Subject: Re: [TEST] txtimestamp.sh pains after netdev foundation migration

From: Jakub Kicinski <[email-masked]>
Date: Thu, 8 Jan 2026 08:06:46 -0800

On Wed, 7 Jan 2026 19:25:11 -0800 Jakub Kicinski wrote:
> On Wed, 07 Jan 2026 19:19:53 -0500 Willem de Bruijn wrote:
> > 17 out of 20 happen in the first SND-USR calculation.
> > One representative example:
> > 
> >     # 7.11 [+0.00] test SND
> >     # 7.11 [+0.00]     USR: 1767443466 s 155019 us (seq=0, len=0)
> >     # 7.19 [+0.08] ERROR: 18600 us expected between 10000 and 18000
> >     # 7.19 [+0.00]     SND: 1767443466 s 173619 us (seq=0, len=10)  (USR +18599 us)
> >     # 7.20 [+0.00]     USR: 1767443466 s 243683 us (seq=0, len=0)
> >     # 7.27 [+0.07]     SND: 1767443466 s 253690 us (seq=1, len=10)  (USR +10006 us)
> >     # 7.27 [+0.00]     USR: 1767443466 s 323746 us (seq=0, len=0)
> >     # 7.35 [+0.08]     SND: 1767443466 s 333752 us (seq=2, len=10)  (USR +10006 us)
> >     # 7.35 [+0.00]     USR: 1767443466 s 403811 us (seq=0, len=0)
> >     # 7.43 [+0.08]     SND: 1767443466 s 413817 us (seq=3, len=10)  (USR +10006 us)
> >     # 7.43 [+0.00]     USR-SND: count=4, avg=12154 us, min=10006 us, max=18599 us  
> 
> Hm, that's the first kernel timestamp vs the timestamp in user space?
> I wonder if we could catch this by re-taking the user stamp after
> sendmsg() returns, if >1msec elapsed something is probably wrong 
> (we got scheduled out before having a chance to complete the send?)

How about:

diff --git a/tools/testing/selftests/net/txtimestamp.c b/tools/testing/selftests/net/txtimestamp.c
index 4b4bbc2ce5c9..abcec47ec2e6 100644
--- a/tools/testing/selftests/net/txtimestamp.c
+++ b/tools/testing/selftests/net/txtimestamp.c
@@ -215,6 +215,24 @@ static void print_timestamp_usr(void)
 	__print_timestamp("  USR", &ts_usr, 0, 0);
 }
 
+static void check_timestamp_usr(void)
+{
+	long long unsigned ts_delta_usec;
+	struct timespec now;
+
+	if (clock_gettime(CLOCK_REALTIME, &now))
+		error(1, errno, "clock_gettime");
+
+	ts_delta_usec = timespec_to_ns64(&now) - timespec_to_ns64(&ts_usr);
+	ts_delta_usec /= 1000;
+	if (ts_delta_usec > cfg_delay_tolerance_usec / 2) {
+		cfg_delay_tolerance_usec =
+			ts_delta_usec + cfg_delay_tolerance_usec / 2;
+		fprintf(stderr, "WARN: sendmsg() took %llu us, increasing delay tolerance to %d us\n",
+			ts_delta_usec, cfg_delay_tolerance_usec);
+	}
+}
+
 static void print_timestamp(struct scm_timestamping *tss, int tstype,
 			    int tskey, int payload_len)
 {
@@ -678,6 +696,8 @@ static void do_test(int family, unsigned int report_opt)
 		if (val != total_len)
 			error(1, errno, "send");
 
+		check_timestamp_usr();
+
 		/* wait for all errors to be queued, else ACKs arrive OOO */
 		if (cfg_sleep_usec)
 			usleep(cfg_sleep_usec);
-- 
2.52.0