Re: [PATCH 1/3] Updated dynamic tick patches - Fix lost tick

Thomas Schlichter
2005-09-01T06:30:37Z

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Hi Srivatsa,

on LKML I did see your patch trying to increase the accuracy of tme pmtmr by
directly converting the PM-timer-ticks to jiffies. I think this is a good
idea but as you already recognized, it is not completely correct...

There are at least these issues:
1. "offset_last" corresponds to the time when the last recognized
jiffyoccoured. So "delta" always corresponds to the time from the last
recognized jiffy to _now_. "monotonic_base" is increased by delta, so after
the first run it will correspond to _now_. But the next time the
offset-time between the last recognized jiffy and the last _now_ is added
_again_. So the monotonic clock ist too fast...
2. "offset_last is modified outside the "monotonic_lock", what is not allowed.

I fixed these issues by using most of the old code, but simply changed "delta"
and "offset_delay" to always contain PM-timer-ticks and compute the lost
jiffies directly using PMTMR_TICKS_PER_JIFFY.

I tested the attached patch during the last night and it sems to work...

Best regards
Thomas Schlichter

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--- linux-2.6.13/arch/i386/kernel/timers/timer_pm.c.orig 2005-08-31 23:34:11.000000000 +0200
+++ linux-2.6.13/arch/i386/kernel/timers/timer_pm.c 2005-09-01 00:08:20.000000000 +0200
@@ -28,6 +28,7 @@
#define PMTMR_TICKS_PER_SEC 3579545
#define PMTMR_EXPECTED_RATE \
((CALIBRATE_LATCH * (PMTMR_TICKS_PER_SEC >> 10)) / (CLOCK_TICK_RATE>>10))
+#define PMTMR_TICKS_PER_JIFFY (PMTMR_TICKS_PER_SEC / HZ)

/* The I/O port the PMTMR resides at.
@@ -128,6 +129,11 @@ pm_good:
return -ENODEV;

init_cpu_khz();
+
+ printk ("Using %u PM timer ticks per jiffy \n", PMTMR_TICKS_PER_JIFFY);
+
+ offset_tick = read_pmtmr();
+
return 0;
}

@@ -151,7 +157,6 @@ static inline u32 cyc2us(u32 cycles)
static void mark_offset_pmtmr(void)
{
u32 lost, delta, last_offset;
- static int first_run = 1;
last_offset = offset_tick;

write_seqlock(&monotonic_lock);
@@ -161,29 +166,23 @@ static void mark_offset_pmtmr(void)
/* calculate tick interval */
delta = (offset_tick - last_offset) & ACPI_PM_MASK;

- /* convert to usecs */
- delta = cyc2us(delta);
-
/* update the monotonic base value */
- monotonic_base += delta * NSEC_PER_USEC;
+ monotonic_base += cyc2us(delta) * NSEC_PER_USEC;
write_sequnlock(&monotonic_lock);

/* convert to ticks */
delta += offset_delay;
- lost = delta / (USEC_PER_SEC / HZ);
- offset_delay = delta % (USEC_PER_SEC / HZ);
+ lost = delta / PMTMR_TICKS_PER_JIFFY;
+ offset_delay = delta % PMTMR_TICKS_PER_JIFFY;

/* compensate for lost ticks */
if (lost >= 2)
jiffies_64 += lost - 1;

- /* don't calculate delay for first run,
- or if we've got less then a tick */
- if (first_run || (lost < 1)) {
- first_run = 0;
+ /* don't calculate delay if we've got less then a tick */
+ if (lost < 1)
offset_delay = 0;
- }
}

@@ -233,9 +232,9 @@ static unsigned long get_offset_pmtmr(vo

offset = offset_tick;
now = read_pmtmr();
- delta = (now - offset)&ACPI_PM_MASK;
+ delta = (now - offset) & ACPI_PM_MASK;

- return (unsigned long) offset_delay + cyc2us(delta);
+ return (unsigned long) cyc2us(delta + offset_delay);
}

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