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

Lecopzer Chen
2022-03-24T14:14:50Z

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@gmail.com>
Sumit Garg <sumit.garg@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@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(-)

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2.25.1