linux-kernel - Re: Linux 2.6.29-rc6

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Date:	Thu, 26 Feb 2009 14:31:23 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Jesper Krogh <jesper@...gh.cc>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: Linux 2.6.29-rc6

On Thu, 2009-02-26 at 23:06 +0100, Thomas Gleixner wrote:
> On Thu, 26 Feb 2009, john stultz wrote:
> > On Thu, 2009-02-26 at 22:35 +0100, Jesper Krogh wrote:
> > > > Also mind sending the full dmesg for both kernels?
> > > 
> > > http://krogh.cc/~jesper/dmesg-2.6.29-rc6.txt
> > > http://krogh.cc/~jesper/dmesg-2.6.26.8.txt
> > 
> > So one interesting difference:
> > 2.6.26.8:	TSC calibrated against PM_TIMER
> > 2.6.29-rc6:	Fast TSC calibration using PIT
> > 
> > Thomas, any thoughts as to why we might be calibrating off the PIT
> > instead of the PM_TIMER w/ 2.6.29?
> 
> Yup, because we introduced the Fast PIT calibration in 2.6.28.

Ah. Ok.

> Is the delta anything NTP might get upset about:
> 
> 2.6.26: time.c: Detected 2311.847 MHz processor.
> 2.6.29: Detected 2310.029 MHz processor.

I wouldn't think so.

Although, I'm recalling on some systems here right after we deploy them
we'll see something similar to the originally reported ntpd "time reset"
noise for a period of time while ntpd tries to find the right freq. For
some reason, I've noticed, having multiple servers in your ntp.conf
seems to increase NTP's difficulty at picking a time and converging. 

So this may be just the slight calibration change is confusing ntp or it
may be the NTP_INTERVAL_LENGTH change from awhile back which would cause
the drift value to change could be doing the same thing (although I
thought that landed in the 2.6.24 timeframe, but I may be forgetting).

I'll kick up some of my own testing between these two releases to see if
I can't find something similar.

Jesper: How long was the box up for when you noticed the ntpd noise?

Also what's the output of the following under the different kernels:
ntpdc -c peers
ntpdc -c kerninfo

thanks
-john

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