Date: Wed, 4 Nov 2009 16:56:41 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Aristeu Rozanski <aris@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] x86: introduce NMI_AUTO as nmi_watchdog option
Message-ID: <20091104155641.GA11444@elte.hu>
References: <20091103171054.GB25437@redhat.com> <20091104114630.GA16993@elte.hu> <20091104151929.GH18613@redhat.com>
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* Aristeu Rozanski <aris@redhat.com> wrote:

> > > 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 ;-)

	Ingo
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