linux-kernel - [PATCH v2 4/7] watchdog/softlockup: Remove logic that tried to prevent repeated reports

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Date:   Thu, 10 Dec 2020 17:00:35 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Laurence Oberman <loberman@...hat.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        Petr Mladek <pmladek@...e.com>
Subject: [PATCH v2 4/7] 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.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/watchdog.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 6259590d6474..dc8a0bf943f5 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;
@@ -410,19 +409,12 @@ 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. */
@@ -452,9 +444,7 @@ 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;
 }
-- 
2.26.2