As we already used hld internally for arm64 since 2020, there still
doesn't have a proper commit on the upstream and we badly need it.

This serise rework on 5.17-rc7 from [1] and the origin author is
Pingfan Liu [off-list ref]
Sumit Garg [off-list ref]

Qoute from [1]:

Hard lockup detector is helpful to diagnose unpaired irq enable/disable.
But the current watchdog framework can not cope with arm64 hw perf event
easily.

On arm64, when lockup_detector_init()->watchdog_nmi_probe(), PMU is not
ready until device_initcall(armv8_pmu_driver_init).  And it is deeply
integrated with the driver model and cpuhp. Hence it is hard to push the
initialization of armv8_pmu_driver_init() before smp_init().

But it is easy to take an opposite approach by enabling watchdog_hld to
get the capability of PMU async. 
The async model is achieved by expanding watchdog_nmi_probe() with
-EBUSY, and a re-initializing work_struct which waits on a
wait_queue_head.

[1] https://lore.kernel.org/lkml/20211014024155.15253-1-kernelfans@gmail.com/ (local)

v2:
  1. Tweak commit message in patch 01/02/04/05 
  2. Remove vobose WARN in patch 04 within watchdog core.
  3. Change from three states variable: detector_delay_init_state to
     two states variable: lockup_detector_pending_init

     Thanks Petr Mladek [off-list ref] for the idea.
     > 1.  lockup_detector_work() called before lockup_detector_check().
     >     In this case, wait_event() will wait until lockup_detector_check()
     >     clears detector_delay_pending_init and calls wake_up().

     > 2. lockup_detector_check() called before lockup_detector_work().
     >    In this case, wait_even() will immediately continue because
     >    it will see cleared detector_delay_pending_init.
  4. Add comment in code in patch 04/05 for two states variable changing.


Lecopzer Chen (4):
  kernel/watchdog: remove WATCHDOG_DEFAULT
  kernel/watchdog: change watchdog_nmi_enable() to void
  kernel/watchdog: Adapt the watchdog_hld interface for async model
  arm64: Enable perf events based hard lockup detector

Pingfan Liu (1):
  kernel/watchdog_hld: Ensure CPU-bound context when creating hardlockup
    detector event

 arch/arm64/Kconfig             |  2 +
 arch/arm64/kernel/Makefile     |  1 +
 arch/arm64/kernel/perf_event.c | 12 +++++-
 arch/sparc/kernel/nmi.c        |  8 ++--
 drivers/perf/arm_pmu.c         |  5 +++
 include/linux/nmi.h            |  5 ++-
 include/linux/perf/arm_pmu.h   |  2 +
 kernel/watchdog.c              | 67 +++++++++++++++++++++++++++++++---
 kernel/watchdog_hld.c          |  8 +++-
 9 files changed, 95 insertions(+), 15 deletions(-)

-- 
2.25.1


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