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

To: "Christopher S. Aker" <caker@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Xen 3.2.2 - Timer ISR/0: Time went backwards
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Tue, 05 Aug 2008 08:03:51 +0100
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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?


>>> "Christopher S. Aker" <caker@xxxxxxxxxxxx> 04.08.08 20:51 >>>
Xen: 3.2.1-rc2 64bit
dom0: 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.


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