Messages in this thread Patch in this message | | | From | Waiman Long <> | Subject | [PATCH v3] clocksource: Warn if too many missing ticks are detected | Date | Mon, 24 Sep 2018 11:36:15 -0400 |
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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
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