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Date:	Mon, 9 Nov 2009 15:02:02 -0500
From:	Aristeu Rozanski <aris@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86: introduce NMI_AUTO as nmi_watchdog option

> > > > NMI_AUTO is a new nmi_watchdog option that makes LAPIC be tried first 
> > > > and if the CPU isn't supported, IOAPIC will be used. It's useful in 
> > > > cases where NMI watchdog is enabled by default in a kernel built for 
> > > > different machines. It can be configured by default or selected with 
> > > > nmi_watchdog=3 or nmi_watchdog=auto parameters.
> > > 
> > > 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.
> > > 
> > > The user would not notice much from this: we'd still have the 
> > > /proc/sys/kernel/nmi_watchdog toggle to turn it on/off, and we'd still 
> > > have the nmi_watchog= boot parameter as well. But the underlying 
> > > implementation would be far more generic and far more usable than the 
> > > current code.
> > > 
> > > Would you be interested in moving the NMI watchdog code in this 
> > > direction? Most of the perf events changes (callbacks, helpers for 
> > > in-kernel event allocations, etc.) are in latest -tip already, so you 
> > > could use that as a base.
> >
> > but that would work only for LAPIC. You're suggesting killing IOAPIC 
> > mode too?
> 
> Would it be a big loss, with all modern systems expected to have a 
> working lapic based NMI source? I wrote the IOAPIC mode originally but i 
> dont feel too attached to it ;-)
ok, fair enough. but since it'll be another implementation, do you mind
applying the patches I submitted so they can be used until the new
implementation is in place?

-- 
Aristeu

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