[PATCH v2] clocksource: Warn if too many missing ticks are detected

[Waiman Long]

The clocksource watchdog, when running, is scheduled on all the CPUs in
the system sequentially on a round-robin fashion with a period of 0.5s.
A bug in the 4.18 kernel is causing missing ticks when nohz_full
is specified. Under some circumstances, this causes the watchdog to
incorrectly state that the TSC is unstable because of counter overflow
in the hpet watchdog clock source after a few minutes delay.

That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched:
idle: Avoid retaining the tick when it has been stopped"). To make it
easier to catch this kind of bug in the future, a check is added to see
if there is too much delay in the invocation of the watchdog callback
and print a warning once if it happens.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/time/clocksource.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 0e6e97a..64b05ab 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -213,6 +213,18 @@ static void clocksource_watchdog(struct timer_list *unused)
 	if (!watchdog_running)
 		goto out;
 
+	/*
+	 * When the timer tick is incorrectly stopped on a CPU with
+	 * pending events, for example, it is possible that the
+	 * clocksource watchdog will stop running for a sufficiently
+	 * long enough time to cause overflow in the delta
+	 * computation leading to incorrect report of unstable clock
+	 * source. So print a warning if there is unusually large
+	 * delay (> 0.5s) in the invocation of the watchdog. That
+	 * can indicate a hidden bug in the timer tick code.
+	 */
+	WARN_ON_ONCE(jiffies - watchdog_timer.expires > WATCHDOG_INTERVAL);
+
 	reset_pending = atomic_read(&watchdog_reset_pending);
 
 	list_for_each_entry(cs, &watchdog_list, wd_list) {
-- 
1.8.3.1