Commit d25a0252 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Thomas Gleixner
Browse files

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: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20210812000133.GA402890@paulmck-ThinkPad-P17-Gen-1/
parent 127c92fe

kernel/time/clocksource-wdtest.c

+2 −3
Original line number Diff line number Diff line
@@ -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*/

kernel/time/jiffies.c

+1 −20
Original line number Diff line number Diff line
@@ -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;

kernel/time/tick-internal.h

+20 −0
Original line number Diff line number Diff line
@@ -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