Re: [PATCH v2 4/5] kernel/watchdog: Adapt the watchdog_hld interface for async model

[Petr Mladek <pmladek@suse.com>]

On Sat 2022-03-19 16:18:22, Lecopzer Chen wrote:
> > On Mon 2022-03-07 23:47:28, Lecopzer Chen wrote:
> > > When lockup_detector_init()->watchdog_nmi_probe(), PMU may be not ready
> > > yet. E.g. on arm64, 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 this
> > > initialization before smp_init().
> > 
> > > --- a/kernel/watchdog.c
> > > +++ b/kernel/watchdog.c
> > > @@ -839,16 +843,70 @@ static void __init watchdog_sysctl_init(void)
> > >  #define watchdog_sysctl_init() do { } while (0)
> > >  #endif /* CONFIG_SYSCTL */
> > >  
> > > +static void lockup_detector_delay_init(struct work_struct *work);
> > > +bool lockup_detector_pending_init __initdata;
> > > +
> > > +struct wait_queue_head hld_detector_wait __initdata =
> > > +		__WAIT_QUEUE_HEAD_INITIALIZER(hld_detector_wait);
> > > +
> > > +static struct work_struct detector_work __initdata =
> > > +		__WORK_INITIALIZER(detector_work, lockup_detector_delay_init);
> > > +
> > > +static void __init lockup_detector_delay_init(struct work_struct *work)
> > > +{
> > > +	int ret;
> > > +
> > > +	wait_event(hld_detector_wait,
> > > +			lockup_detector_pending_init == false);
> > > +
> > > +	/*
> > > +	 * Here, we know the PMU should be ready, so set pending to true to
> > > +	 * inform watchdog_nmi_probe() that it shouldn't return -EBUSY again.
> > > +	 */
> > > +	lockup_detector_pending_init = true;
> > 
> > This does not make sense to me. We are here only when:
> > 
> >    1. lockup_detector_init() queued this work.
> > 
> >    2. Someone cleared @lockup_detector_pending_init and woke the
> >       worker via wait_queue. IT might be either PMU init code
> >       or the late lockup_detector_check().
> > 
> > watchdog_nmi_probe() might still return -EBUSY when PMU init failed.
> > 
> > If you wanted to try the delayed probe once again (3rd attempt) from
> > lockup_detector_check(), you would need to queue the work once again.
> > But you need to be sure that lockup_detector_check() was not called
> > yet. Otherwise, the 2nd work might wait forewer.
> > 
> > IMHO, it is not worth the complexity.
> 
> The original assumption is: nobody should use delayed probe after
> lockup_detector_check() (which has __init attribute).

Good point. It makes perfect sense.

But it was not mentioned anywhere. And the code did not work this way.

> 
> That is, everything including PMU and delayed probe of lock detector must
> finsh before do_initcalls() which means delayed probe can't support with
> external PMU module init.
> 
> Also,
>   1. lockup_detector_check is registered with late_initcall_sync(), so it'd
>      be called in the last order of do_initcalls()).
> 
>   2. watchdog_nmi_probe() and all the delayed relative functions and variables
>      have __init attribute, no one should ever use it after __init section
>      is released.
> 
> The only case is PMU probe function is also late_initcall_sync().

This is the case for PMU. The API for delayed init is generic a should
be safe even for other users.


> How about this one:
>   1. Wrap the wake_up code to reduce the complexity for user side.
> 
>   2. Remove wait queue.
>      Instead queue work when lockup_detector_init(), queue the delayed
>      probe work when arch PMU code finish probe.
> 
> and the flow turns to
> 
>   1. lockup_detector_init() get -EBUSY, set lockup_detector_pending_init=true
> 
>   2. PMU arch code init done, call lockup_detector_queue_work().
> 
>   3. lockup_detector_queue_work() queue the work only when
>      lockup_detector_pending_init=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 lockup_detector_pending_init=false.
> 
>   5. at late_initcall_sync(), lockup_detector_check() call flush_work() first
>      to avoid previous lockup_detector_queue_work() is not scheduled.
>      And then test whether lockup_detector_pending_init 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.

Nice, I like it.

> This remove the complexity of wait event which we were disscussed.
> The draft of the diff code(diff with this series) shows below.
> 
> 
> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> index 77eaefee13ea..c776618fbfa8 100644
> --- a/arch/arm64/kernel/perf_event.c
> +++ b/arch/arm64/kernel/perf_event.c
> @@ -1388,9 +1388,7 @@ static int __init armv8_pmu_driver_init(void)
>  	else
>  		ret = arm_pmu_acpi_probe(armv8_pmuv3_pmu_init);
>  
> -	/* Inform watchdog core we are ready to probe hld by delayed init. */
> -	lockup_detector_pending_init = false;
> -	wake_up(&hld_detector_wait);
> +	lockup_detector_queue_work();

The name is strange. The fact that it uses workqueues is an
implementation detail. I would call it
retry_lockup_detector_init() so that it is more obvious what it does.

>  	return ret;
>  }
>  device_initcall(armv8_pmu_driver_init)
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -876,15 +865,27 @@ static void __init lockup_detector_delay_init(struct work_struct *work)
>  	lockup_detector_pending_init = false;
>  }
>  
> +/* Must call after lockup_detector_init() that we do need delayed probe */
> +void __init lockup_detector_queue_work(void)
> +{
> +	if (!lockup_detector_pending_init)
> +		return;
> +
> +	queue_work_on(__smp_processor_id(), system_wq, &detector_work);
> +}
> +
>  /* Ensure the check is called after the initialization of PMU driver */
>  static int __init lockup_detector_check(void)
>  {
> +	/* Make sure no work is pending. */
> +	flush_work(&detector_work);
> +
>  	if (!lockup_detector_pending_init)
>  		return 0;
>  
>  	pr_info("Delayed init checking failed, retry for once.\n");
> -	lockup_detector_pending_init = false;
> -	wake_up(&hld_detector_wait);
> +	lockup_detector_queue_work();

I would do here

	lockup_detector_pending_init = false;

to make sure that lockup_detector_queue_work() will not longer
queue the work after the final flush.

Maybe, we could rename the variable to allow_lockup_detector_init_retry.

> +	flush_work(&detector_work);
>
>	return 0;
>  }
>  late_initcall_sync(lockup_detector_check);

Best Regards,
Petr