Re: [PATCH 2/2] x86: introduce NMI_AUTO as nmi_watchdog option

From: Ingo Molnar
Date: Wed Nov 04 2009 - 12:43:16 EST

* Valdis.Kletnieks@xxxxxx <Valdis.Kletnieks@xxxxxx> wrote:

> On Wed, 04 Nov 2009 12:46:30 +0100, Ingo Molnar said:
> > What i'd like to see for the NMI watchdog is much more ambitious
> > than this: the use of perf events to run a periodic NMI callback.
> >
> > The NMI watchdog would cause the creation of a per-cpu perf_event
> > structure (in-kernel). All x86 CPUs that have perf event support
> > (the majority of them) will thus be able to have an NMI watchdog
> > using a nice, generic piece of code and we'd be able to phase out
> > the open-coded NMI watchdog code.
> What happens on older/smaller x86 CPUs that don't have any native
> support for perf events?

I think we want to keep their NMI watchdog implementation - but new work
should go towards a perf-events based NMI watchdog.

Note that in practice a working NMI watchdog implementation on an
old/small x86 CPU can be taken and turned into a minimal PMU driver: one
that can provide cycle based NMI events. There's no 'full' PMU support
needed to get a fair amount of perf events functionality - and some of
those CPUs dont even have a PMU.

So there's no hardware barrier of entry.

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