The following commit has been merged into the timers/urgent branch of tip: Commit-ID: 644649553508b9bacf0fc7a5bdc4f9e0165576a5 Gitweb: https://git.kernel.org/tip/644649553508b9bacf0fc7a5bdc4f9e0165576a5 Author: Jiri Wiesner <jwiesner@xxxxxxx> AuthorDate: Mon, 22 Jan 2024 18:23:50 +01:00 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitterDate: Thu, 25 Jan 2024 09:13:16 +01:00 clocksource: Skip watchdog check for large watchdog intervals 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 clocksource: 'tsc' cs_nsec: 14524115132 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 used 200 microseconds for the uncertainty_margin of both the clocksource and watchdog, resulting in a threshold of 400 microseconds (the md variable). Both the cs_nsec and the wd_nsec value indicate that the readout interval was circa 14.5 seconds. The observed behaviour is that watchdog checks failed for large readout intervals on 8 NUMA node machines. This indicates that the size of the skew was directly proportinal to the length of the readout interval on those machines. The measured clocksource skew, 668 microseconds, was evaluated against a threshold (the md variable) that is suited for readout intervals of roughly WATCHDOG_INTERVAL, i.e. HZ >> 1, which is 0.5 second. The intention of 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold") was to tighten the threshold for evaluating skew and set the lower bound for the uncertainty_margin of clocksources to twice WATCHDOG_MAX_SKEW. Later in c37e85c135ce ("clocksource: Loosen clocksource watchdog constraints"), the WATCHDOG_MAX_SKEW constant was increased to 125 microseconds to fit the limit of NTP, which is able to use a clocksource that suffers from up to 500 microseconds of skew per second. Both the TSC and the HPET use default uncertainty_margin. When the readout interval gets stretched the default uncertainty_margin is no longer a suitable lower bound for evaluating skew - it imposes a limit that is far stricter than the skew with which NTP can deal. The root causes of the skew being directly proportinal to the length of the readout interval are: * the inaccuracy of the shift/mult pairs of clocksources and the watchdog * the conversion to nanoseconds is imprecise for large readout intervals Prevent this by skipping the current watchdog check if the readout interval exceeds 2 * WATCHDOG_INTERVAL. Considering the maximum readout interval of 2 * WATCHDOG_INTERVAL, the current default uncertainty margin (of the TSC and HPET) corresponds to a limit on clocksource skew of 250 ppm (microseconds of skew per second). To keep the limit imposed by NTP (500 microseconds of skew per second) for all possible readout intervals, the margins would have to be scaled so that the threshold value is proportional to the length of the actual readout interval. As for why the readout interval may get stretched: 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. Fixes: 2e27e793e280 ("clocksource: Reduce clocksource-skew threshold") Suggested-by: Feng Tang <feng.tang@xxxxxxxxx> Signed-off-by: Jiri Wiesner <jwiesner@xxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Reviewed-by: Feng Tang <feng.tang@xxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Link: https://lore.kernel.org/r/20240122172350.GA740@incl --- kernel/time/clocksource.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index c108ed8..3052b1f 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -99,6 +99,7 @@ static u64 suspend_start; * Interval: 0.5sec. */ #define WATCHDOG_INTERVAL (HZ >> 1) +#define WATCHDOG_INTERVAL_MAX_NS ((2 * WATCHDOG_INTERVAL) * (NSEC_PER_SEC / HZ)) /* * Threshold: 0.0312s, when doubled: 0.0625s. @@ -134,6 +135,7 @@ 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_interval; static inline void clocksource_watchdog_lock(unsigned long *flags) { @@ -399,8 +401,8 @@ static inline void clocksource_reset_watchdog(void) static void clocksource_watchdog(struct timer_list *unused) { u64 csnow, wdnow, cslast, wdlast, delta; + int64_t wd_nsec, cs_nsec, interval; int next_cpu, reset_pending; - int64_t wd_nsec, cs_nsec; struct clocksource *cs; enum wd_read_status read_ret; unsigned long extra_wait = 0; @@ -470,6 +472,27 @@ 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. + * Skew detection may fail for longer watchdog intervals + * on account of fixed margins being used. + * Some clocksources, e.g. acpi_pm, cannot tolerate + * watchdog intervals longer than a few seconds. + */ + interval = max(cs_nsec, wd_nsec); + if (unlikely(interval > WATCHDOG_INTERVAL_MAX_NS)) { + if (system_state > SYSTEM_SCHEDULING && + interval > 2 * watchdog_max_interval) { + watchdog_max_interval = interval; + pr_warn("Long readout interval, 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) {
