The following commit has been merged into the timers/core branch of tip:
Commit-ID: c37e85c135cead4256dc8860073c468d8925c3df
Gitweb: https://git.kernel.org/tip/c37e85c135cead4256dc8860073c468d8925c3df
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate: Tue, 06 Dec 2022 19:36:10 -08:00
Committer: Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Tue, 03 Jan 2023 20:43:45 -08:00
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@xxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
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)
{
/*
|