linux-kernel - Re: printk: selective deactivation of feature ignoring non panic cpu's messages

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Message-ID: <Z8cLEkqLL2IOyNIj@pathway>
Date: Tue, 4 Mar 2025 15:15:46 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Donghyeok Choe <d7271.choe@...sung.com>, 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: printk: selective deactivation of feature ignoring non panic
 cpu's messages

On Tue 2025-03-04 15:05:52, John Ogness wrote:
> On 2025-03-04, Petr Mladek <pmladek@...e.com> wrote:
> > I mean something like:
> >
> > --- a/kernel/printk/printk_ringbuffer.c
> > +++ b/kernel/printk/printk_ringbuffer.c
> > @@ -2143,7 +2143,9 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
> >  			 * But it would have the sequence number returned
> >  			 * by "prb_next_reserve_seq() - 1".
> >  			 */
> > -			if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
> > +			if (this_cpu_in_panic() &&
> > +			    (!printk_debug_non_panic_cpus || legacy_allow_panic_sync) &&
> > +			    ((*seq + 1) < prb_next_reserve_seq(rb)))
> >  				(*seq)++;
> >  			else
> >  				return false;
> 
> Ah, OK. Thanks for the clarification
> 
> > OK, I propose the following changes:
> >
> >   + rename the option to "printk_debug_non_panic_cpus"
> >
> >   + do not skip the messages in _prb_read_valid() when this option
> >     is used before the non-panic CPUs are stopped.
> 
> And of course:
> 
>     + allow non-panic CPUs in panic to store messages when this option
>       is set

Yes.

> I would also keep the dump_stack_lvl() implementation as it is, even if
> it could lead to interweaving of backtraces. Anyone using
> printk_debug_non_panic_cpus should have CONFIG_PRINTK_CALLER enabled.

I agree.

Best Regards,
Petr