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authorMaciej W. Rozycki <macro@linux-mips.org>2008-09-05 14:05:31 -0700
committerIngo Molnar <mingo@elte.hu>2008-09-06 15:31:48 +0200
commit4ff4b9e19a80b73959ebeb28d1df40176686f0a8 (patch)
tree0b9068da53ae13d253d7c21a76a58be9cc5f7fc3 /kernel
parent72d43d9bc9210d24d09202eaf219eac09e17b339 (diff)
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ntp: fix calculation of the next jiffie to trigger RTC sync
We have a bug in the calculation of the next jiffie to trigger the RTC synchronisation. The aim here is to run sync_cmos_clock() as close as possible to the middle of a second. Which means we want this function to be called less than or equal to half a jiffie away from when now.tv_nsec equals 5e8 (500000000). If this is not the case for a given call to the function, for this purpose instead of updating the RTC we calculate the offset in nanoseconds to the next point in time where now.tv_nsec will be equal 5e8. The calculated offset is then converted to jiffies as these are the unit used by the timer. Hovewer timespec_to_jiffies() used here uses a ceil()-type rounding mode, where the resulting value is rounded up. As a result the range of now.tv_nsec when the timer will trigger is from 5e8 to 5e8 + TICK_NSEC rather than the desired 5e8 - TICK_NSEC / 2 to 5e8 + TICK_NSEC / 2. As a result if for example sync_cmos_clock() happens to be called at the time when now.tv_nsec is between 5e8 + TICK_NSEC / 2 and 5e8 to 5e8 + TICK_NSEC, it will simply be rescheduled HZ jiffies later, falling in the same range of now.tv_nsec again. Similarly for cases offsetted by an integer multiple of TICK_NSEC. This change addresses the problem by subtracting TICK_NSEC / 2 from the nanosecond offset to the next point in time where now.tv_nsec will be equal 5e8, effectively shifting the following rounding in timespec_to_jiffies() so that it produces a rounded-to-nearest result. Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/time/ntp.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 5125ddd8196b..1ad46f3df6e7 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -245,7 +245,7 @@ static void sync_cmos_clock(unsigned long dummy)
if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2)
fail = update_persistent_clock(now);
- next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec;
+ next.tv_nsec = (NSEC_PER_SEC / 2) - now.tv_nsec - (TICK_NSEC / 2);
if (next.tv_nsec <= 0)
next.tv_nsec += NSEC_PER_SEC;
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