linux-kernel - Re: [PATCH 1/2] Always probe the NMI watchdog

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Message-Id: <200706260804.l5Q84F2D017936@harpo.it.uu.se>
Date:	Tue, 26 Jun 2007 10:04:15 +0200 (MEST)
From:	Mikael Pettersson <mikpe@...uu.se>
To:	B.Steinbrink@....de, eranian@....hp.com
Cc:	ak@...e.de, ingo@...e.hu, levon@...ementarian.org,
	linux-kernel@...r.kernel.org, oprofile-list@...ts.sourceforge.net,
	perfmon@...ali.hpl.hp.com, wcohen@...hat.com
Subject: Re: [PATCH 1/2] Always probe the NMI watchdog

On Mon, 25 Jun 2007 23:04:25 +0200, B.Steinbrink@....de wrote:
> > I think the tricky part is that we do want to reserve perfctr1 even
> > though the NMI watchdog is not active. This comes from the fact that
> > the NMI watchdog knows about only one counter and if it can't get that
> > one, it probably fails.  By reserving it from the start, we ensure NMI
> > watchdog will work when eventually activated.
> 
> Can you enable it later on at all? It failed for me when I tried,
> because it didn't know which hardware to use. Had to pass the kernel
> parameter to make the proc files do anything. Seems like it has to be
> enable at boot to work at all.
> 
> And AFAICT we never unconditionally reserved a perfctr for the watchdog.

Yes you can dynamically enable/disable the NMI watchdog,
at least if you booted with it enabled.

> In 2.6.21 the nmi watchdog, if enabled, just reserved its perfctrs and
> everything else had to deal with it. Since the cleanup, the watchdog
> will release its perfctr when disabled, so another subsystem can grab
> it. But that also means that that other subsystem must release it again
> before you can reenable the watchdog.

Which is the obvious and correct way to handle a shared resource.

Keeping parts of the PMU HW permanently reserved whether or not
the watchdog is enabled would be a BUG.

/Mikael
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