lkml.org 
[lkml]   [2018]   [Sep]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH v3] clocksource: Warn if too many missing ticks are detected
Date
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@redhat.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
\
 
 \ /
  Last update: 2018-09-24 17:37    [W:0.094 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site