Commit c37e85c1 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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 <riel@surriel.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent beaa1ffe

kernel/time/Kconfig

+5 −1
Original line number Diff line number Diff line
@@ -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

kernel/time/clocksource.c

+9 −6
Original line number Diff line number Diff line
@@ -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)

{

	/*