Re: [PATCH 21.11] examples/vhost: fix retry logic on Rx path

Re: [PATCH 21.11] examples/vhost: fix retry logic on Rx path

[Luca Boccassi]

On Sat, 2022-07-09 at 01:14 +0800, Yuan Wang wrote:
> [ upstream commit 1907ce4baec392a750fbeba5e946920b2f00ae73 ]
> 
> drain_eth_rx() uses rte_vhost_avail_entries() to calculate
> the available entries to determine if a retry is required.
> However, this function only works with split rings, and
> calculating packed rings will return the wrong value and cause
> unnecessary retries resulting in a significant performance penalty.
> 
> This patch fix that by using the difference between tx/rx burst
> as the retry condition.
> 
> Fixes: be800696c26e ("examples/vhost: use burst enqueue and dequeue from lib")
> 
> Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
> Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
> ---
>  examples/vhost/main.c | 79 ++++++++++++++++++-------------------------
>  1 file changed, 33 insertions(+), 46 deletions(-)

Thanks, applied.

-- 
Kind regards,
Luca Boccassi

[PATCH 21.11] examples/vhost: fix retry logic on Rx path

[Yuan Wang]

[ upstream commit 1907ce4baec392a750fbeba5e946920b2f00ae73 ]

drain_eth_rx() uses rte_vhost_avail_entries() to calculate
the available entries to determine if a retry is required.
However, this function only works with split rings, and
calculating packed rings will return the wrong value and cause
unnecessary retries resulting in a significant performance penalty.

This patch fix that by using the difference between tx/rx burst
as the retry condition.

Fixes: be800696c26e ("examples/vhost: use burst enqueue and dequeue from lib")

Signed-off-by: Yuan Wang <yuanx.wang@intel.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
---
 examples/vhost/main.c | 79 ++++++++++++++++++-------------------------
 1 file changed, 33 insertions(+), 46 deletions(-)

diff --git a/examples/vhost/main.c b/examples/vhost/main.c
index 84844da68f..f9e932061f 100644
--- a/examples/vhost/main.c
+++ b/examples/vhost/main.c
@@ -900,31 +900,43 @@ sync_virtio_xmit(struct vhost_dev *dst_vdev, struct vhost_dev *src_vdev,
 	}
 }
 
-static __rte_always_inline void
-drain_vhost(struct vhost_dev *vdev)
+static __rte_always_inline uint16_t
+enqueue_pkts(struct vhost_dev *vdev, struct rte_mbuf **pkts, uint16_t rx_count)
 {
-	uint16_t ret;
-	uint32_t buff_idx = rte_lcore_id() * MAX_VHOST_DEVICE + vdev->vid;
-	uint16_t nr_xmit = vhost_txbuff[buff_idx]->len;
-	struct rte_mbuf **m = vhost_txbuff[buff_idx]->m_table;
+	uint16_t enqueue_count;
 
 	if (builtin_net_driver) {
-		ret = vs_enqueue_pkts(vdev, VIRTIO_RXQ, m, nr_xmit);
+		enqueue_count = vs_enqueue_pkts(vdev, VIRTIO_RXQ, pkts, rx_count);
 	} else if (async_vhost_driver) {
 		uint16_t enqueue_fail = 0;
 
 		complete_async_pkts(vdev);
-		ret = rte_vhost_submit_enqueue_burst(vdev->vid, VIRTIO_RXQ, m, nr_xmit);
-		__atomic_add_fetch(&vdev->pkts_inflight, ret, __ATOMIC_SEQ_CST);
+		enqueue_count = rte_vhost_submit_enqueue_burst(vdev->vid,
+					VIRTIO_RXQ, pkts, rx_count);
+		__atomic_add_fetch(&vdev->pkts_inflight, enqueue_count, __ATOMIC_SEQ_CST);
 
-		enqueue_fail = nr_xmit - ret;
+		enqueue_fail = rx_count - enqueue_count;
 		if (enqueue_fail)
-			free_pkts(&m[ret], nr_xmit - ret);
+			free_pkts(&pkts[enqueue_count], enqueue_fail);
+
 	} else {
-		ret = rte_vhost_enqueue_burst(vdev->vid, VIRTIO_RXQ,
-						m, nr_xmit);
+		enqueue_count = rte_vhost_enqueue_burst(vdev->vid, VIRTIO_RXQ,
+						pkts, rx_count);
 	}
 
+	return enqueue_count;
+}
+
+static __rte_always_inline void
+drain_vhost(struct vhost_dev *vdev)
+{
+	uint16_t ret;
+	uint32_t buff_idx = rte_lcore_id() * MAX_VHOST_DEVICE + vdev->vid;
+	uint16_t nr_xmit = vhost_txbuff[buff_idx]->len;
+	struct rte_mbuf **m = vhost_txbuff[buff_idx]->m_table;
+
+	ret = enqueue_pkts(vdev, m, nr_xmit);
+
 	if (enable_stats) {
 		__atomic_add_fetch(&vdev->stats.rx_total_atomic, nr_xmit,
 				__ATOMIC_SEQ_CST);
@@ -1217,44 +1229,19 @@ drain_eth_rx(struct vhost_dev *vdev)
 	if (!rx_count)
 		return;
 
-	/*
-	 * When "enable_retry" is set, here we wait and retry when there
-	 * is no enough free slots in the queue to hold @rx_count packets,
-	 * to diminish packet loss.
-	 */
-	if (enable_retry &&
-	    unlikely(rx_count > rte_vhost_avail_entries(vdev->vid,
-			VIRTIO_RXQ))) {
-		uint32_t retry;
+	enqueue_count = enqueue_pkts(vdev, pkts, rx_count);
 
-		for (retry = 0; retry < burst_rx_retry_num; retry++) {
+	/* Retry if necessary */
+	if (enable_retry && unlikely(enqueue_count < rx_count)) {
+		uint32_t retry = 0;
+
+		while (enqueue_count < rx_count && retry++ < burst_rx_retry_num) {
 			rte_delay_us(burst_rx_delay_time);
-			if (rx_count <= rte_vhost_avail_entries(vdev->vid,
-					VIRTIO_RXQ))
-				break;
+			enqueue_count += enqueue_pkts(vdev, &pkts[enqueue_count],
+						rx_count - enqueue_count);
 		}
 	}
 
-	if (builtin_net_driver) {
-		enqueue_count = vs_enqueue_pkts(vdev, VIRTIO_RXQ,
-						pkts, rx_count);
-	} else if (async_vhost_driver) {
-		uint16_t enqueue_fail = 0;
-
-		complete_async_pkts(vdev);
-		enqueue_count = rte_vhost_submit_enqueue_burst(vdev->vid,
-					VIRTIO_RXQ, pkts, rx_count);
-		__atomic_add_fetch(&vdev->pkts_inflight, enqueue_count, __ATOMIC_SEQ_CST);
-
-		enqueue_fail = rx_count - enqueue_count;
-		if (enqueue_fail)
-			free_pkts(&pkts[enqueue_count], enqueue_fail);
-
-	} else {
-		enqueue_count = rte_vhost_enqueue_burst(vdev->vid, VIRTIO_RXQ,
-						pkts, rx_count);
-	}
-
 	if (enable_stats) {
 		__atomic_add_fetch(&vdev->stats.rx_total_atomic, rx_count,
 				__ATOMIC_SEQ_CST);
-- 
2.25.1