Re: [PATCH v2] x86/nmi: Enable panic() when NMI handler runs too long

[Borislav Petkov]

On Mon, Oct 19, 2026 at 01:44:30PM -0700, Paul E. McKenney wrote:
> Firmware, device-driver, and configuration issues can result in
> long-running NMI handlers, for example, nmi_cpu_backtrace_handler()
> running for more than two seconds, and with no console issues.  In this
> case, this is not the fault of nmi_cpu_backtrace_handler(), but it is
> the first symptom of the underlying problem.  These NMI handlers result
> in various other problems, including CSD-lock warnings, clocksource
> watchdog false positives, scheduler frequency invariance wobbliness,
> systemd excessive-CPU complaints, and even the occasional RCU CPU
> stall warning.  Eventually, the automation will take the system out of
> service, but with quite a bit of noise obscuring the underlying issue,
> and thus confusing both automation and human beings.
> 
> Therefore, provide a toolong_nmi_panic __setup() parameter that makes
> nmi_check_duration() invoke nmi_panic() if the just-completed NMI handler
> ran for more than the specified number of milliseconds.  This parameter
> defaults to zero, which disables the nmi_panic().  As in, by default
> you get the same behavior that you get today.
> 
> [ paulmck: Apply kernel test robot feedback, add documentation. ]
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: <x86@xxxxxxxxxx>
> 
> ---
>  Documentation/admin-guide/kernel-parameters.txt |    6 ++++++
>  arch/x86/kernel/nmi.c                           |   21 +++++++++++++++++++--
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> Changes since v1:
> 
> o	Add documentation of new toolong_nmi_panic kernel boot parameter.
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 68647ff4bdd24..e7dbd5bf48fc9 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -7642,6 +7642,12 @@ Kernel parameters
>  			Format: {ipi}
>  			ipi: switch to IPI-based TLB flushing
>  
> +	toolong_nmi_panic= [X86]
> +			Cause a panic upon return from an NMI handler
> +			that took longer than the specified number
> +			of milliseconds.  The default value of zero
> +			suppresses these panics.

Cause a panic after an NMI?!

Oh boy, is this trying to solve a Meta fleet problem and you're thinking, why
not upstream it too?

Because I fail to use the usefulness of this...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette