linux-kernel - Re: [patch] disable NMI watchdog by default

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Date:	Mon, 5 Mar 2007 12:42:45 -0500
From:	Len Brown <lenb@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Adrian Bunk <bunk@...sta.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [patch] disable NMI watchdog by default

On Monday 05 March 2007 07:20, Ingo Molnar wrote:
> 
> Linus,
> 
> Andrew sent the patch below (which is now months old and has been in -mm 
> for some time) towards Andi's tree 4 weeks ago, but apparently it fell 
> into a black hole there - the patch is still not upstream!
> 
> This is a must-have for v2.6.21

I agree.
There are multiple machines not booting because of nmi_watchdog.
Some of them are documented here:
http://bugzilla.kernel.org/show_bug.cgi?id=7839

We used to think this was the "nolapic" bug,
but it is actually the "nmi_watchdog=0" bug.

-Len

> Frankly, i find it ridiculous that i had to write more than 10 emails 
> about this stupid topic already, while i'm the original author of this 
> feature to begin with. The "do not enable by default debug features that 
> break certain systems" concept is obvious to me.
> 
> This category of regressions has been introduced by Andi when he made 
> the NMI watchdog the default in certain scenarios at around v2.6.18 - 
> over my repeated objections. Andi Cc:-ed :-)
> 
> 	Ingo
> 
> ----- Forwarded message from Ingo Molnar <mingo@...e.hu> -----
> 
> From: Ingo Molnar <mingo@...e.hu>
> Subject: [patch] disable NMI watchdog by default
> 
> there's a new NMI watchdog related problem: KVM crashes on certain 
> bzImages because ... we enable the NMI watchdog by default (even if the 
> user does not ask for it) , and no other OS on this planet does that so 
> KVM doesnt have emulation for that yet. So KVM injects a #GP, which 
> crashes the Linux guest:
> 
>  general protection fault: 0000 [#1]
>  PREEMPT SMP
>  Modules linked in:
>  CPU:    0
>  EIP:    0060:[<c011a8ae>]    Not tainted VLI
>  EFLAGS: 00000246   (2.6.20-rc5-rt0 #3)
>  EIP is at setup_apic_nmi_watchdog+0x26d/0x3d3
> 
> and no, i did /not/ request an nmi_watchdog on the boot command line!
> 
> Solution: turn off that darn thing! It's a debug tool, not a 'make life 
> harder' tool!!
> 
> with this patch the KVM guest boots up just fine.
> 
> And with this my laptop (Lenovo T60) also stopped its sporadic hard 
> hanging (sometimes in acpi_init(), sometimes later during bootup, 
> sometimes much later during actual use) as well. It hung with both 
> nmi_watchdog=1 and nmi_watchdog=2, so it's generally the fact of NMI 
> injection that is causing problems, not the NMI watchdog variant, nor 
> any particular bootup code.
> 
> The patch is unintrusive.
> 
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> 
> Index: linux/include/asm-i386/nmi.h
> ===================================================================
> --- linux.orig/include/asm-i386/nmi.h
> +++ linux/include/asm-i386/nmi.h
> @@ -33,7 +33,7 @@ extern int nmi_watchdog_tick (struct pt_
>  
>  extern atomic_t nmi_active;
>  extern unsigned int nmi_watchdog;
> -#define NMI_DEFAULT     -1
> +#define NMI_DEFAULT     0
>  #define NMI_NONE	0
>  #define NMI_IO_APIC	1
>  #define NMI_LOCAL_APIC	2
> Index: linux/include/asm-x86_64/nmi.h
> ===================================================================
> --- linux.orig/include/asm-x86_64/nmi.h
> +++ linux/include/asm-x86_64/nmi.h
> @@ -63,7 +63,7 @@ extern int setup_nmi_watchdog(char *);
>  
>  extern atomic_t nmi_active;
>  extern unsigned int nmi_watchdog;
> -#define NMI_DEFAULT	-1
> +#define NMI_DEFAULT	0
>  #define NMI_NONE	0
>  #define NMI_IO_APIC	1
>  #define NMI_LOCAL_APIC	2
> 
> -
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