linux-kernel - Re: Linux 2.6.21-rc5

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Message-ID: <Pine.LNX.4.64.0703281051180.6730@woody.linux-foundation.org>
Date:	Wed, 28 Mar 2007 10:56:37 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <ak@...e.de>
cc:	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>
Subject: Re: Linux 2.6.21-rc5

On Wed, 28 Mar 2007, Andi Kleen wrote:
> 
> Can you test this patch please? 

This patch is totally broken.

> i386/x86-64: Convert nmi reservation to be global
> 
> It doesn't make much sense to have this per CPU, because all
> the services using NMIs run on all CPUs. So make it global.

NO!

If you do this, then you must make all *callers* be global too. But they 
aren't. Right now all callers do per-CPU setup!

See for example enable_lapic_nmi_watchdog():

	on_each_cpu(setup_apic_nmi_watchdog, NULL, 0, 1);

where "setup_apic_nmi_watchdog()" will call "setup_k7_watchdog()", which 
in turn will do a per-CPU reservation of the perfctl for the watchdog.

So I agree in that it probably doesn't make sense to have NMI/perfctl 
reservation per-CPU, but you can't just change the reservation and ignore 
all the *users* of that reservation that assumed that it was per-CPU.

Is that code insane? Probably. But it probably also works. After your 
patch, one CPU will be able to reserve the NMI/perfctl thing (fine so far) 
but then all the other CPU's that try to do it will fail.

			Linus
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