Re: [PATCH] NMI watchdog documentation

[Ville Herva <vherva@niksula.hut.fi>]

On Wed, Jul 30, 2003 at 09:20:33PM +0200, you [Mikael Pettersson] wrote:
> On Tue, 29 Jul 2003 20:53:19 +0300, Ville Herva wrote:
> >On Tue, Jul 29, 2003 at 06:06:30PM +0200, you [Andi Kleen] wrote:
> >> > Andi, you have the numbers mixed up. mode 1 is I/O-APIC, mode 2 is local APIC,
> >> > and x86-64 defaults nmi_watchdog to I/O-APIC mode.
> >> > Now, is it the I/O-APIC or local APIC watchdog that doesn't work in x86-64?
> >> 
> >> Right, 1 and 2 need to be exchanged. Anyways local apic mode does not seem
> >> to work, the kernel always reportss "NMI stuck" at bootup.
> >> IO APIC mode for is default.
> ...
> >+For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
> >+always enabled with perfctr mode. Currently, mode=1 does not work on x86-64.
> 
> Didn't Andi just say it's the other way around? nmi_watchdog=1 (I/O-APIC)
> by default since nmi_watchdog=2 (local APIC) doesn't work.

Ok, you got me confused (thankfully I didn't submit anything for inclusion
yet. :)

Initially, Andi said:

http://marc.theaimsgroup.com/?l=linux-kernel&m=105941508314399&w=2
> x86-64 is the same, except APIC is always compiled in and the nmi watchdog
> is always enabled with perfctr mode. mode=2 seems to also not work
> correctly currently.
>
> However one caveat (even for i386): when you use perfctr mode 1 you lose
> the first performance register which you may need for other things.

To which I replied:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105942026020567&w=2
> So, is something like the following ok by you
>
> +For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
> +always enabled with perfctr mode. Currently, mode=2 does not work on x86-64.
> +
> +Using NMI watchdog (in mode=1) needs the first performance register, so you
> +can't use it for other purposes (such as high precision performance
> +profiling.)

But you pointed out it was the other way around:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105947532631384&w=2
> Andi, you have the numbers mixed up. mode 1 is I/O-APIC, mode 2 is local
> APIC, and x86-64 defaults nmi_watchdog to I/O-APIC mode. Now, is it the
> I/O-APIC or local APIC watchdog that doesn't work in x86-64?

And Andi agreed:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105949540722325&w=2
> Right, 1 and 2 need to be exchanged. Anyways local apic mode does not seem
> to work, the kernel always reportss "NMI stuck" at bootup. IO APIC mode
> for is default.

So I proposed (blindly exchanging the numbers):
http://marc.theaimsgroup.com/?l=linux-kernel&m=105950174531125&w=2
> +For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
> +always enabled with perfctr mode. Currently, mode=1 does not work on x86-64.
> +
> +Using NMI watchdog (in mode=2) needs the first performance register, so you
> +can't use it for other purposes (such as high precision performance
> +profiling.)

So... Should it be something like:

+For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
+always enabled with perctr mode. Currently, mode=2 (local APIC) does not
+work on x86-64. IO APIC mode (mode=1) is the default. Using NMI watchdog
+(mode=1) needs the first performance register, so you can't use it for
+other purposes (such as high precision performance profiling.)

(Is the last sentence only valid for x86-64?)


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