On Mon 2026-06-29 13:54:18, Bradley Morgan wrote: > On 29 June 2026 12:40:52 BST, Feng Tang <[email protected]> > 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: [email protected] > >> > > Signed-off-by: Bradley Morgan <[email protected]> > >> > > --- > >> > > 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 <[email protected]> > >> 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 <[email protected]> > > > >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 <[email protected]> > > 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
