Attachment 317099 Details for Bug 462410 – Fine tune the ACT_HZ value when divider= is enabled.

[patch] Fine tune the ACT_HZ value when divider= is enabled.

tick_nsec_tune.patch (text/plain), 3.52 KB, created by Alok Kataria on 2008-09-18 17:15:27 UTC

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Creator: Alok Kataria

Created: 2008-09-18 17:15:27 UTC

Size: 3.52 KB

patch

obsolete

>[EXPERIMENTAL PATCH]
>
>Fine tune the ACT_HZ value when divider= is enabled.
>
>From: Alok N Kataria <akataria@vmware.com>
>
>As noted by Garret, there is some error in calculation if divider= option is
>passed. The LOGICAL_LATCH value which is based on HZ is calculated to 1193,
>and the LATCH value (based on REAL_HZ) is calculated to 11932, with divider=10.
>
>We use the LATCH value to program the PIT period, but TICK_NSEC (which is
>used to adjust time of day based on jiffies advancement) is computed based
>computed based on LOGICAL_LATCH.
>
>As a result we loose about 2 PIT ticks every interrupt. This is reflected
>in TOD going loosing time at a constant rate of 166ppm.
>Over a run of 5hrs we have seen that TOD looses about 3 sec of time.
>
>The patch below fixes it by fine tuning the ACTHZ value and the TICK_NSEC
>value. The implementation of this proved difficult as TICK_NSEC which
>originally was a constant, is used at lot of places to initialize a value
>or in #ifdefine's. Changing all of these places made the patch very intrusive.
>Instead I know define ACTHZ_FINE which is used to calculate TICK_NSEC_FINE.
>
>This value (TICK_NSEC_FINE) is used in places where TICK_NSEC is used to compute
>TOD.
>
>Analysed-by: Garrett Smith <garrett@vmware.com>
>Signed-off-by: Alok N Kataria <akataria@vmware.com>
>---
>
> include/linux/jiffies.h |    2 ++
> kernel/time/jiffies.c   |    4 ++--
> kernel/timer.c          |    3 +++
> 3 files changed, 7 insertions(+), 2 deletions(-)
>
>
>diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
>index 72a26e7..bef9f48 100644
>--- a/include/linux/jiffies.h
>+++ b/include/linux/jiffies.h
>@@ -63,11 +63,13 @@ extern unsigned int tick_divider;
> 
> /* HZ is the requested value. ACTHZ is actual HZ ("<< 8" is for accuracy) */
> #define ACTHZ (SH_DIV (CLOCK_TICK_RATE, LOGICAL_LATCH, 8))
>+#define ACTHZ_FINE ((SH_DIV (CLOCK_TICK_RATE, LATCH, 8)) * tick_divider)
> 
> #define ACTHZ_HPET (SH_DIV (HPET_TICK_RATE, LOGICAL_LATCH_HPET, 8))
> 
> /* TICK_NSEC is the time between ticks in nsec assuming real ACTHZ */
> #define TICK_NSEC (SH_DIV (1000000UL * 1000, ACTHZ, 8))
>+#define TICK_NSEC_FINE (SH_DIV (1000000UL * 1000, ACTHZ_FINE, 8))
> 
> #define TICK_NSEC_HPET (SH_DIV(1000000UL * 1000, ACTHZ_HPET, 8))
> 
>diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
>index 126bb30..2275735 100644
>--- a/kernel/time/jiffies.c
>+++ b/kernel/time/jiffies.c
>@@ -34,7 +34,7 @@
>  * requested HZ value. It is also not reccomended
>  * for "tick-less" systems.
>  */
>-#define NSEC_PER_JIFFY	((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ))
>+#define NSEC_PER_JIFFY	((u32)((((u64)NSEC_PER_SEC)<<8)/ACTHZ_FINE))
> 
> /* Since jiffies uses a simple NSEC_PER_JIFFY multiplier
>  * conversion, the .shift value could be zero. However
>@@ -60,13 +60,13 @@ struct clocksource clocksource_jiffies = {
> 	.rating		= 0, /* lowest rating*/
> 	.read		= jiffies_read,
> 	.mask		= 0xffffffff, /*32bits*/
>-	.mult		= NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */
> 	.shift		= JIFFIES_SHIFT,
> 	.is_continuous	= 0, /* tick based, not free running */
> };
> 
> static int __init init_jiffies_clocksource(void)
> {
>+	clocksource_jiffies.mult = NSEC_PER_JIFFY << JIFFIES_SHIFT;
> 	return clocksource_register(&clocksource_jiffies);
> }
> 
>diff --git a/kernel/timer.c b/kernel/timer.c
>index 3eb09ed..37d49ee 100644
>--- a/kernel/timer.c
>+++ b/kernel/timer.c
>@@ -1092,6 +1092,9 @@ void __init timekeeping_init(void)
> {
> 	unsigned long flags;
> 
>+#ifdef CONFIG_TICK_DIVIDER
>+	tick_nsec = TICK_NSEC_FINE;
>+#endif
> 	write_seqlock_irqsave(&xtime_lock, flags);
> 	clock = clocksource_get_next();
> 	clocksource_calculate_interval(clock, tick_nsec);

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Attachments on bug 462410: 317099