Re: [PATCH v2] panic: add panic_force_cpu= parameter to redirect panic to a specific CPU

[Andrew Morton]

On Sun,  4 Jan 2026 22:42:10 +0200 Pnina Feder <pnina.feder@xxxxxxxxxxxx> wrote:

> Some platforms require panic handling to execute on a specific CPU for
> crash dump to work reliably. This can be due to firmware limitations,
> interrupt routing constraints, or platform-specific requirements where
> only a single CPU is able to safely enter the crash kernel.
> 
> Add support for redirecting panic execution to a designated CPU via a
> kernel command-line parameter.

Let's tell changelog readers what that command line parameter is called.

> When the parameter is provided, the CPU
> that initially triggers panic forwards the panic context to the target
> CPU, which then proceeds with the normal panic and kexec flow.
> 
> If the specified CPU is invalid, offline, or a panic is already in
> progress on another CPU, the redirection is skipped and panic continues
> on the current CPU.

Well I like it.  Others may not ;)

> Changes since v1:
>  - Replace Kconfig option with a kernel command-line parameter
>  - Fix clang format warning reported by kernel test robot
> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202601041820.6M8cIq2e-lkp@xxxxxxxxx/
> Signed-off-by: Pnina Feder <pnina.feder@xxxxxxxxxxxx>
> ---
>  kernel/panic.c | 93 ++++++++++++++++++++++++++++++++++++++++++++++++++

Please update Documentation/admin-guide/kernel-parameters.txt?

> +__printf(1, 0)
> +static bool panic_force_target_cpu(const char *fmt, va_list args)
> +{
> +	static char panic_redirect_msg[1024];

It's sad to chew 1k of everyone's RAM for this. 
smp_call_function_single() is synchronous, yes?  Can we reduce that
message a lot and use automatic storage?

Or perhaps kmalloc the storage if the user provided the panic_force_cpu
kernel parameter?

> +	int cpu = raw_smp_processor_id();
> +	int target_cpu = panic_force_cpu;
> +
> +	/* Feature not enabled via boot parameter */
> +	if (target_cpu < 0)
> +		return true;
> +
> +	/* Already on target CPU - proceed normally */
> +	if (cpu == target_cpu)
> +		return true;
> +
> +	/* Target CPU is offline, can't redirect */
> +	if (!cpu_online(target_cpu)) {
> +		pr_warn("panic: target CPU %d is offline, proceeding on CPU %d.\n"

"panic: <lowercase>..."

> +			"Crash kernel console output may be unavailable.\n", target_cpu, cpu);
> +		return true;
> +	}
> +
> +	/* Another panic already in progress */
> +	if (panic_in_progress()) {
> +		pr_warn("panic: Another panic in progress on CPU %d, cannot redirect to CPU %d.\n"

"panic: <uppercase>..."

boy, was that a nit!

> +			"Crash kernel console output may be unavailable.\n",
> +			atomic_read(&panic_cpu), target_cpu);
> +		return true;
> +	}
> +
> +	pr_info("panic: Redirecting from CPU %d to CPU %d for crash kernel\n",
> +		cpu, target_cpu);
> +
> +	vsnprintf(panic_redirect_msg, sizeof(panic_redirect_msg), fmt, args);
> +
> +	smp_call_function_single(target_cpu, do_panic_on_target_cpu, panic_redirect_msg, false);
> +
> +	return false;
> +}
> +#else
> +__printf(1, 0)
> +static inline bool panic_force_target_cpu(const char *fmt, va_list args)
> +{
> +	return true;
> +}
> +#endif /* CONFIG_SMP */
> +
>  bool panic_try_start(void)
>  {
>  	int old_cpu, this_cpu;