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

From: Willem de Bruijn <[email-masked]>
Date: Sun, 11 Jan 2026 22:24:17 -0500

Willem de Bruijn wrote:
> Jakub Kicinski wrote:
> > On Thu, 08 Jan 2026 14:02:15 -0500 Willem de Bruijn wrote:
> > > Increasing tolerance should work.
> > > 
> > > The current values are pragmatic choices to be so low as to minimize
> > > total test runtime, but high enough to avoid flakes. Well..
> > > 
> > > If increasing tolerance, we also need to increase the time the test
> > > waits for all notifications to arrive, cfg_sleep_usec.
> > 
> > To be clear the theory is that we got scheduled out between taking the
> > USR timestamp and sending the packet. But once the packet is in the
> > kernel it seems to flow, so AFAIU cfg_sleep_usec can remain untouched.
> > 
> > Thinking about it more - maybe what blocks us is the print? Maybe under
> > vng there's a non-trivial chance that a print to stderr ends up
> > blocking on serial and schedules us out? I mean maybe we should:
> > 
> > diff --git a/tools/testing/selftests/net/txtimestamp.c b/tools/testing/selftests/net/txtimestamp.c
> > index abcec47ec2e6..e2273fdff495 100644
> > --- a/tools/testing/selftests/net/txtimestamp.c
> > +++ b/tools/testing/selftests/net/txtimestamp.c
> > @@ -207,12 +207,10 @@ 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 check_timestamp_usr(void)
> > @@ -636,8 +634,6 @@ 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;
> >  
> > @@ -692,10 +688,14 @@ 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");
> >  
> > +               /* Avoid I/O between taking ts_usr and sendmsg() */
> > +               __print_timestamp("  USR", &ts_usr, 0, 0);
> > +
> >                 check_timestamp_usr();
> >  
> >                 /* wait for all errors to be queued, else ACKs arrive OOO */
> 
> Definitely worth including.
> 
> Could it be helpful to schedule at RR or FIFO prio. Depends on the
> reason for descheduling. And it only affects priority within the VM.
> 
> I'm having trouble reproducing it in vng both locally and on 
> netdev-virt.
> 
> At this point, an initial obviously correct patch and observe how
> much that mitigates the issue is likely the fastest way forward.

Instead of increasing tolerance, how about optionally allowing one
moderate timing error:

@@ -166,8 +167,15 @@ static void validate_timestamp(struct timespec *cur, int min_delay)
        if (cur64 < start64 + min_delay || cur64 > start64 + max_delay) {
                fprintf(stderr, "ERROR: %" PRId64 " us expected between %d and %d\n",
                                cur64 - start64, min_delay, max_delay);
-               if (!getenv("KSFT_MACHINE_SLOW"))
-                       test_failed = true;
+               if (!getenv("KSFT_MACHINE_SLOW")) {
+                       if (cfg_num_max_timing_failures &&
+                           (cur64 <= start64 + (max_delay * 2))) {
+                               cfg_num_max_timing_failures--;
+                               fprintf(stderr, "CONTINUE: ignore 1 timing failure\n");
+                       } else {
+                               test_failed = true;
+                       }
+               }
        }
 }

@@ -746,6 +755,10 @@ static void parse_opt(int argc, char **argv)
                case 'E':
                        cfg_use_epoll = true;
                        cfg_epollet = true;
+                       break;
+               case 'f':
+                       cfg_num_max_timing_failures = strtoul(optarg, NULL, 10);
+                       break;

+++ b/tools/testing/selftests/net/txtimestamp.sh
@@ -30,8 +30,8 @@ run_test_v4v6() {
        # wait for ACK to be queued
        local -r args="$@ -v 10000 -V 60000 -t 8000 -S 80000"
 
-       ./txtimestamp ${args} -4 -L 127.0.0.1
-       ./txtimestamp ${args} -6 -L ::1
+       ./txtimestamp ${args} -f 1 -4 -L 127.0.0.1
+       ./txtimestamp ${args} -f 1 -6 -L ::1
 }

and some boilerplate.

Can fold in the record_timestamp_usr() change too.

I can send this, your alternative with Suggested-by, or let me know if
you prefer to send that.

It's tricky to reproduce, but evidently on some platforms this occurs,
so not unreasonable to give some leeway. A single UDP test runs 12
timing validations: 4 packets * {SND, ENQ, END + SND} setups. A single
TCP test runs additional {ACK, SND + ACK, ENQ + SND + ACK} cases. If
we consider 1/12 skips too high, we could increase packet count. 

txtimestamp.sh runs 3 * 7 * 2 test variants. Alternatively we suppress
1 failure here, rather than in the individual tests.

Any of these approaches should significantly reduce the flake rate
reported on netdev.bots.