[PATCH net-next] selftests: net: reduce txtimestamp deschedule flakes

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

This test occasionally fails due to exceeding timing bounds, as
run in continuous testing on netdev.bots:

  https://netdev.bots.linux.dev/contest.html?test=txtimestamp-sh

A common pattern is a single elevated delay between USR and SND.

    # 8.36 [+0.00] test SND
    # 8.36 [+0.00]     USR: 1767864384 s 240994 us (seq=0, len=0)
    # 8.44 [+0.08] ERROR: 18461 us expected between 10000 and 18000
    # 8.44 [+0.00]     SND: 1767864384 s 259455 us (seq=42, len=10)  (USR +18460 us)
    # 8.52 [+0.07]     SND: 1767864384 s 339523 us (seq=42, len=10)  (USR +10005 us)
    # 8.52 [+0.00]     USR: 1767864384 s 409580 us (seq=0, len=0)
    # 8.60 [+0.08]     SND: 1767864384 s 419586 us (seq=42, len=10)  (USR +10005 us)
    # 8.60 [+0.00]     USR: 1767864384 s 489645 us (seq=0, len=0)
    # 8.68 [+0.08]     SND: 1767864384 s 499651 us (seq=42, len=10)  (USR +10005 us)
    # 8.68 [+0.00]     USR-SND: count=4, avg=12119 us, min=10005 us, max=18460 us

(Note that other delays are nowhere near the large 8ms tolerance.)

One hypothesis is that the task is descheduled between taking the USR
timestamp and sending the packet. Possibly in printing.

Delay taking the timestamp closer to sendmsg, and delay printing until
after sendmsg.

With this change, failure rate is significantly lower in current runs.

Link: https://lore.kernel.org/netdev/20260107110521.1aab55e9@kernel.org/
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 tools/testing/selftests/net/txtimestamp.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/net/txtimestamp.c b/tools/testing/selftests/net/txtimestamp.c
index bcc14688661d..170be192f5c7 100644
--- a/tools/testing/selftests/net/txtimestamp.c
+++ b/tools/testing/selftests/net/txtimestamp.c
@@ -206,12 +206,10 @@ static void __print_timestamp(const char *name, struct timespec *cur,
 	fprintf(stderr, "\n");
 }
 
-static void print_timestamp_usr(void)
+static void record_timestamp_usr(void)
 {
 	if (clock_gettime(CLOCK_REALTIME, &ts_usr))
 		error(1, errno, "clock_gettime");
-
-	__print_timestamp("  USR", &ts_usr, 0, 0);
 }
 
 static void print_timestamp(struct scm_timestamping *tss, int tstype,
@@ -599,8 +597,6 @@ static void do_test(int family, unsigned int report_opt)
 			fill_header_udp(buf + off, family == PF_INET);
 		}
 
-		print_timestamp_usr();
-
 		iov.iov_base = buf;
 		iov.iov_len = total_len;
 
@@ -655,10 +651,14 @@ static void do_test(int family, unsigned int report_opt)
 
 		}
 
+		record_timestamp_usr();
+
 		val = sendmsg(fd, &msg, 0);
 		if (val != total_len)
 			error(1, errno, "send");
 
+		__print_timestamp("  USR", &ts_usr, 0, 0);
+
 		/* wait for all errors to be queued, else ACKs arrive OOO */
 		if (cfg_sleep_usec)
 			usleep(cfg_sleep_usec);
-- 
2.52.0.457.g6b5491de43-goog