linux-kernel - Re: [PATCH] correct inconsistent ntp interval/tick

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Date:	Wed, 30 Jan 2008 18:16:16 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] correct inconsistent ntp interval/tick_length usage


On Thu, 2008-01-31 at 02:55 +0100, Roman Zippel wrote:
> Hi,
> 
> On Tue, 29 Jan 2008, john stultz wrote:
> 
> > +/* Because using NSEC_PER_SEC would be too easy */
> > +#define NTP_INTERVAL_LENGTH ((((s64)TICK_USEC*NSEC_PER_USEC*USER_HZ)+CLOCK_TICK_ADJUST)/NTP_INTERVAL_FREQ)
> 
> Why are you using USER_HZ? Did you test this with HZ!=100?

I'm using USER_HZ, because ntp_update_frequency() uses USER_HZ.

And my tests were run at HZ=1000.

> Anyway, please don't make more complicated than it already is.
> What I said previously about the update interval is still valid, so the 
> correct solution is to use the simpler NTP_INTERVAL_LENGTH calculation 
> from my last mail and to omit the correction for NO_HZ.

I'm not sure I follow how having two separate and totally different
definitions of NTP_INTERVAL_LENGTH is less complicated then my last
patch.

Maybe I'm missing something from your suggestion? Or maybe could you
contribute your suggestion as a patch?

My concern is we state the accumulation interval is X ns long. Then
current_tick_length() is to return (X + ntp_adjustment), so each
accumulation interval we can keep track of the error and adjust our
interval length.

So if ntp_update_frequency() sets tick_length_base to be:

	u64 second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)
			<< TICK_LENGTH_SHIFT;
	second_length += (s64)CLOCK_TICK_ADJUST << TICK_LENGTH_SHIFT;
	second_length += (s64)time_freq
				<< (TICK_LENGTH_SHIFT - SHIFT_NSEC);

	tick_length_base = second_length;
	do_div(tick_length_base, NTP_INTERVAL_FREQ);


The above is basically (X + part of ntp_adjustment)

So I'm trying to calculate the X as: 

#define NTP_INTERVAL_LENGTH ((((s64)TICK_USEC*NSEC_PER_USEC*USER_HZ)+CLOCK_TICK_ADJUST)/NTP_INTERVAL_FREQ)


thanks
-john


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