linux-kernel - Re: [PATCH v2] panic: Remove redundant panic-cpu backtrace

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Message-ID: <aLj0FYAAzT6FwEgG@U-2FWC9VHC-2323.local>
Date: Thu, 4 Sep 2025 10:06:13 +0800
From: Feng Tang <feng.tang@...ux.alibaba.com>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	John Ogness <john.ogness@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] panic: Remove redundant panic-cpu backtrace

On Wed, Sep 03, 2025 at 12:04:18PM +0200, Petr Mladek wrote:
> From: Sergey Senozhatsky <senozhatsky@...omium.org>
> 
> Backtraces from all CPUs are printed during panic() when
> SYS_INFO_ALL_CPU_BT is set. It shows the backtrace for
> the panic-CPU even when it has already been explicitly
> printed before.
> 
> Do not change the legacy code which prints the backtrace
> in various context, for example, as part of Oops report,
> right after panic message. It will always be visible in
> the crash dump.
> 
> Instead, remember when the backtrace was printed, and skip
> it when dumping the optional backtraces on all CPUs.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> Link: https://lore.kernel.org/r/20250731030314.3818040-1-senozhatsky@chromium.org
> [pmladek@...e.com: Handle situations when the backtrace was not printed for the panic CPU.]
> Signed-off-by: Petr Mladek <pmladek@...e.com>

It worked as expected and had the similar effect as Sergey's
original patch, upon my test. Feel free to add:

Tested-by: Feng Tang <feng.tang@...ux.alibaba.com>

Thanks,
Feng

> ---
> Hi,
> 
> I resend my proposal [1] as a proper patch.
> 
> Changes against v1:
> 
>   - Handle situations when the backtrace was not printed for
>     the panic CPU.
> 
> [1] https://lore.kernel.org/all/aJs7p_UjPIfb_XYd@pathway/
> 
> kernel/panic.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 72fcbb5a071b..e3cec9bc05ef 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -67,6 +67,7 @@ static unsigned int warn_limit __read_mostly;
>  static bool panic_console_replay;
>  
>  bool panic_triggering_all_cpu_backtrace;
> +bool panic_this_cpu_backtrace_printed;
>  
>  int panic_timeout = CONFIG_PANIC_TIMEOUT;
>  EXPORT_SYMBOL_GPL(panic_timeout);
> @@ -328,6 +329,19 @@ void check_panic_on_warn(const char *origin)
>  		      origin, limit);
>  }
>  
> +static void panic_trigger_all_cpu_backtrace(void)
> +{
> +	/* Temporary allow non-panic CPUs to write their backtraces. */
> +	panic_triggering_all_cpu_backtrace = true;
> +
> +	if (panic_this_cpu_backtrace_printed)
> +		trigger_allbutcpu_cpu_backtrace(raw_smp_processor_id());
> +	else
> +		trigger_all_cpu_backtrace();
> +
> +	panic_triggering_all_cpu_backtrace = false;
> +}
> +
>  /*
>   * Helper that triggers the NMI backtrace (if set in panic_print)
>   * and then performs the secondary CPUs shutdown - we cannot have
> @@ -335,12 +349,8 @@ void check_panic_on_warn(const char *origin)
>   */
>  static void panic_other_cpus_shutdown(bool crash_kexec)
>  {
> -	if (panic_print & SYS_INFO_ALL_CPU_BT) {
> -		/* Temporary allow non-panic CPUs to write their backtraces. */
> -		panic_triggering_all_cpu_backtrace = true;
> -		trigger_all_cpu_backtrace();
> -		panic_triggering_all_cpu_backtrace = false;
> -	}
> +	if (panic_print & SYS_INFO_ALL_CPU_BT)
> +		panic_trigger_all_cpu_backtrace();
>  
>  	/*
>  	 * Note that smp_send_stop() is the usual SMP shutdown function,
> @@ -422,13 +432,15 @@ void vpanic(const char *fmt, va_list args)
>  		buf[len - 1] = '\0';
>  
>  	pr_emerg("Kernel panic - not syncing: %s\n", buf);
> -#ifdef CONFIG_DEBUG_BUGVERBOSE
>  	/*
>  	 * Avoid nested stack-dumping if a panic occurs during oops processing
>  	 */
> -	if (!test_taint(TAINT_DIE) && oops_in_progress <= 1)
> +	if (test_taint(TAINT_DIE) || oops_in_progress > 1) {
> +		panic_this_cpu_backtrace_printed = true;
> +	} else if (IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE)) {
>  		dump_stack();
> -#endif
> +		panic_this_cpu_backtrace_printed = true;
> +	}
>  
>  	/*
>  	 * If kgdb is enabled, give it a chance to run before we stop all
> -- 
> 2.50.1