[PATCH 08/12] net/pcap: optimize calculation of receive timestamp

Stephen Hemminger
2026-01-06T18:34:05Z

Avoid doing slow instructions in receive path when calculating
timestamp. Give all packets in the same rx burst the same timestamp.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
drivers/net/pcap/pcap_ethdev.c | 23 +++++++++++++----------
1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/drivers/net/pcap/pcap_ethdev.c b/drivers/net/pcap/pcap_ethdev.c
index 175d6998f9..e283fb3787 100644
--- a/drivers/net/pcap/pcap_ethdev.c
+++ b/drivers/net/pcap/pcap_ethdev.c
@@ -20,6 +20,7 @@
#include <bus_vdev_driver.h>
#include <rte_os_shim.h>
#include <rte_time.h>
+#include <rte_reciprocal.h>

#include "pcap_osdep.h"

@@ -41,7 +42,7 @@

static struct timespec start_time;
static uint64_t start_cycles;
-static uint64_t hz;
+static struct rte_reciprocal_u64 hz_inv;

static uint64_t timestamp_rx_dynflag;
static int timestamp_dynfield_offset = -1;
@@ -362,8 +363,6 @@ eth_null_rx(void *queue __rte_unused,
return 0;
}

-#define NSEC_PER_SEC 1000000000L
-
/*
* This function stores nanoseconds in `tv_usec` field of `struct timeval`,
* because `ts` goes directly to nanosecond-precision dump.
@@ -374,8 +373,10 @@ calculate_timestamp(struct timeval *ts) {
struct timespec cur_time;

cycles = rte_get_timer_cycles() - start_cycles;
- cur_time.tv_sec = cycles / hz;
- cur_time.tv_nsec = (cycles % hz) * NSEC_PER_SEC / hz;
+ cur_time.tv_sec = rte_reciprocal_divide_u64(cycles, &hz_inv);
+ /* compute remainder */
+ cycles -= cur_time.tv_sec * rte_get_timer_hz();
+ cur_time.tv_nsec = rte_reciprocal_divide_u64(cycles * NS_PER_S, &hz_inv);

ts->tv_sec = start_time.tv_sec + cur_time.tv_sec;
ts->tv_usec = start_time.tv_nsec + cur_time.tv_nsec;
@@ -394,6 +395,7 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
unsigned int i;
struct pmd_process_private *pp;
struct pcap_tx_queue *dumper_q = queue;
+ struct pcap_pkthdr header;
uint16_t num_tx = 0;
uint32_t tx_bytes = 0;
pcap_dumper_t *dumper;
@@ -406,13 +408,14 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
if (unlikely(dumper == NULL || nb_pkts == 0))
return 0;

- /* writes the nb_pkts packets to the previously opened pcap file
- * dumper */
+ /* all packets in burst have same timestamp */
+ calculate_timestamp(&header.ts);
+
+ /* writes the nb_pkts packets to the previously opened pcap file dumper */
for (i = 0; i < nb_pkts; i++) {
struct rte_mbuf *mbuf = bufs[i];
size_t len = rte_pktmbuf_pkt_len(mbuf);
uint8_t temp_data[RTE_ETH_PCAP_SNAPLEN];
- struct pcap_pkthdr header;

if (unlikely(len > mtu))
continue;
@@ -420,7 +423,6 @@ eth_pcap_tx_dumper(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
if ((mbuf->ol_flags & RTE_MBUF_F_TX_VLAN) && rte_vlan_insert(&mbuf))
continue;

- calculate_timestamp(&header.ts);
header.len = len;
header.caplen = len;

@@ -1530,7 +1532,8 @@ pmd_pcap_probe(struct rte_vdev_device *dev)

timespec_get(&start_time, TIME_UTC);
start_cycles = rte_get_timer_cycles();
- hz = rte_get_timer_hz();
+
+ hz_inv = rte_reciprocal_value_u64(rte_get_timer_hz());

if (rte_eal_process_type() == RTE_PROC_SECONDARY) {
eth_dev = rte_eth_dev_attach_secondary(name);
--
2.51.0