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

[Stephen Hemminger <stephen@networkplumber.org>]

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