| From | Paul Gortmaker <> | Subject | [34-longterm 075/179] time: Compensate for rounding on odd-frequency clocksources | Date | Mon, 14 May 2012 22:12:51 -0400 |
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From: Kasper Pedersen <kkp2010@kasperkp.dk>
------------------- This is a commit scheduled for the next v2.6.34 longterm release. http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git If you see a problem with using this for longterm, please comment. -------------------
commit a386b5af8edda1c742ce9f77891e112eefffc005 upstream.
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> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> --- kernel/time/timekeeping.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 39f6177..268020d 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; @@ -772,7 +776,8 @@ 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; -- 1.7.9.6
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