[RFC PATCH V1] watchdog: Add boot-time selection for hard lockup detector

Jinchao Wang
2025-09-16T14:51:33Z

Currently, the hard lockup detector is selected at compile time via
Kconfig, which requires a kernel rebuild to switch implementations.
This is inflexible, especially on systems where a perf event may not
be available or may be needed for other tasks.

This commit refactors the hard lockup detector to replace a rigid
compile-time choice with a flexible build-time and boot-time solution.
The patch supports building the kernel with either detector
independently, or with both. When both are built, a new boot parameter
`hardlockup_detector="perf|buddy"` allows the selection at boot time.
This is a more robust and user-friendly design.

This patch is a follow-up to the discussion on the kernel mailing list
regarding the preference and future of the hard lockup detectors. It
implements a flexible solution that addresses the community's need to
select an appropriate detector at boot time.

The core changes are:
- The `perf` and `buddy` watchdog implementations are separated into
distinct functions (e.g., `watchdog_perf_hardlockup_enable`).
- Global function pointers are introduced (`watchdog_hardlockup_enable_ptr`)
to serve as a single API for the entire feature.
- A new `hardlockup_detector=` boot parameter is added to allow the
user to select the desired detector at boot time.
- The Kconfig options are simplified by removing the complex
`HARDLOCKUP_DETECTOR_PREFER_BUDDY` and allowing both detectors to be
built without mutual exclusion.
- The weak stubs are updated to call the new function pointers,
centralizing the watchdog logic.

Link: https://lore.kernel.org/all/20250915035355.10846-1-cuiyunhui@bytedance.com/
Link: https://lore.kernel.org/all/CAD=FV=WWUiCi6bZCs_gseFpDDWNkuJMoL6XCftEo6W7q6jRCkg@mail.gmail.com/

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
.../admin-guide/kernel-parameters.txt | 7 +++
include/linux/nmi.h | 6 +++
kernel/watchdog.c | 46 ++++++++++++++++++-
kernel/watchdog_buddy.c | 7 +--
kernel/watchdog_perf.c | 10 ++--
lib/Kconfig.debug | 37 +++++++--------
6 files changed, 85 insertions(+), 28 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5a7a83c411e9..0af214ee566c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1828,6 +1828,13 @@
backtraces on all cpus.
Format: 0 | 1

+ hardlockup_detector=
+ [perf, buddy] Selects the hard lockup detector to use at
+ boot time.
+ Format: <string>
+ - "perf": Use the perf-based detector.
+ - "buddy": Use the buddy-based detector.
+
hash_pointers=
[KNL,EARLY]
By default, when pointers are printed to the console
diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index cf3c6ab408aa..9298980ce572 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -100,6 +100,9 @@ void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs);
#endif

#if defined(CONFIG_HARDLOCKUP_DETECTOR_PERF)
+void watchdog_perf_hardlockup_enable(unsigned int cpu);
+void watchdog_perf_hardlockup_disable(unsigned int cpu);
+extern int watchdog_perf_hardlockup_probe(void);
extern void hardlockup_detector_perf_stop(void);
extern void hardlockup_detector_perf_restart(void);
extern void hardlockup_config_perf_event(const char *str);
@@ -120,6 +123,9 @@ void watchdog_hardlockup_disable(unsigned int cpu);
void lockup_detector_reconfigure(void);

#ifdef CONFIG_HARDLOCKUP_DETECTOR_BUDDY
+void watchdog_buddy_hardlockup_enable(unsigned int cpu);
+void watchdog_buddy_hardlockup_disable(unsigned int cpu);
+int watchdog_buddy_hardlockup_probe(void);
void watchdog_buddy_check_hardlockup(int hrtimer_interrupts);
#else
static inline void watchdog_buddy_check_hardlockup(int hrtimer_interrupts) {}
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 80b56c002c7f..85451d24a77d 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -55,6 +55,37 @@ unsigned long *watchdog_cpumask_bits = cpumask_bits(&watchdog_cpumask);

