linux-kernel - Re: [PATCH 3/3] [RFC] nmi_watchdog: config option to enable new nmi

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Date:	Tue, 2 Feb 2010 08:29:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, gorcunov@...il.com,
	aris@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] [RFC] nmi_watchdog: config option to enable new
 nmi_watchdog


* Don Zickus <dzickus@...hat.com> wrote:

> On Fri, Jan 29, 2010 at 09:12:27AM +0100, Ingo Molnar wrote:
> > 
> > * Don Zickus <dzickus@...hat.com> wrote:
> > 
> > > On Thu, Jan 28, 2010 at 03:54:54PM +0100, Peter Zijlstra wrote:
> > > > On Wed, 2010-01-27 at 15:03 -0500, Don Zickus wrote:
> > > > >
> > > > > These are the bits that enable the new nmi_watchdog and safely 
> > > > > isolate the old nmi_watchdog.  Only one or the other can run, not 
> > > > > both at the same time.
> > > > 
> > > > perf disables the lapic watchdog when it wants the pmu, so there 
> > > > shouldn't be a problem having both built in.
> > > 
> > > Yes it does disable but does not prevent nmi_watchdog_tick from running 
> > > nor the /proc interface from being loaded.  So perhaps my description 
> > > isn't very good.  The idea with the new watchdog was to re-use some of 
> > > the bits of the old one, but having them both compiled in seemed to 
> > > stomp on each other.  That is what I was trying to prevent.
> > > 
> > > I can certainly change the behaviour, just makes the code a little more 
> > > messy I think.
> > 
> > I think that's a good idea - and i think we want to be bold and just have 
> > the new code run seemlessly. (and fix bugs, if any.)
> 
> Ok.  I guess I am confused what you are suggesting here, to do as Peter 
> suggested and run both at the same time?

I dont think we want to run old and new code at once, the old NMI watchdog 
code is really a hardcoded minimal PMU driver generating a cycles based NMI 
tick once per second.

> > What do you think?
> 
> I will need to give you an updated patch that properly sets the frequency 
> of the NMI and I probably should still implement a code path that uses the 
> software perf counters in the cases where the hardware perf counters are 
> not available.
> 
> It seems like you are ok with my approach.  If that is so, I can test on 
> more machines to iron out some more bugs.  Or did you want to take my 
> patches as is and have me throw fixes on top?

Well, all known bugs/showstoppers should be fixed - but otherwise if you 
think it works fine we can certainly apply it and then iterate it from that 
point on to increase coverage and add features.

	Ingo
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