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clocksource: Loosen clocksource watchdog constraints

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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 <riel@surriel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-12-06 19:36:10 -08:00
parent beaa1ffe55
commit c37e85c135
2 changed files with 14 additions and 7 deletions
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6
kernel/time/Kconfig
View File

@ -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

15
kernel/time/clocksource.c
View File

@ -95,6 +95,11 @@ static char override_name[CS_NAME_LEN];
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)
{
/*