[PATCH clocksource-wdtest] Print time values for short udelay(1)

[Paul E. McKenney]

When a pair of clocksource reads separated by a udelay(1) claim less
than a full microsecond of elapsed time, print the measured delay as
part of the splat.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: John Stultz <jstultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>

diff --git a/kernel/time/clocksource-wdtest.c b/kernel/time/clocksource-wdtest.c
index 62e73444ffe45..38dae590b29f5 100644
--- a/kernel/time/clocksource-wdtest.c
+++ b/kernel/time/clocksource-wdtest.c
@@ -137,7 +137,8 @@ static int wdtest_func(void *arg)
 	udelay(1);
 	j2 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
 	pr_info("--- tsc-like times: %lu - %lu = %lu.\n", j2, j1, j2 - j1);
-	WARN_ON_ONCE(time_before(j2, j1 + NSEC_PER_USEC));
+	WARN_ONCE(time_before(j2, j1 + NSEC_PER_USEC),
+		  "Expected at least 1000ns, got %lu.\n", j2 - j1);
 
 	/* Verify tsc-like stability with various numbers of errors injected. */
 	max_retries = clocksource_get_max_watchdog_retry();