time: apply NTP frequency/tick changes immediately

Since the GENERIC_TIME changes landed, the adjtimex behavior changed
for struct timex.tick and .freq changed. When the tick or freq value
is set, we adjust the tick_length_base in ntp_update_frequency().
However, this new value doesn't get applied to tick_length until the
next second (via second_overflow).

This means some applications that do quick time tweaking do not see the
requested change made as quickly as expected.

I've run a few tests with this change, and ntpd still functions fine.
Signed-off-by: NJohn Stultz <johnstul@us.ibm.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/time/ntp.c

@@ -51,6 +51,7 @@ static long ntp_tick_adj;
 
 static void ntp_update_frequency(void)
 {
+	u64 old_tick_length_base = tick_length_base;
 	u64 second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)
 				<< NTP_SCALE_SHIFT;
 	second_length += (s64)ntp_tick_adj << NTP_SCALE_SHIFT;
@@ -60,6 +61,12 @@ static void ntp_update_frequency(void)
 
 	tick_nsec = div_u64(second_length, HZ) >> NTP_SCALE_SHIFT;
 	tick_length_base = div_u64(tick_length_base, NTP_INTERVAL_FREQ);
+
+	/*
+	 * Don't wait for the next second_overflow, apply
+	 * the change to the tick length immediately
+	 */
+	tick_length += tick_length_base - old_tick_length_base;
 }
 
 static void ntp_update_offset(long offset)