[PATCH] vhost-net: add limitation of sent packets for tx polling

["haibinzhang(张海斌)" <haibinzhang@tencent.com>]

handle_tx will delay rx for a long time when tx busy polling udp packets
with small length(e.g. 1byte udp payload), because setting VHOST_NET_WEIGHT
takes into account only sent-bytes but no single packet length.

Tests were done between two Virtual Machines using netperf(UDP_STREAM, len=1),
then another machine pinged the client. Result shows as follow:

Packet#       Ping-Latency(ms)
              min     avg     max
Origin      3.319  18.489  57.503
64          1.643   2.021   2.552
128         1.825   2.600   3.224
256         1.997   2.710   4.295
512*        1.860   3.171   4.631
1024        2.002   4.173   9.056
2048        2.257   5.650   9.688
4096        2.093   8.508  15.943

512 is selected, which is multi-VRING_SIZE and close to VHOST_NET_WEIGHT/MTU.
To evaluate this change, another tests were done using netperf(RR, TX) between
two machines with Intel(R) Xeon(R) Gold 6133 CPU @ 2.50GHz. Result as follow
does not show obvious changes:

TCP_RR

size/sessions/+thu%/+normalize%
   1/       1/  -7%/        -2%
   1/       4/  +1%/         0%
   1/       8/  +1%/        -2%
  64/       1/  -6%/         0%
  64/       4/   0%/        +2%
  64/       8/   0%/         0%
 256/       1/  -3%/        -4%
 256/       4/  +3%/        +4%
 256/       8/  +2%/         0%

UDP_RR

size/sessions/+thu%/+normalize%
   1/       1/  -5%/        +1%
   1/       4/  +4%/        +1%
   1/       8/  -1%/        -1%
  64/       1/  -2%/        -3%
  64/       4/  -5%/        -1%
  64/       8/   0%/        -1%
 256/       1/  +7%/        +1%
 256/       4/  +1%/        +1%
 256/       8/  +2%/        +2%

TCP_STREAM

size/sessions/+thu%/+normalize%
  64/       1/   0%/        -3%
  64/       4/  +3%/        -1%
  64/       8/  +9%/        -4%
 256/       1/  +1%/        -4%
 256/       4/  -1%/        -1%
 256/       8/  +7%/        +5%
 512/       1/  +1%/         0%
 512/       4/  +1%/        -1%
 512/       8/  +7%/        -5%
1024/       1/   0%/        -1%
1024/       4/  +3%/         0%
1024/       8/  +8%/        +5%
2048/       1/  +2%/        +2%
2048/       4/  +1%/         0%
2048/       8/  -2%/         0%
4096/       1/  -2%/         0%
4096/       4/  +2%/         0%
4096/       8/  +9%/        -2%

Signed-off-by: Haibin Zhang <haibinzhang@tencent.com>
Signed-off-by: Yunfang Tai <yunfangtai@tencent.com>
Signed-off-by: Lidong Chen <lidongchen@tencent.com>
---
 drivers/vhost/net.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8139bc70ad7d..13a23f3f3ea4 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -44,6 +44,10 @@ MODULE_PARM_DESC(experimental_zcopytx, "Enable Zero Copy TX;"
  * Using this limit prevents one virtqueue from starving others. */
 #define VHOST_NET_WEIGHT 0x80000
 
+/* Max number of packets transferred before requeueing the job.
+ * Using this limit prevents one virtqueue from starving rx. */
+#define VHOST_NET_PKT_WEIGHT 512
+
 /* MAX number of TX used buffers for outstanding zerocopy */
 #define VHOST_MAX_PEND 128
 #define VHOST_GOODCOPY_LEN 256
@@ -473,6 +477,7 @@ static void handle_tx(struct vhost_net *net)
 	struct socket *sock;
 	struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
 	bool zcopy, zcopy_used;
+	int sent_pkts = 0;
 
 	mutex_lock(&vq->mutex);
 	sock = vq->private_data;
@@ -580,7 +585,8 @@ static void handle_tx(struct vhost_net *net)
 		else
 			vhost_zerocopy_signal_used(net, vq);
 		vhost_net_tx_packet(net);
-		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+		if (unlikely(total_len >= VHOST_NET_WEIGHT) ||
+		    unlikely(++sent_pkts >= VHOST_NET_PKT_WEIGHT)) {
 			vhost_poll_queue(&vq->poll);
 			break;
 		}
-- 
2.12.3