#ifdef CONFIG_HARDLOCKUP_DETECTOR

+#ifdef CONFIG_HARDLOCKUP_DETECTOR_PERF
+/* The global function pointers */
+void (*watchdog_hardlockup_enable_ptr)(unsigned int cpu) = watchdog_perf_hardlockup_enable;
+void (*watchdog_hardlockup_disable_ptr)(unsigned int cpu) = watchdog_perf_hardlockup_disable;
+int (*watchdog_hardlockup_probe_ptr)(void) = watchdog_perf_hardlockup_probe;
+#elif defined(CONFIG_HARDLOCKUP_DETECTOR_BUDDY)
+void (*watchdog_hardlockup_enable_ptr)(unsigned int cpu) = watchdog_buddy_hardlockup_enable;
+void (*watchdog_hardlockup_disable_ptr)(unsigned int cpu) = watchdog_buddy_hardlockup_disable;
+int (*watchdog_hardlockup_probe_ptr)(void) = watchdog_buddy_hardlockup_probe;
+#endif
+
+#ifdef CONFIG_HARDLOCKUP_DETECTOR_MULTIPLE
+static char *hardlockup_detector_type = "perf"; /* Default to perf */
+static int __init set_hardlockup_detector_type(char *str)
+{
+ if (!strncmp(str, "perf", 4)) {
+ watchdog_hardlockup_enable_ptr = watchdog_perf_hardlockup_enable;
+ watchdog_hardlockup_disable_ptr = watchdog_perf_hardlockup_disable;
+ watchdog_hardlockup_probe_ptr = watchdog_perf_hardlockup_probe;
+ } else if (!strncmp(str, "buddy", 5)) {
+ watchdog_hardlockup_enable_ptr = watchdog_buddy_hardlockup_enable;
+ watchdog_hardlockup_disable_ptr = watchdog_buddy_hardlockup_disable;
+ watchdog_hardlockup_probe_ptr = watchdog_buddy_hardlockup_probe;
+ }
+ return 1;
+}
+
+__setup("hardlockup_detector=", set_hardlockup_detector_type);
+
+#endif
+
# ifdef CONFIG_SMP
int __read_mostly sysctl_hardlockup_all_cpu_backtrace;
# endif /* CONFIG_SMP */
@@ -262,9 +293,17 @@ static inline void watchdog_hardlockup_kick(void) { }
* softlockup watchdog start and stop. The detector must select the
* SOFTLOCKUP_DETECTOR Kconfig.
*/
-void __weak watchdog_hardlockup_enable(unsigned int cpu) { }
+void __weak watchdog_hardlockup_enable(unsigned int cpu)
+{
+ if (watchdog_hardlockup_enable_ptr)
+ watchdog_hardlockup_enable_ptr(cpu);
+}

-void __weak watchdog_hardlockup_disable(unsigned int cpu) { }
+void __weak watchdog_hardlockup_disable(unsigned int cpu)
+{
+ if (watchdog_hardlockup_disable_ptr)
+ watchdog_hardlockup_disable_ptr(cpu);
+}

/*
* Watchdog-detector specific API.
@@ -275,6 +314,9 @@ void __weak watchdog_hardlockup_disable(unsigned int cpu) { }
*/
int __weak __init watchdog_hardlockup_probe(void)
{
+ if (watchdog_hardlockup_probe_ptr)
+ return watchdog_hardlockup_probe_ptr();
+
return -ENODEV;
}

diff --git a/kernel/watchdog_buddy.c b/kernel/watchdog_buddy.c
index ee754d767c21..390d89bfcafa 100644
--- a/kernel/watchdog_buddy.c
+++ b/kernel/watchdog_buddy.c
@@ -19,15 +19,16 @@ static unsigned int watchdog_next_cpu(unsigned int cpu)
return next_cpu;
}

-int __init watchdog_hardlockup_probe(void)
+int __init watchdog_buddy_hardlockup_probe(void)
{
return 0;
}

-void watchdog_hardlockup_enable(unsigned int cpu)
+void watchdog_buddy_hardlockup_enable(unsigned int cpu)
{
unsigned int next_cpu;

+ pr_info("ddddd %s\n", __func__);
/*
* The new CPU will be marked online before the hrtimer interrupt
* gets a chance to run on it. If another CPU tests for a
@@ -58,7 +59,7 @@ void watchdog_hardlockup_enable(unsigned int cpu)
cpumask_set_cpu(cpu, &watchdog_cpus);
}

