[PATCH] 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 watchdog invocation and print a
warning once if it happens.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/time/clocksource.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 0e6e97a..2ea5db0 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -140,6 +140,7 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags)
  * Interval: 0.5sec Threshold: 0.0625s
  */
 #define WATCHDOG_INTERVAL (HZ >> 1)
+#define WATCHDOG_INTERNVAL_NS (NSEC_PER_SEC >> 1)
 #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4)
 
 static void clocksource_watchdog_work(struct work_struct *work)
@@ -242,6 +243,18 @@ static void clocksource_watchdog(struct timer_list *unused)
 		wd_nsec = clocksource_cyc2ns(delta, watchdog->mult,
 					     watchdog->shift);
 
+		/*
+		 * 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(!wd_nsec || wd_nsec > 2*WATCHDOG_INTERNVAL_NS);
+
 		delta = clocksource_delta(csnow, cs->cs_last, cs->mask);
 		cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);
 		wdlast = cs->wd_last; /* save these in case we print them */
-- 
1.8.3.1