Re: [PATCH v3 5/5] arm64: Enable perf events based hard lockup detector

[Lecopzer Chen <lecopzer.chen@mediatek.com>]

> > The call path for hardlockup_detector_perf_init() is really complicated,
> > 
> > I have some approach about this:
> >   1. abstract second variable with Kconfig.
> >     a. Add a ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT
> >        (the naming is a little bit long, may have better naming)
> >        in "lib/Kconfig.debug" if ARCH knew they do need delayed init for
> >        lockup detector.
> > 
> >        + select ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT if HAVE_HARDLOCKUP_DETECTOR_PERF
> > 
> >     b. and the watchdog_nmi_probe would look like.
> > 
> >     +int __init watchdog_nmi_probe(void)
> >     +{
> >     +	int ret;
> >     +
> >     + /* comment here... */
> >     +	if (!arm_pmu_irq_is_nmi())
> >     +		return -ENODEV;
> >     +
> >     +	ret = hardlockup_detector_perf_init();
> >     +	if (ret &&
> >     +		  IS_ENABLED(ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT))
> >     +		return -EBUSY;
> >     +
> >     + return ret;
> >     +}
> > 
> >     and than we can have only one variable (allow_lockup_detector_init_retry)
> >     in 4th patch.
> > 
> >  
> >   2. base on ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT, change
> >      inside hardlockup_detector_perf_init().
> > 
> > int __init hardlockup_detector_perf_init(void)
> > {
> > 	int ret = hardlockup_detector_event_create();
> > 
> > 	if (ret) {
> > 		pr_info("Perf NMI watchdog permanently disabled\n");
> > +
> > +		/* comment here... */
> > +		if (IS_ENABLED(ARCH_SUPPORTS_HARDLOCKUP_DETECTOR_DLAYED_INIT))
> > +			ret = -EBUSY;
> > 	} else {
> > 		perf_event_release_kernel(this_cpu_read(watchdog_ev));
> > 		this_cpu_write(watchdog_ev, NULL);
> > 	}
> > 	return ret;
> > }
> > 
> >   3. Don't add any other config, try to find a proper location
> >      to return -EBUSY in hardlockup_detector_event_create().
> >      IMHO, this may involve the PMU subsys and should be
> >      the hardest approach.
> 
> Honestly, everything looks a bit ugly and complicated to me.
> 
> OKAY, is the return value actually important?
> 
> What about just introducing the API that will allow to try to
> initialize the hardlockup detector later:
> 
> /*
>  * Retry hardlockup detector init. It is useful when it requires some
>  * functionality that has to be initialized later on a particular
>  * platform.
>  */
> void __init retry_lockup_detector_init(void)
> {
> 	/* Must be called before late init calls. */
> 	if (!allow_lockup_detector_init_retry)
> 		return 0;
> 
> 	queue_work_on(__smp_processor_id(), system_wq, &detector_work);
> }
> 
> /*
>  * Ensure that optional delayed hardlockup init is proceed before
>  * the init code and memory is freed.
>  */
> static int __init lockup_detector_check(void)
> {
> 	/* Prevent any later retry. */
> 	allow_lockup_detector_init_retry = false;
> 
> 	/* Make sure no work is pending. */
> 	flush_work(&detector_work);
> }
> late_initcall_sync(lockup_detector_check);
> 
> You could leave lockup_detector_init() as it is. It does not really
> matter what was the exact error value returned by watchdog_nmi_probe().
> 
> Then you could call retry_lockup_detector_init() in
> armv8_pmu_driver_init() and be done with it.
> 
> It will be universal API that might be used on any architecture
> for any reason. If nobody calls retry_lockup_detector_init()
> then nohing will happen and the code will work as before.
> 
> It might make sense to provide the API only on architectures that
> really need it. We could hide it under
> 
>    ARCH_NEED_DELAYED_HARDLOCKUP_DETECTOR_INIT
> 
> , similar to ARCH_NEEDS_CPU_IDLE_COUPLE.
> 

During implementation, if I add ARCH_NEED_DELAYED_..., there will contain
many enclosed ifdef-endif and is a little bit ugly.
Also, I didn't find a must-have reason to this Kconfig after I rebase from
your suggestion.

The one calls retry_lockup_detector_init() must fail at lockup_detector_init,
so I think anyone who has aleady failed at lockup_detector_init() has right
to retry no matter HW, SW or Arch reason.
Thus I might not introduce a new Kconfig in v4, and I would be glad to see
if any further commet on this.