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>

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 6bc5113..b8d4182 100644 (file)
--- 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;
 }