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

From: Jinchao Wang <[email-masked]>
Date: Wed, 17 Sep 2025 09:47:37 +0800

On Tue, Sep 16, 2025 at 05:03:48PM -0700, Ian Rogers wrote:
> On Tue, Sep 16, 2025 at 7:51 AM Jinchao Wang <[email-masked]> wrote:
> >
> > 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.
> 
> What is the impact on  /proc/sys/kernel/nmi_watchdog ? Is that
> enabling and disabling whatever the boot time choice was? I'm not sure
> why this has to be a boot time option given the ability to configure
> via /proc/sys/kernel/nmi_watchdog.
The new hardlockup_detector boot parameter and the existing
/proc/sys/kernel/nmi_watchdog file serve different purposes.

The boot parameter selects the type of hard lockup detector (perf or buddy).
This choice is made once at boot.

 /proc/sys/kernel/nmi_watchdog, on the other hand, is only a simple on/off
switch for the currently selected detector. It does not change the detector's
type.
> 
> > 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);
> >  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.
> 
> I'd say this option should be the default for kernel developers but
> shouldn't be used by default to free the perf event and due to the
> extra power overhead.
> 
> Thanks,
> Ian
> 
> >  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
> >

-- 
Jinchao