On July 2, 2026 7:13:26 PM GMT+01:00, Bradley Morgan <[email protected]> wrote: >On July 2, 2026 10:09:41 AM GMT+01:00, Petr Mladek <[email protected]> >wrote: >>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 >> > > >sup petr, here's my poc > > >This should make my entire thing make sense
Actually, looks like churn to me. we shouldn't do that. It is on that gist though. As in, I'm thinking about doing a new API to perhaps, make sys_info better? But it's very complicated, and may need discussion, feel free to ignore that POC... crappy old me, need a coffee heh. Thanks!
