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

scott.k.mitch1
2026-01-28T09:36:28Z

From: Scott Mitchell <scott.k.mitch1@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@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)