linux-kernel - [PATCH][RFC(again)] Apply NTP frequency/tick changes immediately.

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Date:	Wed, 18 Feb 2009 16:02:22 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Clark Williams <williams@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH][RFC(again)] Apply NTP frequency/tick changes immediately.

On Thu, 2009-02-12 at 19:02 -0800, john stultz wrote:
> Hey Roman,
> 	It was brought to my attention that since the GENERIC_TIME changes
> landed, the adjtimex behavior changed for struct timex.tick and .freq
> changes. 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.
> 
> Looking at the code, it doesn't seem like it would be too hard to
> immediately apply the change to the tick_length value, so its changed in
> the following NTP_INTERVAL, which this patch does.
> 
> I've run a few tests with this change, and ntpd still functions fine. I
> do however note that the drift value for my test system changed from
> ~170ppm to ~18ppm, which I didn't quite expect, and needs some
> additional research.
> 
> Anyway, I just wanted to see if you had any thoughts on this sort of
> change.

Hey Roman, Just wanted to ping you again to see if you had any
objections to this change.

I did find the cause of my test system's drift switching from 170ppm to
18ppm, and it ends up its due to my bouncing between kernel versions.
The 170ppm drift was established after running w/ a older 2.6.24 based
kernel for awhile, and after the NTP_INTERVAL_LENGTH changes
(10a398d04c4a1fc395840f4d040493375f562302) landed the ppm value was
expected to change. Comparing the same kernel 2.6.29-rc4 with and
without this patch, the ppm value did not change.

Anyway, here it is again.

thanks
-john


Apply NTP tick/frequency adjustments immediately instead of waiting for
the next second to pass.

Signed-off-by: John Stultz <johnstul@...ibm.com>

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index f5f793d..9fd0d15 100644
--- a/kernel/time/ntp.c
+++ b/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 +61,11 @@ 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)


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