Messages in this thread Patch in this message | | | From | "Paul E. McKenney" <> | Subject | [PATCH clocksource 1/3] clocksource: Reject bogus watchdog clocksource measurements | Date | Mon, 14 Nov 2022 15:28:24 -0800 |
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One remaining clocksource-skew issue involves extreme CPU overcommit, which can cause the clocksource watchdog measurements to be delayed by tens of seconds. This in turn means that a clock-skew criterion that is appropriate for a 500-millisecond interval will instead give lots of false positives.
Therefore, check for the watchdog clocksource reporting much larger or much less than the time specified by WATCHDOG_INTERVAL. In these cases, print a pr_warn() warning and refrain from marking the clocksource under test as being unstable.
Reported-by: Chris Mason <clm@meta.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Cc: John Stultz <jstultz@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Stephen Boyd <sboyd@kernel.org> Cc: Feng Tang <feng.tang@intel.com> Cc: Waiman Long <longman@redhat.com> --- kernel/time/clocksource.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 8058bec87acee..dcaf38c062161 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -386,7 +386,7 @@ EXPORT_SYMBOL_GPL(clocksource_verify_percpu); static void clocksource_watchdog(struct timer_list *unused) { - u64 csnow, wdnow, cslast, wdlast, delta; + u64 csnow, wdnow, cslast, wdlast, delta, wdi; int next_cpu, reset_pending; int64_t wd_nsec, cs_nsec; struct clocksource *cs; @@ -440,6 +440,17 @@ static void clocksource_watchdog(struct timer_list *unused) if (atomic_read(&watchdog_reset_pending)) continue; + /* Check for bogus measurements. */ + wdi = jiffies_to_nsecs(WATCHDOG_INTERVAL); + if (wd_nsec < (wdi >> 2)) { + pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced only %lld ns during %d-jiffy time interval, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL); + continue; + } + if (wd_nsec > (wdi << 2)) { + pr_warn("timekeeping watchdog on CPU%d: Watchdog clocksource '%s' advanced an excessive %lld ns during %d-jiffy time interval, probable CPU overutilization, skipping watchdog check.\n", smp_processor_id(), watchdog->name, wd_nsec, WATCHDOG_INTERVAL); + continue; + } + /* Check the deviation from the watchdog clocksource. */ md = cs->uncertainty_margin + watchdog->uncertainty_margin; if (abs(cs_nsec - wd_nsec) > md) { -- 2.31.1.189.g2e36527f23
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