Re: [GIT PULL clocksource] Clocksource watchdog commits for v5.15
From: Paul E. McKenney
Date: Thu Aug 12 2021 - 16:01:15 EST
On Thu, Aug 12, 2021 at 07:11:55PM +0200, Thomas Gleixner wrote:
> On Thu, Aug 12 2021 at 09:37, Paul E. McKenney wrote:
>
> > On Thu, Aug 12, 2021 at 03:46:42PM +0200, Thomas Gleixner wrote:
> >> On Wed, Aug 11 2021 at 17:01, Paul E. McKenney wrote:
> >> > This pull request contains a single change that prevents clocksource
> >> > watchdog testing on systems with HZ < 100, thus preventing the integer
> >> > underflow that can occur on leisurely HZed systems. This has been
> >> > posted to LKML:
> >> >
> >> > https://lore.kernel.org/lkml/20210721212755.GA2066078@paulmck-ThinkPad-P17-Gen-1/
> >>
> >> So with HZ < 100 .mult overflows, but why not simply adjusting the
> >> mult, shift value to be
> >>
> >> .mult = TICK_NSEC,
> >> .shift = 0,
> >>
> >> which is effectively the same as
> >>
> >> .mult = TICK_NSEC << 8,
> >> .shift = 8,
> >>
> >> Hmm?
> >
> > Another option would be for me to be less lazy and to move this code:
> >
> > /* 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
> >
> > from kernel/time/jiffies.c to include/linux/clocksource.h.
>
> No need to expose this globaly.
>
> kernel/time/tick-internal.h or kernel/time/jiffies.h
How about like this?
Thanx, Paul
------------------------------------------------------------------------
clocksource-wdtest.c | 5 ++---
jiffies.c | 21 +--------------------
tick-internal.h | 20 ++++++++++++++++++++
3 files changed, 23 insertions(+), 23 deletions(-)
commit 2419f70beeb885ad96b832c64648e42559d7cf1d
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date: Thu Aug 12 09:31:28 2021 -0700
clocksource: Make clocksource-wdtest.c safe for slow-HZ systems
Currently, clocksource-wdtest.c sets a local JIFFIES_SHIFT macro for
operation at HZ>=67, which can cause this test suite to fail on systems
with HZ<67. Therefore, move the HZ-based definitions of JIFFIES_SHIFT
from kernel/time/jiffies.c to kernel/time/tick-internal.h, allowing
the local JIFFIES_SHIFT macro to be removed from clocksource-wdtest.c
in favor of a properly HZ-based definition. This in turn makes
clocksource-wdtest.c safe for slow-HZ systems.
[ paulmck: Moved JIFFIES_SHIFT from include/linux/clocksource.h. ]
Link: https://lore.kernel.org/lkml/20210812000133.GA402890@paulmck-ThinkPad-P17-Gen-1/
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
Cc: Rong Chen <rong.a.chen@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/kernel/time/clocksource-wdtest.c b/kernel/time/clocksource-wdtest.c
index b72a969f7b938..7e82500c400b9 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@xxxxxxxxxx>");
@@ -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 01935aafdb460..bc4db9e5ab70c 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 6a742a29e545f..5459bab2f4f70 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -165,3 +165,23 @@ DECLARE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases);
extern u64 get_next_timer_interrupt(unsigned long basej, u64 basem);
void timer_clear_idle(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