From: Petr Mladek pmladek@suse.com
[ Upstream commit 1bc503cb4a2638fb1c57801a7796aca57845ce63 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/watchdog.c | 14 ++------------ 1 file changed, 2 insertions(+), 12 deletions(-)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c index 6bc5113d3d74..b8d4182d14cc 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -179,7 +179,6 @@ static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts); static DEFINE_PER_CPU(unsigned long, watchdog_report_ts); static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer); static DEFINE_PER_CPU(bool, softlockup_touch_sync); -static DEFINE_PER_CPU(bool, soft_watchdog_warn); static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts); static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved); static unsigned long soft_lockup_nmi_warn; @@ -411,19 +410,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) if (kvm_check_and_clear_guest_paused()) return HRTIMER_RESTART;
- /* only warn once */ - if (__this_cpu_read(soft_watchdog_warn) == true) - return HRTIMER_RESTART; - if (softlockup_all_cpu_backtrace) { /* Prevent multiple soft-lockup reports if one cpu is already * engaged in dumping cpu back traces */ - if (test_and_set_bit(0, &soft_lockup_nmi_warn)) { - /* Someone else will report us. Let's give up */ - __this_cpu_write(soft_watchdog_warn, true); + if (test_and_set_bit(0, &soft_lockup_nmi_warn)) return HRTIMER_RESTART; - } }
/* Start period for the next softlockup warning. */ @@ -453,9 +445,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK); if (softlockup_panic) panic("softlockup: hung tasks"); - __this_cpu_write(soft_watchdog_warn, true); - } else - __this_cpu_write(soft_watchdog_warn, false); + }
return HRTIMER_RESTART; }