From c37e85c135cead4256dc8860073c468d8925c3df Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 6 Dec 2022 19:36:10 -0800 Subject: [PATCH] clocksource: Loosen clocksource watchdog constraints Currently, MAX_SKEW_USEC is set to 100 microseconds, which has worked reasonably well. However, NTP is willing to tolerate 500 microseconds of skew per second, and a clocksource that is good enough for NTP should be good enough for the clocksource watchdog. The watchdog's skew is controlled by MAX_SKEW_USEC and the CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option. However, these values are doubled before being associated with a clocksource's ->uncertainty_margin, and the ->uncertainty_margin values of the pair of clocksource's being compared are summed before checking against the skew. Therefore, set both MAX_SKEW_USEC and the default for the CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option to 125 microseconds of skew per second, resulting in 500 microseconds of skew per second in the clocksource watchdog's skew comparison. Suggested-by Rik van Riel Signed-off-by: Paul E. McKenney --- kernel/time/Kconfig | 6 +++++- kernel/time/clocksource.c | 15 +++++++++------ 2 files changed, 14 insertions(+), 7 deletions(-) diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig index a41753b..bae8f11 100644 --- a/kernel/time/Kconfig +++ b/kernel/time/Kconfig @@ -200,10 +200,14 @@ config CLOCKSOURCE_WATCHDOG_MAX_SKEW_US int "Clocksource watchdog maximum allowable skew (in μs)" depends on CLOCKSOURCE_WATCHDOG range 50 1000 - default 100 + default 125 help Specify the maximum amount of allowable watchdog skew in microseconds before reporting the clocksource to be unstable. + The default is based on a half-second clocksource watchdog + interval and NTP's maximum frequency drift of 500 parts + per million. If the clocksource is good enough for NTP, + it is good enough for the clocksource watchdog! endmenu endif diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 4a2c3bb..a3d19f6 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -96,6 +96,11 @@ static int finished_booting; static u64 suspend_start; /* + * Interval: 0.5sec. + */ +#define WATCHDOG_INTERVAL (HZ >> 1) + +/* * Threshold: 0.0312s, when doubled: 0.0625s. * Also a default for cs->uncertainty_margin when registering clocks. */ @@ -106,11 +111,14 @@ static u64 suspend_start; * clocksource surrounding a read of the clocksource being validated. * This delay could be due to SMIs, NMIs, or to VCPU preemptions. Used as * a lower bound for cs->uncertainty_margin values when registering clocks. + * + * The default of 500 parts per million is based on NTP's limits. + * If a clocksource is good enough for NTP, it is good enough for us! */ #ifdef CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US #define MAX_SKEW_USEC CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US #else -#define MAX_SKEW_USEC 100 +#define MAX_SKEW_USEC (125 * WATCHDOG_INTERVAL / HZ) #endif #define WATCHDOG_MAX_SKEW (MAX_SKEW_USEC * NSEC_PER_USEC) @@ -140,11 +148,6 @@ static inline void clocksource_watchdog_unlock(unsigned long *flags) static int clocksource_watchdog_kthread(void *data); static void __clocksource_change_rating(struct clocksource *cs, int rating); -/* - * Interval: 0.5sec. - */ -#define WATCHDOG_INTERVAL (HZ >> 1) - static void clocksource_watchdog_work(struct work_struct *work) { /* -- 2.7.4