[PATCH clocksource 2/3] clocksource: Add comments to classify bogus measurements

[Paul E. McKenney]

An extremely busy system can delay the clocksource watchdog, so that
the corresponding too-long bogus-measurement error does not necessarily
imply an error in the system.  However, a too-short bogus-measurement
error likely indicates a bug in hardware, firmware or software.

Therefore, add comments clarifying these bogus-measurement pr_warn()s.

Reported-by: Feng Tang <feng.tang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
Reviewed-by: Feng Tang <feng.tang@xxxxxxxxx>
Cc: John Stultz <jstultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
---
 kernel/time/clocksource.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index dcaf38c062161..3f5317faf891f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -443,10 +443,12 @@ static void clocksource_watchdog(struct timer_list *unused)
 		/* Check for bogus measurements. */
 		wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL);
 		if (wd_nsec < (wdi >> 2)) {
+			/* This usually indicates broken timer code or hardware. */
 			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
 			continue;
 		}
 		if (wd_nsec > (wdi << 2)) {
+			/* This can happen on busy systems, which can delay the watchdog. */
 			pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL);
 			continue;
 		}
-- 
2.31.1.189.g2e36527f23