[PATCH clocksource 2/6] clocksource: Loosen clocksource watchdog constraints

Paul E. McKenney
2023-01-05T01:07:27Z

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>
---
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 a41753be1a2bf..bae8f11070bef 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 4a2c3bb92e2e9..a3d19f6660ac7 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -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)
{
/*
--
2.31.1.189.g2e36527f23