Re: NMI handling rework for x86

[John Levon]

On Fri, Nov 15, 2002 at 08:13:51AM -0600, Corey Minyard wrote:

> I don't think that's a good idea for two reasons:
> 
>    * If the oprofile code is only using the counter that the NMI
>      watchdog is not using, it will silently cause the NMI watchdog to
>      stop working.  I know that's not the case now, but it could be in
>      the future.

Uh, this is fine. We always call the NMI watchdog handler, so it will
see apic irqs get stuck, and work anyway.

>    * The oprofile code will always reset the counter, so the NMI
>      watchdog will never see the timeout, so it doesn't matter.

wrong. If we are using counters 0 and 1, and 1 overflows, oprofile
resets that, then 0 overflows, the NMI watchdog will see it and
incorrectly reset it. You HAVE to avoid the reset - you can test it if
you don't believe me.

> It's currently kind of an unnatural relationship.  IMHO, it would be 
> better to have a separate handler for the perf counters that they both 
> use.  But that's beyond the scope of this right now.

Yes.

> +/* This is for I/O APIC, until we can figure out how to tell if it's from the
> +   I/O APIC.  If the NMI  was not handled before now, we handle it. */
> +static int dummy_watchdog_reset(int handled)
> +{
> +	return !handled;
> +}

And if it was handled previously, you reset it to not handled ? Uh ?

regards
john