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

From: Jinchao Wang <[email-masked]>
Date: Wed, 17 Sep 2025 14:54:49 +0800

On Wed, Sep 17, 2025 at 08:08:57AM +0200, Christophe Leroy wrote:
> 
> 
> Le 16/09/2025 à 16:50, Jinchao Wang a écrit :
> > 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/[email-masked]/
> > Link: https://lore.kernel.org/all/CAD=FV=[email-masked]/
> > 
> > Signed-off-by: Jinchao Wang <[email-masked]>
> > ---
> >   .../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);
> 
> No 'extern' on function prototypes, this is pointless.
Got it.
> 
> >   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;
> 
> As this is set only once at startup, can we use static_call instead of
> function pointers ?
> 
> Also, can it me made __ro_after_init ?
Not really, this is just an RFC patch, and there is no consensus yet.
If it is included in the final consensus, I will handle it in the next version.
> 
> > +#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)) {
> 
> Why strncmp ?
Copy from hardlockup_panic_setup().
> 
> What if I set 'hardlockup_detector=performance" ?
I think that is acceptable in this case.
> 
> 
> > +		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);
> > +}
> 
> This is a weak function so it can be overloaded. What happens then, for
> instance if the sparc architecture version of watchdog_hardlockup_enable()
> is called instead ?
It is a historical problem; I prefer using an #if condition instead. 
I had considered sparc arch, if sparc version is called, it is expected.
Because the __weak functions only handle perf & buddy watchdog not the sparc
watchdog.

I think we should first resolve the consensus issue:
- Should we keep both perf and buddy watchdogs? (probably yes already)
- Should the watchdog type be changeable at boot time?
- Should the watchdog type be changeable at runtime?

How we handle these different watchdog types(maybe including sparc type)
depends on the answers to these questions.
What do you think?

> 
> > -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__);
> 
> Leftover from debuging ?
Forgot to delete the log, will fix if a v2 is needed.
> 
> >   	/*
> >   	 * 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
>