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 a092ff0f90cae22b2ac8028ecd2c6f6c1a9e4601
	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>

1 files changed, 7 insertions, 2 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 49010d822f72..5bb86da82003 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -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;