From: Waiman Long <longman@redhat.com>
To: John Stultz <john.stultz@linaro.org>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Stephen Boyd <sboyd@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Waiman Long <longman@redhat.com>
Subject: [PATCH v2] clocksource: Warn if too many missing ticks are detected
Date: Tue, 18 Sep 2018 21:19:39 -0400 [thread overview]
Message-ID: <1537319979-6139-1-git-send-email-longman@redhat.com> (raw)
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@redhat.com>
---
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
next reply other threads:[~2018-09-19 1:19 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-19 1:19 Waiman Long [this message]
2018-09-19 7:53 ` [PATCH v2] clocksource: Warn if too many missing ticks are detected Thomas Gleixner
2018-09-19 9:17 ` Peter Zijlstra
2018-09-19 16:32 ` Waiman Long
2018-09-24 15:35 ` Waiman Long
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