.

On Thu, Jun 02, 2016 at 12:08:06PM -0700, David Miller wrote:

From: Jason Wang <redacted>
Date: Wed,  1 Jun 2016 01:56:32 -0400

quoted

This series tries to optimize vhost_net polling at two points:

- Stop rx polling for reduicng the unnecessary wakeups during
  handle_rx().
- Conditonally enable tx polling for reducing the unnecessary
  traversing and spinlock touching.

Test shows about 17% improvement on rx pps.

Please review

Changes from V2:
- Don't enable rx vq if we meet an err or rx vq is empty
Changes from V1:
- use vhost_net_disable_vq()/vhost_net_enable_vq() instead of open
  coding.
- Add a new patch for conditionally enable tx polling.

Michael, please review this patch series.

Thanks.

thanks for the reminder, I plan to review next week.

On Wed, Jun 01, 2016 at 01:56:33AM -0400, Jason Wang wrote:

We don't stop rx polling socket during rx processing, this will lead
unnecessary wakeups from under layer net devices (E.g
sock_def_readable() form tun). Rx will be slowed down in this
way. This patch avoids this by stop polling socket during rx
processing. A small drawback is that this introduces some overheads in
light load case because of the extra start/stop polling, but single
netperf TCP_RR does not notice any change. In a super heavy load case,
e.g using pktgen to inject packet to guest, we get about ~8.8%
improvement on pps:

before: ~1240000 pkt/s
after:  ~1350000 pkt/s

Signed-off-by: Jason Wang <redacted>

I guess this works though I suspect it's even faster to
maintain some state in the vq structure so we don't need
to play with waitq all the time.

For now

Acked-by: Michael S. Tsirkin <mst@redhat.com>

On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:

This seems rather risky. What if TX failed for some other reason?
Polling won't ever be re-enabled ...

-- 
1.8.3.1

From: Jason Wang <redacted>
Date: Wed,  1 Jun 2016 01:56:33 -0400

We don't stop rx polling socket during rx processing, this will lead
unnecessary wakeups from under layer net devices (E.g
sock_def_readable() form tun). Rx will be slowed down in this
way. This patch avoids this by stop polling socket during rx
processing. A small drawback is that this introduces some overheads in
light load case because of the extra start/stop polling, but single
netperf TCP_RR does not notice any change. In a super heavy load case,
e.g using pktgen to inject packet to guest, we get about ~8.8%
improvement on pps:

before: ~1240000 pkt/s
after:  ~1350000 pkt/s

Signed-off-by: Jason Wang <redacted>

Applied.

On 2016年06月07日 20:26, Michael S. Tsirkin wrote:

On Wed, Jun 01, 2016 at 01:56:34AM -0400, Jason Wang wrote:

quoted

We always poll tx for socket, this is sub optimal since:

- it will be only used when we exceed the sndbuf of the socket.
- since we use two independent polls for tx and vq, this will slightly
   increase the waitqueue traversing time and more important, vhost
   could not benefit from commit
   9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 ("net-tun: restructure
   tun_do_read for better sleep/wakeup efficiency") even if we've
   stopped rx polling during handle_rx since tx poll were still left in
   the waitqueue.

Fix this by conditionally enable tx polling only when -EAGAIN were
met.

Test shows about 8% improvement on guest rx pps.

Before: ~1350000
After:  ~1460000

Signed-off-by: Jason Wang <redacted>
---
  drivers/vhost/net.c | 3 +++
  1 file changed, 3 insertions(+)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 1d3e45f..e75ffcc 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -378,6 +378,7 @@ static void handle_tx(struct vhost_net *net)
  		goto out;
  
  	vhost_disable_notify(&net->dev, vq);
+	vhost_net_disable_vq(net, vq);
  
  	hdr_size = nvq->vhost_hlen;
  	zcopy = nvq->ubufs;

@@ -459,6 +460,8 @@ static void handle_tx(struct vhost_net *net)
  					% UIO_MAXIOV;
  			}
  			vhost_discard_vq_desc(vq, 1);
+			if (err == -EAGAIN)
+				vhost_net_enable_vq(net, vq);
  			break;
  		}
  		if (err != len)

This seems rather risky. What if TX failed for some other reason?
Polling won't ever be re-enabled ...

But why we need to enable tx poll in this case? Even if we enable it, we 
wont' get any wakeup.

quoted

-- 
1.8.3.1

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