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SubjectRe: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

* David Miller <davem@davemloft.net> wrote:

> While working on an NMI watchdog implementation on sparc64 I noticed
> what seems to be a peculiar behavior of the NMI timer int oprofile
> support on x86.
>
> When the NMI watchdog tests itself at boot timer we start with nmi_hz
> equal to HZ.
>
> After the NMI watchdog self-test passes, nmi_hz is reduced down to '1'.
>
> The NMI timer int oprofile support simply uses DIE_NMI notifiers for
> it's implementation. But I don't see anything in the code of
> arch/x86/oprofile/nmi_timer_int.c nor the NMI watchdog infrastructure
> which will re-adjust nmi_hz back to HZ or something similar.
>
> Am I missing something?

Reducing it to 1 HZ was kind of a performance hack: running NMIs at HZ
needlessly interrupts the CPU HZ times a second. It's more than enough to
have 1 nmi-watchdog tick per second to notice deadlocks that take longer
than 5 seconds.

Can you see a problem with that approach, or was this just a question
about why it's reduced to 1 Hz?

Ingo


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