[PATCH 0/2] Latencystat optimization and fix

[PATCH 0/2] Latencystat optimization and fix

[Stephen Hemminger]

After investigating latencystat test failures, discovered it poorly
designed to handle bursts and multiple queues.

Stephen Hemminger (2):
  latencystats: fix receive sample MP issues
  latencystats: optimize locking on transmit

 lib/latencystats/rte_latencystats.c | 64 +++++++++++++++++++----------
 1 file changed, 42 insertions(+), 22 deletions(-)

-- 
2.47.2

[PATCH 1/2] latencystats: fix receive sample MP issues

[Stephen Hemminger]

The receive callback was not safe with multiple queues.
If one receive queue callback decides to take a sample it
needs to add that sample and do atomic update to the previous
TSC sample value. Add a new lock for that.

Also, add code to handle TSC wraparound in comparison.
Perhaps this should move to rte_cycles.h?

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Fixes: 5cd3cac9ed22 ("latency: added new library for latency stats")
Cc: stable@dpdk.org
---
 lib/latencystats/rte_latencystats.c | 47 ++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff --git a/lib/latencystats/rte_latencystats.c b/lib/latencystats/rte_latencystats.c
index 6873a44a92..181c53dd0e 100644
--- a/lib/latencystats/rte_latencystats.c
+++ b/lib/latencystats/rte_latencystats.c
@@ -45,10 +45,19 @@ timestamp_dynfield(struct rte_mbuf *mbuf)
 			timestamp_dynfield_offset, rte_mbuf_timestamp_t *);
 }
 
+/* Compare two 64 bit timer counter but deal with wraparound correctly. */
+static inline bool tsc_after(uint64_t t0, uint64_t t1)
+{
+	return (int64_t)(t1 - t0) < 0;
+}
+
+#define tsc_before(a, b) tsc_after(b, a)
+
 static const char *MZ_RTE_LATENCY_STATS = "rte_latencystats";
 static int latency_stats_index;
+
+static rte_spinlock_t sample_lock = RTE_SPINLOCK_INITIALIZER;
 static uint64_t samp_intvl;
-static uint64_t timer_tsc;
 static uint64_t prev_tsc;
 
 #define LATENCY_AVG_SCALE     4
@@ -147,25 +156,27 @@ add_time_stamps(uint16_t pid __rte_unused,
 		void *user_cb __rte_unused)
 {
 	unsigned int i;
-	uint64_t diff_tsc, now;
+	uint64_t now = rte_rdtsc();
 
-	/*
-	 * For every sample interval,
-	 * time stamp is marked on one received packet.
-	 */
-	now = rte_rdtsc();
-	for (i = 0; i < nb_pkts; i++) {
-		diff_tsc = now - prev_tsc;
-		timer_tsc += diff_tsc;
-
-		if ((pkts[i]->ol_flags & timestamp_dynflag) == 0
-				&& (timer_tsc >= samp_intvl)) {
-			*timestamp_dynfield(pkts[i]) = now;
-			pkts[i]->ol_flags |= timestamp_dynflag;
-			timer_tsc = 0;
+	/* Check without locking */
+	if (likely(tsc_before(now, prev_tsc + samp_intvl)))
+		return nb_pkts;
+
+	/* Try and get sample, skip if sample is being done by other core. */
+	if (likely(rte_spinlock_trylock(&sample_lock))) {
+		for (i = 0; i < nb_pkts; i++) {
+			struct rte_mbuf *m = pkts[i];
+
+			/* skip if already timestamped */
+			if (unlikely(m->ol_flags & timestamp_dynflag))
+				continue;
+
+			m->ol_flags |= timestamp_dynflag;
+			*timestamp_dynfield(m) = now;
+			prev_tsc = now;
+			break;
 		}
-		prev_tsc = now;
-		now = rte_rdtsc();
+		rte_spinlock_unlock(&sample_lock);
 	}
 
 	return nb_pkts;
-- 
2.47.2

[PATCH 2/2] latencystats: optimize locking on transmit

[Stephen Hemminger]

