Re: [PATCH v4] panic: add panic_force_cpu= parameter to redirect panic to a specific CPU

[Baoquan He <bhe@redhat.com>]

On 01/07/26 at 11:56pm, Pnina Feder 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,

Thanks for fixing this. Could you kindly reveal which platform this
issue is from?

> interrupt routing constraints, or platform-specific requirements where
> only a single CPU is able to safely enter the crash kernel.
> 
> Add the panic_force_cpu= kernel command-line parameter to redirect panic
> execution to a designated CPU. When the parameter is provided, the CPU
> that initially triggers panic forwards the panic context to the target
> CPU via IPI, 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.

What if both the original CPU and specified CPU are not able to function
well on panic jumping? The crash dumping will fail in this case?

> 
> Changes since v3:
>  - Dump original CPU's stack before redirecting to preserve debug info
>  - Add Documentation/admin-guide/kernel-parameters.txt entry
>  - Use smp_call_function_single_async() to avoid blocking in csd_lock()
>  - Add CONFIG_CRASH_DUMP dependency
>  - Reuse vpanic()'s static buffer instead of separate allocation
>  - Remove verbose warning messages
> 
> Signed-off-by: Pnina Feder <pnina.feder@mobileye.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  13 +++
>  kernel/panic.c                                | 104 ++++++++++++++++++
>  2 files changed, 117 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a8d0afde7f85..561ce09a6ae1 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4753,6 +4753,19 @@ Kernel parameters
>  	panic_on_warn=1	panic() instead of WARN().  Useful to cause kdump
>  			on a WARN().
>  
> +	panic_force_cpu=
> +			[KNL,SMP] Force panic handling to execute on a specific CPU.
> +			Format: <cpu number>
> +			Some platforms require panic handling to occur on a
> +			specific CPU for the crash kernel to function correctly.
> +			This can be due to firmware limitations, interrupt routing
> +			constraints, or platform-specific requirements where only
> +			a particular CPU can safely enter the crash kernel.
> +			When set, panic() will redirect execution to the specified
> +			CPU before proceeding with the normal panic and kexec flow.
> +			If the target CPU is offline or unavailable, panic proceeds
> +			on the current CPU.
> +
>  	panic_print=	Bitmask for printing system info when panic happens.
>  			User can chose combination of the following bits:
>  			bit 0: print all tasks info
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 0d52210a9e2b..414ef60d2306 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -300,6 +300,103 @@ void __weak crash_smp_send_stop(void)
>  
>  atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
>  
> +#if defined(CONFIG_SMP) && defined(CONFIG_CRASH_DUMP)
> +/* CPU to redirect panic to, or -1 if disabled */
> +static int panic_force_cpu = -1;
> +static call_single_data_t panic_csd;
> +
> +static int __init panic_force_cpu_setup(char *str)
> +{
> +	int cpu;
> +
> +	if (!str)
> +		return -EINVAL;
> +
> +	if (kstrtoint(str, 0, &cpu) || cpu < 0) {
> +		pr_warn("panic_force_cpu: invalid value '%s'\n", str);
> +		return -EINVAL;
> +	}
> +
> +	panic_force_cpu = cpu;
> +	return 0;
> +}
> +early_param("panic_force_cpu", panic_force_cpu_setup);
> +
> +static void do_panic_on_target_cpu(void *info)
> +{
> +	panic("%s", (char *)info);
> +}
> +
> +/**
> + * panic_force_target_cpu - Redirect panic to a specific CPU for crash kernel
> + * @fmt: panic message format string
> + * @args: arguments for format string
> + *
> + * Some platforms require panic handling to occur on a specific CPU
> + * for the crash kernel to function correctly. This function redirects
> + * panic handling to the CPU specified via the panic_redirect_cpu= boot parameter.
> + *
> + * Returns true if panic should proceed on current CPU.
> + * Returns false (never returns) if panic was redirected.
> + */
> +__printf(3, 0)
> +static bool panic_force_target_cpu(char *buf, int buf_size, const char *fmt, va_list args)
> +{
> +	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))
> +		return true;
> +
> +	/* Another panic already in progress */
> +	if (panic_in_progress())
> +		return true;
> +
> +	vsnprintf(buf, buf_size, fmt, args);
> +
> +	console_verbose();
> +	bust_spinlocks(1);
> +
> +	pr_emerg("panic: Redirecting from CPU %d to CPU %d for crash kernel\n",
> +		cpu, target_cpu);
> +
> +	/* Dump original CPU's stack before redirecting */
> +	if (test_taint(TAINT_DIE) || oops_in_progress > 1) {
> +		panic_this_cpu_backtrace_printed = true;
> +	} else if (IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE)) {
> +		dump_stack();
> +		panic_this_cpu_backtrace_printed = true;
> +	}
> +
> +	printk_legacy_allow_panic_sync();
> +	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
> +
> +	panic_csd.func = do_panic_on_target_cpu;
> +	panic_csd.info = buf;
> +
> +	/* Use smp_call_function_single_async() to avoid blocking in csd_lock(). */
> +	if (smp_call_function_single_async(target_cpu, &panic_csd) != 0)
> +		return true;
> +
> +	return false;
> +}
> +#else
> +__printf(3, 0)
> +static inline bool panic_force_target_cpu(char *buf, int buf_size, const char *fmt, va_list args)
> +{
> +	return true;
> +}
> +#endif /* CONFIG_SMP && CONFIG CRASH_DUMP */
> +
>  bool panic_try_start(void)
>  {
>  	int old_cpu, this_cpu;
> @@ -451,6 +548,13 @@ void vpanic(const char *fmt, va_list args)
>  	local_irq_disable();
>  	preempt_disable_notrace();
>  
> +	/*
> +	 * Redirect panic to target CPU if configured via panic_force_cpu=.
> +	 * Returns false and never returns if panic was redirected.
> +	 */
> +	if (!panic_force_target_cpu(buf, sizeof(buf), fmt, args))
> +		panic_smp_self_stop();
> +
>  	/*
>  	 * It's possible to come here directly from a panic-assertion and
>  	 * not have preempt disabled. Some functions called from here want
> -- 
> 2.43.0
>