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

[Bradley Morgan]

On 29 June 2026 12:40:52 BST, Feng Tang <feng.tang@xxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx
>> > > Signed-off-by: Bradley Morgan <include@xxxxxxxxx>
>> > > ---
>> > >  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@xxxxxxxx>
>> 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@xxxxxxxx>
>
>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@xxxxxxxxxxxxxxxxx>

Thanks,

im feeling a new file to do all the force panic jazz, but putting tape
on sys_info.c isn't bd either.

>> ---
>>  arch/powerpc/kernel/watchdog.c | 13 ++++++++++---
>>  include/linux/sys_info.h       |  3 +++
>>  kernel/hung_task.c             |  2 ++
>>  kernel/panic.c                 |  4 +++-
>>  kernel/watchdog.c              | 10 ++++++++--
>>  lib/sys_info.c                 | 30 +++++++++++++++++++++++++++++-
>>  6 files changed, 55 insertions(+), 7 deletions(-)
>

Thanks!