linux-kernel - Re: [PATCH v2] printk/panic: Add option to allow non-panic CPUs to write to the ring buffer.

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Message-ID: <Z9wz9Wk28lmvy4Dg@pathway.suse.cz>
Date: Thu, 20 Mar 2025 16:27:49 +0100
From: Petr Mladek <pmladek@...e.com>
To: Donghyeok Choe <d7271.choe@...sung.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org, takakura@...inux.co.jp,
	youngmin.nam@...sung.com, hajun.sung@...sung.com,
	seungh.jung@...sung.com, jh1012.choi@...sung.com
Subject: Re: [PATCH v2] printk/panic: Add option to allow non-panic CPUs to
 write to the ring buffer.

On Tue 2025-03-18 11:23:20, Donghyeok Choe wrote:
> Commit 779dbc2e78d7 ("printk: Avoid non-panic CPUs writing to ringbuffer")
> aimed to isolate panic-related messages. However, when panic() itself
> malfunctions, messages from non-panic CPUs become crucial for debugging.
> 
> While commit bcc954c6caba ("printk/panic: Allow cpu backtraces to
> be written into ringbuffer during panic") enables non-panic CPU
> backtraces, it may not provide sufficient diagnostic information.
> 
> Introduce the "printk_debug_non_panic_cpus" command-line option,
> enabling non-panic CPU messages to be stored in the ring buffer during
> a panic. This also prevents discarding non-finalized messages from
> non-panic CPUs during console flushing, providing a more comprehensive
> view of system state during critical failures.
> 
> Link: https://lore.kernel.org/all/Z8cLEkqLL2IOyNIj@pathway/
> Signed-off-by: Donghyeok Choe <d7271.choe@...sung.com>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> Link: https://lore.kernel.org/all/20250305044046.1249972-1-d7271.choe@samsung.com/

I was about to push this patch but I have realized that we forgot
to update Documentation/admin-guide/kernel-parameters.txt

Also I found that the prefix "printk_" did not fit well with
the existing options. There were only module parameters
which started with "printk." prefix.

So, I decided to remove the "printk_" prefix and add also
the module_paramter, so that we have both:

early_param("debug_non_panic_cpus", debug_non_panic_cpus_setup);
module_param(debug_non_panic_cpus, bool, 0644);
MODULE_PARM_DESC(debug_non_panic_cpus,
		 "allow messages from non-panic CPUs in panic()");

It is similar to "ignore_loglevel".

I had a dilemma whether to send it for review or just push it.
I decided to push it because nobody did really care about
the name and the changes were straightforward. Also it did not
make much sense to miss the upcoming merge window just
because of these cosmetic issues.


Result:

The patch has been comitted into printk/linux.git,
branch for-6.15, see
https://web.git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git/commit/?h=for-6.15&id=c1aa3daa517292303d98ff61f0440c354669f948

Feel free to ask me take it back. I am going to wait few days
before sending pull request for 6.15.

Best Regards,
Petr