From: Petr Mladek <pmladek@xxxxxxxx>
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@xxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Laurence Oberman <loberman@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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;
}
_
