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SubjectRe: x86's nmi_hz wrt. oprofile's nmi_timer_int.c
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From: Andi Kleen <andi@firstfloor.org>
Date: Mon, 23 Feb 2009 05:52:00 +0100

> > Again, the code in nmi_timer_int.c doesn't.
> >
> > It uses the NMI watchdog timer interrupts, it catches DIE_NMI
> > events.
> >
> > > Does that answer your question?
> >
> > Not really.
>
> Ah see what you mean now. The nmi_timer_int code can only
> be active ever when the cpu is not known to nmi_int.c and
> when the nmi watchdog is in io apic mode. But IO apic mode
> doesn't use the fast check/slowdown because it always runs
> at HZ frequency. That only happens in LAPIC mode.
>
> The standard fallback mode for unknown CPU is the non NMI timer
> fallback in oprofile_init, the IO APIC mode happens near never in practice.

Look at the fallback logic, the pure NMI profiler can fail for
a number of reasons, not just because the watchdog is in
I/O APIC mode.

No matter what the failure reason, nmi_int.c is used.

And in some of those cases, the nmi_hz has been decreased to '1'.


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