RE: [Xen-devel] Xen 3.2.2 - Timer ISR/0: Time went backwards

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

> 20-50+% timer interrupts. The moment this rate exceeds about 50%,
> platform time calibration breaks (as it sets the timer to 
> half the overflow period). 

I've looked at that code in local_time_calibration() a few times
and even added debug code once to see if it occurs.  It
didn't on my machine, but I can see how it would cause problems
if it did happen.

Keir, would you accept a patch (or just add the two lines yourself)
to printk a warning if that "goto out" ever occurs and/or maybe
if the "scale factor is clamped"?

(Chris, this might not be your problem so apologies for the topic
drift, but if the printk had been there awhile ago, we'd at least
know if it is or is not the problem.)

Dan

P.S. This is also what led to the separate thread about measuring
interrupt latency.  If this problem is due to huge periods with
interrupts off, it would be nice to know.

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Jan Beulich
> Sent: Tuesday, August 05, 2008 1:04 AM
> To: Christopher S. Aker
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen 3.2.2 - Timer ISR/0: Time went backwards
> 
> 
> This looks very similar to bug report we've got from IBM I'm currently
> trying to research (difficult, as I can't touch the 
> hardware). What I know
> so far is that we're losing, starting a few seconds after 
> dom0 boot began,
> 20-50+% timer interrupts. The moment this rate exceeds about 50%,
> platform time calibration breaks (as it sets the timer to 
> half the overflow
> period). Since jiffies aren't used much elsewhere, this loss 
> of timer ticks
> doesn't seem to matter much elsewhere.
> 
> I've got no real clue so far *why* there's such a high rate 
> of lost interrupts,
> though. The only (albeit small, since appearing very 
> unlikely) possibility
> would be frequent and extensive SMM entries after ACPI mode got
> enabled on the system.
> 
> Btw., does -unstable exhibit the same behavior?
> 
> Jan
> 
> >>> "Christopher S. Aker" <caker@xxxxxxxxxxxx> 04.08.08 20:51 >>>
> Hardware:
> Xen: 3.2.1-rc2 64bit
> dom0: 2.6.18.8 at changeset 622, PAE
> 
> # xm dmesg | grep -e sync -e timer
> (XEN) checking TSC synchronization across 8 CPUs: passed.
> (XEN) Platform timer overflows in 234 jiffies.
> (XEN) Platform timer is 3.579MHz ACPI PM Timer
> (XEN) Machine check exception polling timer started.
> 
> Spools one of these to console every few seconds:
> 
> Timer ISR/0: Time went backwards: delta=-4270576170971 
> delta_cpu=254829029 shadow=2037844042151244163 off=261710497 
> processed=2037848312989081849 cpu_processed=2037844042158081849
>   0: 2037844042158081849
>   1: 2037828468354081849
>   2: 2037848312989081849
>   3: 2037837726866081849
>   4: 2037842059197081849
>   5: 2037840075526081849
>   6: 2037845844663081849
>   7: 2037841593777081849
> 
> A few t's into Xen's console:
> 
> (XEN) *** Serial input -> Xen (type 'CTRL-a' three times to 
> switch input 
> to DOM0)
> (XEN) Min = 2037829427350793281 ; Max = 2037848310626701146 ; Diff = 
> 18883275907865 (18883275907 microseconds)
> (XEN) Min = 2037829428349256182 ; Max = 2037848311625163843 ; Diff = 
> 18883275907661 (18883275907 microseconds)
> (XEN) Min = 2037829428565188930 ; Max = 2037848311841096807 ; Diff = 
> 18883275907877 (18883275907 microseconds)
> 
> This particular box does this with 3.2.0 - 3.2.2-rc2.  I have another 
> box doing the same thing, except the delta is more sane (0 - 2 
> microseconds), however eventually dom0 freezes.
> 
> -Chris
> 
> 
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