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From: john stultz <johnstul@us.ibm.com>
To: Roman Zippel <zippel@linux-m68k.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] correct inconsistent ntp interval/tick_length usage
Date: Mon, 28 Jan 2008 18:28:06 -0800	[thread overview]
Message-ID: <1201573686.6766.13.camel@localhost> (raw)
In-Reply-To: <Pine.LNX.4.64.0801251349120.17507@scrub.home>


On Fri, 2008-01-25 at 15:07 +0100, Roman Zippel wrote:
> Hi,
> 
> On Wed, 23 Jan 2008, john stultz wrote:
> 
> > This difference in calculation was causing the clocksource correction
> > code to apply a correction factor to the clocksource so the two
> > intervals were the same, however this results in the actual frequency of
> > the clocksource to be made incorrect. I believe this difference would
> > affect all clocksources, although to differing degrees depending on the
> > clocksource resolution.
> 
> Let's look at why the correction is done in first place. The update steps 
> don't add up precisely to 1sec (LATCH*HZ != CLOCK_TICK_RATE), so a small 
> addjustment is used to make up for it. The problem here is that if the 
> update frequency changes, the addjustment isn't correct anymore.
> The simple fix is to just omit the addjustment in these cases in ntp.c:
> 
> #if NTP_INTERVAL_FREQ == HZ
> ...
> #else
> #define CLOCK_TICK_ADJUST	0
> #endif

Hmmm, although this doesn't explain why the issue is seen when
NTP_INTERVAL_FREQ == HZ (as it is in my system's case). Or am I missing
something?

Regardless, current_tick_length() really is the base interval we're
using in the error accumulation loop, so it seems the cleanest interface
to use (just to avoid redundancy at least) when establishing the
clocksource's interval length. Or do you not agree?

thanks
-john







  reply	other threads:[~2008-01-29  2:28 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-24  2:38 john stultz
2008-01-24  9:14 ` Valdis.Kletnieks
2008-01-25 14:07 ` Roman Zippel
2008-01-29  2:28   ` john stultz [this message]
2008-01-29  4:02     ` Roman Zippel
2008-01-30  2:14       ` john stultz
2008-01-31  1:55         ` Roman Zippel
2008-01-31  2:16           ` john stultz
2008-01-31  5:02             ` Roman Zippel
2008-02-02  1:02               ` John Stultz
2008-02-08 17:33                 ` Roman Zippel
2008-02-09  2:17                   ` john stultz
2008-02-09  4:47                     ` Roman Zippel
2008-02-09  5:04                       ` Andrew Morton
2008-02-09 14:13                         ` Roman Zippel
2008-02-10 18:45                     ` Roman Zippel
2008-02-12  0:09                       ` john stultz
2008-02-12 14:36                         ` Roman Zippel
2008-02-14  4:36                           ` john stultz
2008-02-16  4:24                             ` Roman Zippel
2008-02-19  1:02                               ` john stultz
2008-02-19  4:04                                 ` Roman Zippel
2008-02-20  1:50                                   ` john stultz
2008-02-20 17:08                                     ` Roman Zippel
2008-02-22  2:39                                       ` john stultz
2008-02-25 14:44                                         ` Roman Zippel
2008-02-29  4:49                                         ` [PATCH] Remove obsolete CLOCK_TICK_ADJUST Roman Zippel
2008-02-29  6:25                                           ` Ray Lee
2008-02-29 13:31                                             ` Roman Zippel
2008-02-29 22:08                                           ` Andrew Morton
2008-02-29 22:27                                             ` Roman Zippel
2008-02-29 22:38                                               ` Andrew Morton
2008-02-29 23:11                                               ` john stultz
2008-02-29 23:11                                           ` john stultz
2008-03-02  4:03                                             ` Roman Zippel
2008-02-29 18:54                                         ` [PATCH] correct inconsistent ntp interval/tick_length usage Jörg-Volker Peetz

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