Re: [suse-amd64] False "lost ticks" on dual-Opteron system (=> timer twice as fast)

[Bernd Paysan <bernd.paysan@mikron.de>]

On Tuesday 10 May 2005 13:12, Andi Kleen wrote:
> > So that explains why nobody sees this problem. But the TSC-based
> > fallback timekeeping is still broken on SMP systems with PowerNow and
> > distributed IRQ handling, which both together seem to be rare enough
> > ;-).
>
> There is a patch pending for the TSC problem - using the pmtimer instead
> in this case.
>
> But the distributed timer interrupt problem is weird. It should not
> happen. You sure it was IRQ 0 that was duplicated and not "LOC" ?

Yes. Only one CPU actually gets and handles the timer interrupt, but which 
one is somewhat random (for about 10 seconds, it's the same CPU, then it 
switches over).

> When you watch -n1 cat /proc/interrupts does the rate roughly match
> up to 1000Hz?

Yes, and this is confirmed over longer time:

# grep timer /proc/interrupts; uptime
  0:   40347440   40582285    IO-APIC-edge  timer
  1:26pm  an  22:28,  1 user,  Durchschnittslast: 0,00, 0,01, 0,04
# echo $[(3600*22+28*60)*1000] $[40347440+40582285]
80880000 80929725

Given that uptime is only accurate to the minute, this sounds very 
reasonable. The distribution also is close to 50:50. That's (almost) true 
for all interrupt sources:

 # cat /proc/interrupts 
           CPU0       CPU1       
  0:   40523846   40753939    IO-APIC-edge  timer
  1:          3        189    IO-APIC-edge  i8042
  8:        261        280    IO-APIC-edge  rtc
  9:          0          0   IO-APIC-level  acpi
 15:     364369     364479    IO-APIC-edge  ide1
169:      59195      55498   IO-APIC-level  3w-9xxx
177:     618198     604643   IO-APIC-level  3w-9xxx
185:    8195891    8147619   IO-APIC-level  aic79xx, eth1
193:          0         30   IO-APIC-level  aic79xx
201:          0          0   IO-APIC-level  ohci_hcd, ohci_hcd
NMI:       1184       1013 
LOC:   81273966   81271958 
ERR:          0
MIS:          0

-- 
Bernd Paysan
http://www.mikron.de/

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