-void watchdog_hardlockup_disable(unsigned int cpu)
+void watchdog_buddy_hardlockup_disable(unsigned int cpu)
{
unsigned int next_cpu = watchdog_next_cpu(cpu);

diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
index 9c58f5b4381d..270110e58f20 100644
--- a/kernel/watchdog_perf.c
+++ b/kernel/watchdog_perf.c
@@ -153,10 +153,12 @@ static int hardlockup_detector_event_create(void)
* watchdog_hardlockup_enable - Enable the local event
* @cpu: The CPU to enable hard lockup on.
*/
-void watchdog_hardlockup_enable(unsigned int cpu)
+void watchdog_perf_hardlockup_enable(unsigned int cpu)
{
WARN_ON_ONCE(cpu != smp_processor_id());

+ pr_info("ddddd %s\n", __func__);
+
if (hardlockup_detector_event_create())
return;

@@ -172,7 +174,7 @@ void watchdog_hardlockup_enable(unsigned int cpu)
* watchdog_hardlockup_disable - Disable the local event
* @cpu: The CPU to enable hard lockup on.
*/
-void watchdog_hardlockup_disable(unsigned int cpu)
+void watchdog_perf_hardlockup_disable(unsigned int cpu)
{
struct perf_event *event = this_cpu_read(watchdog_ev);

@@ -257,10 +259,12 @@ bool __weak __init arch_perf_nmi_is_available(void)
/**
* watchdog_hardlockup_probe - Probe whether NMI event is available at all
*/
-int __init watchdog_hardlockup_probe(void)
+int __init watchdog_perf_hardlockup_probe(void)
{
int ret;

+ pr_info("ddddd %s\n", __func__);
+
if (!arch_perf_nmi_is_available())
return -ENODEV;

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index dc0e0c6ed075..443353fad1c1 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1167,36 +1167,33 @@ config HARDLOCKUP_DETECTOR
#
# Note that arch-specific variants are always preferred.
#
-config HARDLOCKUP_DETECTOR_PREFER_BUDDY
- bool "Prefer the buddy CPU hardlockup detector"
- depends on HARDLOCKUP_DETECTOR
- depends on HAVE_HARDLOCKUP_DETECTOR_PERF && HAVE_HARDLOCKUP_DETECTOR_BUDDY
- depends on !HAVE_HARDLOCKUP_DETECTOR_ARCH
- help
- Say Y here to prefer the buddy hardlockup detector over the perf one.
-
- With the buddy detector, each CPU uses its softlockup hrtimer
- to check that the next CPU is processing hrtimer interrupts by
- verifying that a counter is increasing.
-
- This hardlockup detector is useful on systems that don't have
- an arch-specific hardlockup detector or if resources needed
- for the hardlockup detector are better used for other things.
-
config HARDLOCKUP_DETECTOR_PERF
- bool
+ bool "Enable perf-based hard lockup detector (preferred)"
depends on HARDLOCKUP_DETECTOR
- depends on HAVE_HARDLOCKUP_DETECTOR_PERF && !HARDLOCKUP_DETECTOR_PREFER_BUDDY
+ depends on HAVE_HARDLOCKUP_DETECTOR_PERF
depends on !HAVE_HARDLOCKUP_DETECTOR_ARCH
select HARDLOCKUP_DETECTOR_COUNTS_HRTIMER
+ help
+ This detector uses a perf event on the CPU to detect when a CPU
+ has become non-maskable interrupt (NMI) stuck. This is the
+ preferred method on modern systems as it can detect lockups on
+ all CPUs at the same time.

config HARDLOCKUP_DETECTOR_BUDDY
- bool
+ bool "Enable buddy-based hard lockup detector"
depends on HARDLOCKUP_DETECTOR
depends on HAVE_HARDLOCKUP_DETECTOR_BUDDY
- depends on !HAVE_HARDLOCKUP_DETECTOR_PERF || HARDLOCKUP_DETECTOR_PREFER_BUDDY
depends on !HAVE_HARDLOCKUP_DETECTOR_ARCH
select HARDLOCKUP_DETECTOR_COUNTS_HRTIMER
+ help
+ This is an alternative lockup detector that uses a heartbeat
+ mechanism between CPUs to detect when one has stopped responding.
+ It is less precise than the perf-based detector and cannot detect
+ all-CPU lockups, but it does not require a perf counter.
+
+config CONFIG_HARDLOCKUP_DETECTOR_MULTIPLE
+ bool
+ depends on HARDLOCKUP_DETECTOR_PERF && HARDLOCKUP_DETECTOR_BUDDY

config HARDLOCKUP_DETECTOR_ARCH
bool
--
2.43.0