[PATCH v2 2/4] net/af_packet: RX/TX unlikely, bulk free, prefetch

[scott.k.mitch1 at gmail.com]

From: Scott Mitchell <scott.k.mitch1 at gmail.com>

- Add rte_prefetch0() to prefetch next frame/mbuf while processing
  current packet, reducing cache miss latency
- Use rte_pktmbuf_free_bulk() in TX path instead of individual
  rte_pktmbuf_free() calls for better batch efficiency
- Add unlikely() hints for error paths (oversized packets, VLAN
  insertion failures, sendto errors) to optimize branch prediction
- Remove unnecessary early nb_pkts == 0 when loop handles this
  and app may never call with 0 frames.

Signed-off-by: Scott Mitchell <scott.k.mitch1 at gmail.com>
---
 drivers/net/af_packet/rte_eth_af_packet.c | 65 ++++++++++++-----------
 1 file changed, 34 insertions(+), 31 deletions(-)

diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
index 6c276bb7fc..e357ae168b 100644
--- a/drivers/net/af_packet/rte_eth_af_packet.c
+++ b/drivers/net/af_packet/rte_eth_af_packet.c
@@ -161,9 +161,6 @@ eth_af_packet_rx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 	uint32_t tp_status;
 	unsigned int framecount, framenum;
 
-	if (unlikely(nb_pkts == 0))
-		return 0;
-
 	/*
 	 * Reads the given number of packets from the AF_PACKET socket one by
 	 * one and copies the packet data into a newly allocated mbuf.
@@ -177,6 +174,14 @@ eth_af_packet_rx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 		if ((tp_status & TP_STATUS_USER) == 0)
 			break;
 
+		unsigned int next_framenum = framenum + 1;
+		if (next_framenum >= framecount)
+			next_framenum = 0;
+
+		/* prefetch the next frame for the next loop iteration */
+		if (likely(i + 1 < nb_pkts))
+			rte_prefetch0(pkt_q->rd[next_framenum].iov_base);
+
 		/* allocate the next mbuf */
 		mbuf = rte_pktmbuf_alloc(pkt_q->mb_pool);
 		if (unlikely(mbuf == NULL)) {
@@ -210,8 +215,7 @@ eth_af_packet_rx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 
 		/* release incoming frame and advance ring buffer */
 		tpacket_write_status(&ppd->tp_status, TP_STATUS_KERNEL);
-		if (++framenum >= framecount)
-			framenum = 0;
+		framenum = next_framenum;
 		mbuf->port = pkt_q->in_port;
 
 		/* account for the receive frame */
@@ -261,9 +265,6 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 	uint32_t num_tx_bytes = 0;
 	uint16_t i;
 
-	if (unlikely(nb_pkts == 0))
-		return 0;
-
 	memset(&pfd, 0, sizeof(pfd));
 	pfd.fd = pkt_q->sockfd;
 	pfd.events = POLLOUT;
@@ -271,22 +272,25 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 
 	framecount = pkt_q->framecount;
 	framenum = pkt_q->framenum;
-	ppd = (struct tpacket2_hdr *) pkt_q->rd[framenum].iov_base;
 	for (i = 0; i < nb_pkts; i++) {
-		mbuf = *bufs++;
-
-		/* drop oversized packets */
-		if (mbuf->pkt_len > pkt_q->frame_data_size) {
-			rte_pktmbuf_free(mbuf);
-			continue;
+		unsigned int next_framenum = framenum + 1;
+		if (next_framenum >= framecount)
+			next_framenum = 0;
+
+		/* prefetch the next source mbuf and destination TPACKET */
+		if (likely(i + 1 < nb_pkts)) {
+			rte_prefetch0(bufs[i + 1]);
+			rte_prefetch0(pkt_q->rd[next_framenum].iov_base);
 		}
 
-		/* insert vlan info if necessary */
-		if (mbuf->ol_flags & RTE_MBUF_F_TX_VLAN) {
-			if (rte_vlan_insert(&mbuf)) {
-				rte_pktmbuf_free(mbuf);
-				continue;
-			}
+		mbuf = bufs[i];
+		ppd = (struct tpacket2_hdr *)pkt_q->rd[framenum].iov_base;
+
+		/* Drop oversized packets. Insert VLAN if necessary */
+		if (unlikely(mbuf->pkt_len > pkt_q->frame_data_size ||
+			    ((mbuf->ol_flags & RTE_MBUF_F_TX_VLAN) != 0 &&
+			     rte_vlan_insert(&mbuf) != 0))) {
+			continue;
 		}
 
 		/*
@@ -312,6 +316,9 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 
 		pbuf = (uint8_t *)ppd + ETH_AF_PACKET_FRAME_OVERHEAD;
 
+		ppd->tp_len = mbuf->pkt_len;
+		ppd->tp_snaplen = mbuf->pkt_len;
+
 		struct rte_mbuf *tmp_mbuf = mbuf;
 		do {
 			uint16_t data_len = rte_pktmbuf_data_len(tmp_mbuf);
@@ -320,23 +327,19 @@ eth_af_packet_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 			tmp_mbuf = tmp_mbuf->next;
 		} while (tmp_mbuf);
 
-		ppd->tp_len = mbuf->pkt_len;
-		ppd->tp_snaplen = mbuf->pkt_len;
-
 		/* release incoming frame and advance ring buffer */
 		tpacket_write_status(&ppd->tp_status, TP_STATUS_SEND_REQUEST);
-		if (++framenum >= framecount)
-			framenum = 0;
-		ppd = (struct tpacket2_hdr *) pkt_q->rd[framenum].iov_base;
-
+		framenum = next_framenum;
 		num_tx++;
 		num_tx_bytes += mbuf->pkt_len;
-		rte_pktmbuf_free(mbuf);
 	}
 
+	rte_pktmbuf_free_bulk(&bufs[0], i);
+
 	/* kick-off transmits */
-	if (sendto(pkt_q->sockfd, NULL, 0, MSG_DONTWAIT, NULL, 0) == -1 &&
-			errno != ENOBUFS && errno != EAGAIN) {
+	if (unlikely(num_tx > 0 &&
+		     sendto(pkt_q->sockfd, NULL, 0, MSG_DONTWAIT, NULL, 0) == -1 &&
+		     errno != ENOBUFS && errno != EAGAIN)) {
 		/*
 		 * In case of a ENOBUFS/EAGAIN error all of the enqueued
 		 * packets will be considered successful even though only some
-- 
2.39.5 (Apple Git-154)