[PATCH v3 0/5] Support hld delayed init based on Pseudo-NMI for arm64

[Lecopzer Chen]

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 from [1] and the origin author is
Pingfan Liu <kernelfans at gmail.com>
Sumit Garg <sumit.garg at linaro.org>

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.

Provide an API - retry_lockup_detector_init() for anyone who needs
to delayed init lockup detector.

The original assumption is: nobody should use delayed probe after
lockup_detector_check() (which has __init attribute).
That is, anyone uses this API must call between lockup_detector_init()
and lockup_detector_check(), and the caller must have __init attribute

The delayed init flow is:
1. lockup_detector_init() get -EBUSY from arch code, set
   allow_lockup_detector_init_retry=true

2. PMU arch code init done, call retry_lockup_detector_init().

3. retry_lockup_detector_init() queue the work only when
   allow_lockup_detector_init_retry=true which means nobody should call
   this before lockup_detector_init().

4. the work lockup_detector_delay_init() is doing without wait event.
   if probe success, set allow_lockup_detector_init_retry=false.

5. at late_initcall_sync(), lockup_detector_check() call flush_work() first
   to avoid previous retry_lockup_detector_init() is not scheduled.
   And then test whether allow_lockup_detector_init_retry is false, if it's
   true, means we have pending init un-finished, than forcely queue work
   again and flush_work to make sure the __init section won't be freed
   before the work done.

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

v3:
  1. Tweak commit message in patch 04 
	2. Remove wait event
  3. s/lockup_detector_pending_init/allow_lockup_detector_init_retry/
  4. provide api retry_lockup_detector_init()


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: allow_lockup_detector_init_retry

     Thanks Petr Mladek <pmladek at suse.com> 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.
https://lore.kernel.org/lkml/20220307154729.13477-1-lecopzer.chen@mediatek.com/

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