[tip: timers/core] clocksource: Make clocksource watchdog test safe for slow-HZ systems

[tip: timers/core] clocksource: Make clocksource watchdog test safe for slow-HZ systems

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     d25a025201ed98f4b93775e0999a3f2135702106
Gitweb:        https://git.kernel.org/tip/d25a025201ed98f4b93775e0999a3f2135702106
Author:        Paul E. McKenney <paulmck@kernel.org>
AuthorDate:    Thu, 12 Aug 2021 09:31:28 -07:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Sat, 28 Aug 2021 17:01:32 +02:00

clocksource: Make clocksource watchdog test safe for slow-HZ systems

The clocksource watchdog test sets a local JIFFIES_SHIFT macro and assumes
that HZ is >= 100. For smaller HZ values this shift value is too large and
causes undefined behaviour.

Move the HZ-based definitions of JIFFIES_SHIFT from kernel/time/jiffies.c
to kernel/time/tick-internal.h so the clocksource watchdog test can utilize
them, which makes it work correctly with all HZ values.

[ tglx: Resolved conflicts and massaged changelog ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20210812000133.GA402890@paulmck-ThinkPad-P17-Gen-1/
---
 kernel/time/clocksource-wdtest.c |  5 ++---
 kernel/time/jiffies.c            | 21 +--------------------
 kernel/time/tick-internal.h      | 20 ++++++++++++++++++++
 3 files changed, 23 insertions(+), 23 deletions(-)

diff --git a/kernel/time/clocksource-wdtest.c b/kernel/time/clocksource-wdtest.c
index 01df123..df922f4 100644
--- a/kernel/time/clocksource-wdtest.c
+++ b/kernel/time/clocksource-wdtest.c
@@ -19,6 +19,8 @@
 #include <linux/prandom.h>
 #include <linux/cpu.h>
 
+#include "tick-internal.h"
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Paul E. McKenney <paulmck@kernel.org>");
 
@@ -34,9 +36,6 @@ static u64 wdtest_jiffies_read(struct clocksource *cs)
 	return (u64)jiffies;
 }
 
-/* Assume HZ > 100. */
-#define JIFFIES_SHIFT	8
-
 static struct clocksource clocksource_wdtest_jiffies = {
 	.name			= "wdtest-jiffies",
 	.rating			= 1, /* lowest valid rating*/
diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index 01935aa..bc4db9e 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -10,28 +10,9 @@
 #include <linux/init.h>
 
 #include "timekeeping.h"
+#include "tick-internal.h"
 
 
-/* Since jiffies uses a simple TICK_NSEC multiplier
- * conversion, the .shift value could be zero. However
- * this would make NTP adjustments impossible as they are
- * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
- * shift both the nominator and denominator the same
- * amount, and give ntp adjustments in units of 1/2^8
- *
- * The value 8 is somewhat carefully chosen, as anything
- * larger can result in overflows. TICK_NSEC grows as HZ
- * shrinks, so values greater than 8 overflow 32bits when
- * HZ=100.
- */
-#if HZ < 34
-#define JIFFIES_SHIFT	6
-#elif HZ < 67
-#define JIFFIES_SHIFT	7
-#else
-#define JIFFIES_SHIFT	8
-#endif
-
 static u64 jiffies_read(struct clocksource *cs)
 {
 	return (u64) jiffies;
diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 3548f08..649f2b4 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -177,3 +177,23 @@ void clock_was_set(unsigned int bases);
 void clock_was_set_delayed(void);
 
 void hrtimers_resume_local(void);
+
+/* Since jiffies uses a simple TICK_NSEC multiplier
+ * conversion, the .shift value could be zero. However
+ * this would make NTP adjustments impossible as they are
+ * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
+ * shift both the nominator and denominator the same
+ * amount, and give ntp adjustments in units of 1/2^8
+ *
+ * The value 8 is somewhat carefully chosen, as anything
+ * larger can result in overflows. TICK_NSEC grows as HZ
+ * shrinks, so values greater than 8 overflow 32bits when
+ * HZ=100.
+ */
+#if HZ < 34
+#define JIFFIES_SHIFT	6
+#elif HZ < 67
+#define JIFFIES_SHIFT	7
+#else
+#define JIFFIES_SHIFT	8
+#endif