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: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index a41753b..bae8f11 100644 (file)
--- 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 (file)
--- 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)
 {
        /*