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time: Compensate for rounding on odd-frequency clocksources 

When the clocksource is not a multiple of HZ, the clock will be off.  For
acpi_pm, HZ=1000 the error is 127.111 ppm:

The rounding of cycle_interval ends up generating a false error term in
ntp_error accumulation since xtime_interval is not exactly 1/HZ.  So, we
subtract out the error caused by the rounding.

This has been visible since 2.6.32-rc2
	commit a092ff0f90
	time: Implement logarithmic time accumulation
That commit raised NTP_INTERVAL_FREQ and exposed the rounding error.

testing tool: http://n1.taur.dk/permanent/testpmt.c
Also tested with ntpd and a frequency counter.

Signed-off-by: Kasper Pedersen <kkp2010@kasperkp.dk>
Acked-by: john stultz <johnstul@us.ibm.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Kasper Pedersen 2010-10-20 15:55:15 -07:00 committed by Thomas Gleixner
parent 8af3c153ba
commit a386b5af8e
1 changed files with 7 additions and 2 deletions
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9
kernel/time/timekeeping.c
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@ -32,6 +32,8 @@ struct timekeeper {
cycle_t cycle_interval;
/* Number of clock shifted nano seconds in one NTP interval. */
u64 xtime_interval;
/* shifted nano seconds left over when rounding cycle_interval */
s64 xtime_remainder;
/* Raw nano seconds accumulated per NTP interval. */
u32 raw_interval;
@ -62,7 +64,7 @@ struct timekeeper timekeeper;
static void timekeeper_setup_internals(struct clocksource *clock)
{
cycle_t interval;
u64 tmp;
u64 tmp, ntpinterval;
timekeeper.clock = clock;
clock->cycle_last = clock->read(clock);
@ -70,6 +72,7 @@ static void timekeeper_setup_internals(struct clocksource *clock)
/* Do the ns -> cycle conversion first, using original mult */
tmp = NTP_INTERVAL_LENGTH;
tmp <<= clock->shift;
ntpinterval = tmp;
tmp += clock->mult/2;
do_div(tmp, clock->mult);
if (tmp == 0)
@ -80,6 +83,7 @@ static void timekeeper_setup_internals(struct clocksource *clock)
/* Go back from cycles -> shifted ns */
timekeeper.xtime_interval = (u64) interval * clock->mult;
timekeeper.xtime_remainder = ntpinterval - timekeeper.xtime_interval;
timekeeper.raw_interval =
((u64) interval * clock->mult) >> clock->shift;
@ -719,7 +723,8 @@ static cycle_t logarithmic_accumulation(cycle_t offset, int shift)
/* Accumulate error between NTP and clock interval */
timekeeper.ntp_error += tick_length << shift;
timekeeper.ntp_error -= timekeeper.xtime_interval <<
timekeeper.ntp_error -=
(timekeeper.xtime_interval + timekeeper.xtime_remainder) <<
(timekeeper.ntp_error_shift + shift);
return offset;