Commit 1bc503cb authored by Petr Mladek's avatar Petr Mladek Committed by Linus Torvalds
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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 58687acb
("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 ed235875
("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: default avatarPetr 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent fef06efc

kernel/watchdog.c

+2 −12
Original line number Diff line number Diff line
@@ -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;
@@ -411,20 +410,13 @@ 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. */

		update_report_ts();
@@ -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;

}