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H A D | watchdog.c | 1bc503cb Fri Apr 30 00:54:30 CDT 2021 Petr Mladek <pmladek@suse.com> watchdog/softlockup: remove logic that tried to prevent repeated reports
The softlockup detector does some gymnastic with the variable soft_watchdog_warn. It was added by the commit 58687acba59266735ad ("lockup_detector: Combine nmi_watchdog and softlockup detector").
The purpose is not completely clear. There are the following clues. They describe the situation how it looked after the above mentioned commit:
1. The variable was checked with a comment "only warn once".
2. The variable was set when softlockup was reported. It was cleared only when the CPU was not longer in the softlockup state.
3. watchdog_touch_ts was not explicitly updated when the softlockup was reported. Without this variable, the report would normally be printed again during every following watchdog_timer_fn() invocation.
The logic has got even more tangled up by the commit ed235875e2ca98 ("kernel/watchdog.c: print traces for all cpus on lockup detection"). After this commit, soft_watchdog_warn is set only when softlockup_all_cpu_backtrace is enabled. But multiple reports from all CPUs are prevented by a new variable soft_lockup_nmi_warn.
Conclusion:
The variable probably never worked as intended. In each case, it has not worked last many years because the softlockup was reported repeatedly after the full period defined by watchdog_thresh.
The reason is that watchdog gets touched in many known slow paths, for example, in printk_stack_address(). This code is called also when printing the softlockup report. It means that the watchdog timestamp gets updated after each report.
Solution:
Simply remove the logic. People want the periodic report anyway.
Link: https://lkml.kernel.org/r/20210311122130.6788-5-pmladek@suse.com Signed-off-by: Petr Mladek <pmladek@suse.com> Cc: Ingo Molnar <mingo@kernel.org> Cc: Laurence Oberman <loberman@redhat.com> Cc: Michal Hocko <mhocko@suse.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vincent Whitchurch <vincent.whitchurch@axis.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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