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Message-Id: <1537803375-31667-1-git-send-email-longman@redhat.com>
Date: Mon, 24 Sep 2018 11:36:15 -0400
From: Waiman Long <longman@...hat.com>
To: John Stultz <john.stultz@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Stephen Boyd <sboyd@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Waiman Long <longman@...hat.com>
Subject: [PATCH v3] clocksource: Warn if too many missing ticks are detected
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.
v3: Do the check only when SYSTEM_RUNNING & print the delay as well.
Signed-off-by: Waiman Long <longman@...hat.com>
---
kernel/time/clocksource.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 0e6e97a..e5d2e38 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -208,11 +208,40 @@ static void clocksource_watchdog(struct timer_list *unused)
u64 csnow, wdnow, cslast, wdlast, delta;
int64_t wd_nsec, cs_nsec;
int next_cpu, reset_pending;
+ static bool prev_running; /* Set if previously in SYSTEM_RUNNING */
spin_lock(&watchdog_lock);
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.
+ *
+ * This check is performed only when the system is previously
+ * running in the SYSTEM_RUNNING state as large delay may happen
+ * when running in other states, especially when self-tests are
+ * being run. If the watchdog was previously in the running state,
+ * that will make sure that the current timer expiry happened in
+ * that state too.
+ */
+ if (prev_running) {
+ unsigned long delay = jiffies - watchdog_timer.expires;
+
+ if (delay > WATCHDOG_INTERVAL) {
+ pr_warn("watchdog delayed by %ld ticks!\n", delay);
+ WARN_ON_ONCE(1);
+ }
+ } else if (system_state == SYSTEM_RUNNING) {
+ prev_running = true;
+ }
+
reset_pending = atomic_read(&watchdog_reset_pending);
list_for_each_entry(cs, &watchdog_list, wd_list) {
--
1.8.3.1
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