If transmit callback is called, and there are no packets
in the burst with timestamp set, then the expensive operations
of locking and accessing TSC can be skipped.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/latencystats/rte_latencystats.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/lib/latencystats/rte_latencystats.c b/lib/latencystats/rte_latencystats.c
index 181c53dd0e..dc415c4682 100644
--- a/lib/latencystats/rte_latencystats.c
+++ b/lib/latencystats/rte_latencystats.c
@@ -190,17 +190,26 @@ calc_latency(uint16_t pid __rte_unused,
 		void *_ __rte_unused)
 {
 	unsigned int i;
-	uint64_t now, latency;
+	uint64_t ts_flags = 0;
 	static uint64_t prev_latency;
 
-	now = rte_rdtsc();
+	for (i = 0; i < nb_pkts; i++)
+		ts_flags |= (pkts[i]->ol_flags & timestamp_dynflag);
 
+	/* no samples in this burst, skip locking */
+	if (likely(ts_flags == 0))
+		return nb_pkts;
+
+	uint64_t now = rte_rdtsc();
 	rte_spinlock_lock(&glob_stats->lock);
 	for (i = 0; i < nb_pkts; i++) {
-		if (!(pkts[i]->ol_flags & timestamp_dynflag))
+		struct rte_mbuf *m = pkts[i];
+		uint64_t latency;
+
+		if (!(m->ol_flags & timestamp_dynflag))
 			continue;
 
-		latency = now - *timestamp_dynfield(pkts[i]);
+		latency = now - *timestamp_dynfield(m);
 
 		if (glob_stats->samples++ == 0) {
 			glob_stats->min_latency = latency;
-- 
2.47.2

RE: [PATCH 0/2] Latencystat optimization and fix

[Varghese, Vipin]

[Public]

> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Friday, June 13, 2025 6:04 AM
> To: dev@dpdk.org
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Subject: [PATCH 0/2] Latencystat optimization and fix
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> After investigating latencystat test failures, discovered it poorly designed to handle
> bursts and multiple queues.

Acked-by: Vipin.Varghese@amd.com
Reviewed-by: Vipin.Varghese@amd.com
Tested-by: Thiyagarajan.P@amd.com

We tested the changes using DPDK-TestPmd, following is the summary of the test shared below

1. before vs after patch with 1Q: min: -10ns, max: -5000ns, avg: -10ns, jitter: +5ns
2. before vs after patch with 4Q: min: -10ns, max: -7000ns, avg: +5ns, jitter: +80ns

Since we see a greater variation with multiple queues compared to single queue, we also collect HW Traffic Generator before vs after.
Values we got are ` min: -50000ns, max: -62000ns, avg: -5300ns, jitter: 6ns `.

NIC: Intel E810 2CQ-DA2 (used 1 100Gbps port), firmware: 4.7, DDP: default
Processor: AMD EPYC 8534P 64-Core Processor
Traffic Rate: 100Gbps

Application CMD:
1. DPDK: ` sudo build/app/dpdk-testpmd -l 11,16,17,18,19 -a 42:00.0 --file-prefix tr01 --proc-type=primary --force-max-simd-bitwidth=512 -- --nb-cores=4 --nb-ports=1 --txq=4 --rxq=4 --txd=2048 --rxd=2048 --burst=64 --rss-udp --latencystats=16  -i -a `
2. Proc-Info: ` sudo build/app/dpdk-proc-info --file-prefix=tr01 --proc-type=secondary -a 42:00.0 -- --metrics `

>
> Stephen Hemminger (2):
>   latencystats: fix receive sample MP issues
>   latencystats: optimize locking on transmit
>
>  lib/latencystats/rte_latencystats.c | 64 +++++++++++++++++++----------
>  1 file changed, 42 insertions(+), 22 deletions(-)
>
> --
> 2.47.2