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. >> --- >> 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!
