linux-kernel - Re: Clock jumps

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Date:	Thu, 27 May 2010 17:46:45 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Bernhard Schmidt <berni@...kenwald.de>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Clock jumps

On Fri, 2010-05-28 at 02:33 +0200, Bernhard Schmidt wrote:
> On 28.05.2010 02:00, john stultz wrote:
> > Looking at the diff:
> > --- dmesg-lenny 2010-05-27 16:45:33.000000000 -0700
> > +++ dmesg-squeeze       2010-05-27 16:46:14.000000000 -0700
> > @@ -132,8 +132,8 @@
> >   console [ttyS1] enabled
> >   hpet clockevent registered
> >   Fast TSC calibration using PIT
> > -Detected 2660.398 MHz processor.
> > -Calibrating delay loop (skipped), value calculated using timer frequency.. 5320.79 BogoMIPS (lpj=10641592)
> > +Detected 2613.324 MHz processor.
> > +Calibrating delay loop (skipped), value calculated using timer frequency.. 5226.64 BogoMIPS (lpj=10453296)
> >   Security Framework initialized
> >   Dentry cache hash table entries: 524288 (order: 10, 4194304 bytes)
> >   Inode-cache hash table entries: 262144 (order: 9, 2097152 bytes)
> > @@ -160,7 +160,7 @@
> >   CPU0: Intel(R) Xeon(R) CPU            3075  @ 2.66GHz stepping 0b
> >   Booting Node   0, Processors  #1
> >   Brought up 2 CPUs
> > -Total of 2 processors activated (10640.79 BogoMIPS).
> > +Total of 2 processors activated (10546.63 BogoMIPS).
> >   NET: Registered protocol family 16
> >   ACPI: bus type pci registered
> >   PCI: MMCONFIG for domain 0000 [bus 00-ff] at [mem 0xe0000000-0xefffffff] (base 0xe0000000)
> >
> > So you can see in the above the during the second boot the TSC
> > calibration was badly mis-calculated. This was the cause of the skew.
> >
> > Not sure how that might be linked to the distro upgrade. It could have
> > been something like SMI damage during the calibration time, but I
> > thought the calibration loop watched for that.
> >
> > Bernhard: I expect with all those vms, this machine isn't rebooted
> > frequently. So could you look at the logs to see how much the  "Detected
> > xxxx.yyy MHz processor." line varies by across a few other boots (if
> > they still exist?).
> 
> Correct, the box isn't rebooted often, but I do have a few dmesg outputs 
> laying around. lpj was always almost the same until the very last boot 
> which screwed up the clock.
> 
> dmesg:[    0.000000] Linux version 2.6.33 (root@...02) (gcc version 
> 4.3.2 (Debian 4.3.2-1.1) ) #1 SMP Sun Mar 7 23:01:45 CET 2010
> dmesg:[    0.008005] Calibrating delay loop (skipped), value calculated 
> using timer frequency.. 5226.64 BogoMIPS (lpj=10453296)
> dmesg:[    0.288002] Total of 2 processors activated (10546.63 BogoMIPS).

Yea. The bogomips/loops per jiffies are actually calculated with a
different chunk of code (although its interesting it miscalculated in
both cases).

Could you send the "Detected xxxx.yyy MHz processor." lines as well?

thanks
-john


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