[patch 025/178] watchdog/softlockup: remove logic that tried to prevent repeated reports

[Andrew Morton <akpm@linux-foundation.org>]

From: Petr Mladek <pmladek@suse.com>
Subject: 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>
---

 kernel/watchdog.c |   14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

--- a/kernel/watchdog.c~watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports
+++ a/kernel/watchdog.c
@@ -179,7 +179,6 @@ static DEFINE_PER_CPU(unsigned long, wat
 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_tim
 		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_tim
 		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;
 }
_