Re: [PATCH v3 4/4] panic: use sys_info_with_filter() to avoid duplicate backtraces

[Petr Mladek <pmladek@suse.com>]

On Mon 2026-06-29 13:54:18, Bradley Morgan wrote:
> On 29 June 2026 12:40:52 BST, Feng Tang <feng.tang@linux.alibaba.com>
> wrote:
> >On Fri, Jun 26, 2026 at 02:14:14PM +0200, Petr Mladek wrote:
> >> On Fri 2026-06-26 12:23:50, Petr Mladek wrote:
> >> > On Thu 2026-06-25 15:25:58, Bradley Morgan wrote:
> >> > > panic_other_cpus_shutdown() handles SYS_INFO_ALL_BT before stopping
> >the
> >> > > other CPUs. Do not ask sys_info() to handle that bit again later in
> >the
> >> > > panic path.
> >> > > 
> >> > > Use sys_info_with_filter() so panic_print=all_bt does not request
> >more
> >> > > output after the CPUs are stopped.
> >> > > 
> >> > > Fixes: a9af76a78760 ("watchdog: add sys_info sysctls to dump sys
> >info on system lockup")
> >> > > Cc: stable@vger.kernel.org
> >> > > Signed-off-by: Bradley Morgan <include@grrlz.net>
> >> > > ---
> >> > >  kernel/panic.c | 2 +-
> >> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> > > 
> >> > > diff --git a/kernel/panic.c b/kernel/panic.c
> >> > > index 213725b612aa..eb842823df61 100644
> >> > > --- a/kernel/panic.c
> >> > > +++ b/kernel/panic.c
> >> > > @@ -680,7 +680,7 @@ void vpanic(const char *fmt, va_list args)
> >> > >  	 */
> >> > >  	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
> >> > >  
> >> > > -	sys_info(panic_print);
> >> > > +	sys_info_with_filter(panic_print, SYS_INFO_ALL_BT);
> >> > 
> >> > Hmm, this prevents printing backtraces from all CPUs completely.
> >> > But what if they were not printed?
> >> > 
> >> > They might be printed by:
> >> > 
> >> > static void panic_other_cpus_shutdown(bool crash_kexec)
> >> > {
> >> > 	if (panic_print & SYS_INFO_ALL_BT)
> >> > 		panic_trigger_all_cpu_backtrace();
> >> > 
> >> > [...]
> >> > }
> >> > 
> >> > But it checks only "panic_print" variable. It won't do anything
> >> > when (panic_print == 0).
> >> > 
> >> > In this case, we might still want to print the backraces when
> >> > SYS_INFO_ALL_BT is set in kernel_si_info.
> >> > 
> >> > >  	kmsg_dump_desc(KMSG_DUMP_PANIC, buf);
> >> > 
> >> > Of course, we might fix panic_other_cpus_shutdown() to check also
> >> > kernel_si_info.
> >> > 
> >> > But it all becomes very hairy. We have several levels:
> >> > 
> >> >    + watchdog-all_bt-specific option, e.g.
> >sysctl_hardlockup_all_cpu_backtrace
> >> > 
> >> >    + watchdog-specific si_info preferences, e.g. hardlockup_si_mask
> >> > 
> >> >    + panic-specific si_info: panic_print
> >> > 
> >> >    + universal fallback for any layer: kernel_si_info
> >> > 
> >> > Now, we try to check all these variables back and forth to
> >> > trigger all backtraces or to avoid triggering them.
> >> > And it clearly does not work well and the code is more and more
> >> > hairy.
> >> > 
> >> > I think about another approach. The word "waterfall" comes to my mind.
> >> > Instead of checking all the settings back and forth, let's process
> >> > each setting one by one and just remember what has been done and
> >> > skip this in the next level.
> >> > 
> >> > All the si_info actions seems to dump a global system state.
> >> > So, it would make sense to remember the state in a global variable
> >> > even when it might be modified by more CPUs in parallel.
> >> > 
> >> > I am going to think more about it.
> >> 
> >> I have created a POC using Gemini. I haven't tested it.
> >> But it looks acceptable. And the logic seems to be more
> >> straightforward.
> >> 
> >> One drawback is that it requires adding the _reset()
> >> call for all sys_info() callers. It is fine in principle
> >> but it might complicate back-porting because all changes
> >> have to be done in one patch.
> >> 
> >> But honestly, this is a nice to have fix. Most people could
> >> live happily without it.
> >> 
> >> From 3c66436d9978030845a96bfaedd6b914536e2ac4 Mon Sep 17 00:00:00 2001
> >> From: Petr Mladek <pmladek@suse.com>
> >> Date: Fri, 26 Jun 2026 13:55:41 +0200
> >> Subject: [POC] sys_info: Introduce state-tracking APIs to prevent
> >duplicate
> >>  backtraces
> >> 
> >> In watchdog, panic, and hung task detection scenarios, sys_info() can
> >> be called multiple times or alongside direct backtrace triggers like
> >> trigger_allbutcpu_cpu_backtrace(). This results in identical backtraces
> >> being dumped repeatedly from all CPUs, cluttering the kernel log and
> >> delaying or obscuring critical debug details.
> >> 
> >> Introduce a state tracking bitmask and associated helpers:
> >> - sys_info_done(mask): Marks specific sys_info bits as already printed.
> >> - sys_info_reset(): Resets the tracking state.
> >> - sys_info_is_done(mask): Checks if all bits in the mask have been
> >printed.
> >> 
> >> Update sys_info() to automatically filter out already printed bits
> >> using this state. Integrate these APIs with the generic hardlockup
> >> and softlockup watchdogs, the PowerPC watchdog, the hung task detector,
> >> and the panic core. This ensures that each piece of system information
> >> and backtrace output is printed at most once per lockup/panic event,
> >> and the state is reset cleanly when a lockup does not trigger a panic.
> >> 
> >> Races between sys_info() callers are ignored. It should be acceptable
> >> because the output from various watchdogs has never been synchronized.
> >> And panic() never returns.
> >> 
> >> Assisted-by: gemini-1.5-flash
> >> Signed-off-by: Petr Mladek <pmladek@suse.com>
> >
> >Yep. There are cases that people want panic on task-hung or sw/hw lockup,
> >and this could remove much duplication of sys info dump, thanks!
> >
> >Reviewed-by: Feng Tang <feng.tang@linux.alibaba.com>
> 
> Thanks,
> 
> im feeling a new file to do all the force panic jazz, but putting tape
> on sys_info.c isn't bd either.

I wonder how to move forward with this.

Honestly, I am not sure what exactly you mean by creating another
API for tracking the reports so I could not judge it. Feel free
to sent some POC.

Otherwise, I would go with my proposal to remember the printed states
by the sys_info API. I am not sure whether I should send a proper
patch or you would like to somehow improve it.

Best Regards,
Petr