[PATCH] clocksource: Skip watchdog check for large watchdog intervals

[Jiri Wiesner]

There have been reports of the watchdog marking clocksources unstable on
machines with 8 NUMA nodes:
> clocksource: timekeeping watchdog on CPU373: Marking clocksource 'tsc' as unstable because the skew is too large:
> clocksource:   'hpet' wd_nsec: 14523447520 wd_now: 5a749706 wd_last: 45adf1e0 mask: ffffffff
> clocksource:   'tsc' cs_nsec: 14524115132 cs_now: 515ce2c5a96caa cs_last: 515cd9a9d83918 mask: ffffffffffffffff
> clocksource:   'tsc' is current clocksource.
> tsc: Marking TSC unstable due to clocksource watchdog
> TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.
> sched_clock: Marking unstable (1950347883333462, 79649632569)<-(1950428279338308, -745776594)
> clocksource: Checking clocksource tsc synchronization from CPU 400 to CPUs 0,46,52,54,138,208,392,397.
> clocksource: Switched to clocksource hpet

The measured clocksource skew - the absolute difference between cs_nsec
and wd_nsec - was 668 microseconds:
> cs_nsec - wd_nsec = 14524115132 - 14523447520 = 667612

The kernel (based on 5.14.21) used 200 microseconds for the
uncertainty_margin of both the clocksource and watchdog, resulting in a
threshold of 400 microseconds.  The discrepancy is that the measured
clocksource skew was evaluated against a threshold suited for watchdog
intervals of roughly WATCHDOG_INTERVAL, i.e. HZ >> 1, which is 0.5 second.
Both the cs_nsec and the wd_nsec value indicate that the actual watchdog
interval was circa 14.5 seconds. Since the watchdog is executed in softirq
context the expiration of the watchdog timer can get severely delayed on
account of a ksoftirqd thread not getting to run in a timely manner.
Surely, a system with such belated softirq execution is not working well
and the scheduling issue should be looked into but the clocksource
watchdog should be able to deal with it accordingly.

The solution in this patch skips the current watchdog check if the
watchdog interval exceeds 1.5 * WATCHDOG_INTERVAL. Considering the maximum
watchdog interval of 1.5 * WATCHDOG_INTERVAL, the current default
uncertainty margin (of the TSC and HPET) corresponds to a limit on
clocksource skew of 333 ppm (microseconds of skew per second). To keep the
limit imposed by NTP (500 microseconds of skew per second) for all
possible watchdog intervals, the margins would have to be scaled so that
the threshold value is proportional to the length of the actual watchdog
interval.

Fixes: 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold")
Suggested-by: Feng Tang <feng.tang@xxxxxxxxx>
Signed-off-by: Jiri Wiesner <jwiesner@xxxxxxx>
---
 kernel/time/clocksource.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index c108ed8a9804..ac5cb0ff278b 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -98,7 +98,9 @@ static u64 suspend_start;
 /*
  * Interval: 0.5sec.
  */
-#define WATCHDOG_INTERVAL (HZ >> 1)
+#define WATCHDOG_INTERVAL	(HZ >> 1)
+#define WATCHDOG_INTR_MAX_NS	((WATCHDOG_INTERVAL + (WATCHDOG_INTERVAL >> 1))\
+				 * NSEC_PER_SEC / HZ)
 
 /*
  * Threshold: 0.0312s, when doubled: 0.0625s.
@@ -134,6 +136,7 @@ static DECLARE_WORK(watchdog_work, clocksource_watchdog_work);
 static DEFINE_SPINLOCK(watchdog_lock);
 static int watchdog_running;
 static atomic_t watchdog_reset_pending;
+static int64_t watchdog_max_intr;
 
 static inline void clocksource_watchdog_lock(unsigned long *flags)
 {
@@ -400,7 +403,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 {
 	u64 csnow, wdnow, cslast, wdlast, delta;
 	int next_cpu, reset_pending;
-	int64_t wd_nsec, cs_nsec;
+	int64_t wd_nsec, cs_nsec, interval;
 	struct clocksource *cs;
 	enum wd_read_status read_ret;
 	unsigned long extra_wait = 0;
@@ -470,6 +473,27 @@ static void clocksource_watchdog(struct timer_list *unused)
 		if (atomic_read(&watchdog_reset_pending))
 			continue;
 
+		/*
+		 * The processing of timer softirqs can get delayed (usually
+		 * on account of ksoftirqd not getting to run in a timely
+		 * manner), which causes the watchdog interval to stretch.
+		 * Some clocksources, e.g. acpi_pm, cannot tolerate
+		 * watchdog intervals longer than a few seconds.
+		 * Skew detection may fail for longer watchdog intervals
+		 * on account of fixed margins being used.
+		 */
+		interval = max(cs_nsec, wd_nsec);
+		if (unlikely(interval > WATCHDOG_INTR_MAX_NS)) {
+			if (system_state > SYSTEM_SCHEDULING &&
+			    interval > 2 * watchdog_max_intr) {
+				watchdog_max_intr = interval;
+				pr_warn("Skipping watchdog check: cs_nsec: %lld wd_nsec: %lld\n",
+					cs_nsec, wd_nsec);
+			}
+			watchdog_timer.expires = jiffies;
+			continue;
+		}
+
 		/* Check the deviation from the watchdog clocksource. */
 		md = cs->uncertainty_margin + watchdog->uncertainty_margin;
 		if (abs(cs_nsec - wd_nsec) > md) {
-- 
2.35.3


-- 
Jiri Wiesner
SUSE Labs