[PATCH net-next v4 10/10] virtio-net: xsk zero copy xmit kick by threshold

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

After testing, the performance of calling kick every time is not stable.
And if all the packets are sent and kicked again, the performance is not
good. So add a module parameter to specify how many packets are sent to
call a kick.

8 is a relatively stable value with the best performance.

Here is the pps of the test of xsk_kick_thr under different values (from
1 to 64).

thr  PPS             thr PPS             thr PPS
1    2924116.74247 | 23  3683263.04348 | 45  2777907.22963
2    3441010.57191 | 24  3078880.13043 | 46  2781376.21739
3    3636728.72378 | 25  2859219.57656 | 47  2777271.91304
4    3637518.61468 | 26  2851557.9593  | 48  2800320.56575
5    3651738.16251 | 27  2834783.54408 | 49  2813039.87599
6    3652176.69231 | 28  2847012.41472 | 50  3445143.01839
7    3665415.80602 | 29  2860633.91304 | 51  3666918.01281
8    3665045.16555 | 30  2857903.5786  | 52  3059929.2709
9    3671023.2401  | 31  2835589.98963 | 53  2831515.21739
10   3669532.23274 | 32  2862827.88706 | 54  3451804.07204
11   3666160.37749 | 33  2871855.96696 | 55  3654975.92385
12   3674951.44813 | 34  3434456.44816 | 56  3676198.3188
13   3667447.57331 | 35  3656918.54177 | 57  3684740.85619
14   3018846.0503  | 36  3596921.16722 | 58  3060958.8594
15   2792773.84505 | 37  3603460.63903 | 59  2828874.57191
16   3430596.3602  | 38  3595410.87666 | 60  3459926.11027
17   3660525.85806 | 39  3604250.17819 | 61  3685444.47599
18   3045627.69054 | 40  3596542.28428 | 62  3049959.0809
19   2841542.94177 | 41  3600705.16054 | 63  2806280.04013
20   2830475.97348 | 42  3019833.71191 | 64  3448494.3913
21   2845655.55789 | 43  2752951.93264 |
22   3450389.84365 | 44  2753107.27164 |

It can be found that when the value of xsk_kick_thr is relatively small,
the performance is not good, and when its value is greater than 13, the
performance will be more irregular and unstable. It looks similar from 3
to 13, I chose 8 as the default value.

The test environment is qemu + vhost-net. I modified vhost-net to drop
the packets sent by vm directly, so that the cpu of vm can run higher.
By default, the processes in the vm and the cpu of softirqd are too low,
and there is no obvious difference in the test data.

During the test, the cpu of softirq reached 100%. Each xsk_kick_thr was
run for 300s, the pps of every second was recorded, and the average of
the pps was finally taken. The vhost process cpu on the host has also
reached 100%.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
---
 drivers/net/virtio_net.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c441d6bf1510..4e360bfc2cf0 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -28,9 +28,11 @@ static int napi_weight = NAPI_POLL_WEIGHT;
 module_param(napi_weight, int, 0444);
 
 static bool csum = true, gso = true, napi_tx = true;
+static int xsk_kick_thr = 8;
 module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 module_param(napi_tx, bool, 0644);
+module_param(xsk_kick_thr, int, 0644);
 
 /* FIXME: MTU in config. */
 #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
@@ -2690,6 +2692,7 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
 	struct xdp_desc desc;
 	int err, packet = 0;
 	int ret = -EAGAIN;
+	int need_kick = 0;
 
 	if (sq->xsk.last_desc.addr) {
 		if (sq->vq->num_free < 2 + MAX_SKB_FRAGS)
@@ -2700,6 +2703,7 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
 			return -EBUSY;
 
 		++packet;
+		++need_kick;
 		--budget;
 		sq->xsk.last_desc.addr = 0;
 	}
@@ -2723,11 +2727,22 @@ static int virtnet_xsk_xmit_batch(struct send_queue *sq,
 		}
 
 		++packet;
+		++need_kick;
+		if (need_kick > xsk_kick_thr) {
+			if (virtqueue_kick_prepare(sq->vq) &&
+			    virtqueue_notify(sq->vq))
+				++stats->kicks;
+
+			need_kick = 0;
+		}
 	}
 
 	if (packet) {
-		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
-			++stats->kicks;
+		if (need_kick) {
+			if (virtqueue_kick_prepare(sq->vq) &&
+			    virtqueue_notify(sq->vq))
+				++stats->kicks;
+		}
 
 		*done = packet;
 		stats->xdp_tx += packet;
-- 
2.31.0