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

[Lecopzer Chen]

> On Thu 2022-03-24 22:14:05, Lecopzer Chen wrote:
> > With the recent feature added to enable perf events to use pseudo NMIs
> > as interrupts on platforms which support GICv3 or later, its now been
> > possible to enable hard lockup detector (or NMI watchdog) on arm64
> > platforms. So enable corresponding support.
> > 
> > One thing to note here is that normally lockup detector is initialized
> > just after the early initcalls but PMU on arm64 comes up much later as
> > device_initcall(). To cope with that, overriding watchdog_nmi_probe() to
> > let the watchdog framework know PMU not ready, and inform the framework
> > to re-initialize lockup detection once PMU has been initialized.
> > 
> > [1]: http://lore.kernel.org/linux-arm-kernel/1610712101-14929-1-git-send-email-sumit.garg@xxxxxxxxxx
> > 
> > --- /dev/null
> > +++ b/arch/arm64/kernel/watchdog_hld.c
> > @@ -0,0 +1,37 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <linux/nmi.h>
> > +#include <linux/cpufreq.h>
> > +#include <linux/perf/arm_pmu.h>
> > +
> > +/*
> > + * Safe maximum CPU frequency in case a particular platform doesn't implement
> > + * cpufreq driver. Although, architecture doesn't put any restrictions on
> > + * maximum frequency but 5 GHz seems to be safe maximum given the available
> > + * Arm CPUs in the market which are clocked much less than 5 GHz. On the other
> > + * hand, we can't make it much higher as it would lead to a large hard-lockup
> > + * detection timeout on parts which are running slower (eg. 1GHz on
> > + * Developerbox) and doesn't possess a cpufreq driver.
> > + */
> > +#define SAFE_MAX_CPU_FREQ	5000000000UL // 5 GHz
> > +u64 hw_nmi_get_sample_period(int watchdog_thresh)
> > +{
> > +	unsigned int cpu = smp_processor_id();
> > +	unsigned long max_cpu_freq;
> > +
> > +	max_cpu_freq = cpufreq_get_hw_max_freq(cpu) * 1000UL;
> > +	if (!max_cpu_freq)
> > +		max_cpu_freq = SAFE_MAX_CPU_FREQ;
> > +
> > +	return (u64)max_cpu_freq * watchdog_thresh;
> > +}
> 
> This change is not mentioned in the commit message.
> Please, put it into a separate patch.


Actully, This cames from
[1]: http://lore.kernel.org/linux-arm-kernel/1610712101-14929-1-git-send-email-sumit.garg@xxxxxxxxxx
And I didn't touch the commit message from the origin patch.
But of course, I could imporve it with proper description if
anyone thinks it's not good enough.

Would you mean put this function hw_nmi_get_sample_period() in patch 6th?
In the view of "arm64 uses delayed init with all the functionality it need to set up",
IMO, this make sense for me to put into a single patch.

But if you still think this should put into a separate patch, I'll do it:)


> 
> > +int __init watchdog_nmi_probe(void)
> > +{
> > +	if (!allow_lockup_detector_init_retry)
> > +		return -EBUSY;
> 
> How do you know that you should return -EBUSY
> when retry in not enabled?
> 
> I guess that it is an optimization to make it fast
> during the first call. But the logic is far from
> obvious.
> 

Yes, you can see this as an optimization, because arm64 PMU is not ready
during lockup_detector_init(), so the watchdog_nmi_probe() must fail.

Thus we only want to do watchdog_nmi_probe() in delayed init,
so if not in the state (allow_lockup_detector_init_retry=true), just tell

if it's unclear, maybe a brief comment can be add like this:

+	/* arm64 is only able to initialize lockup detecor during delayed init */
+	if (!allow_lockup_detector_init_retry)
+		return -EBUSY;




> > +
> > +	if (!arm_pmu_irq_is_nmi())
> > +		return -ENODEV;
> > +
> > +	return hardlockup_detector_perf_init();
> > +}
> 
> Is this just an optimization or is it really needed?
> Why this was not needed in v2 patchset?
> 
> If it is just an optimization then I would remove it.
> IMHO, it just adds confusion and it is not worth it.
> 

It was a mistake when I rebased v2, This should be included in v2
but I missed it.

For arm_pmu_irq_is_nmi() checking, we do need it, becasue arm64 needs
explictly turns on Pseudo-NMI to support base function for NMI.

hardlockup_detector_perf_init() will success even if we haven't had
Pseudo-NMI turns on, however, the pmu interrupts will act like a
normal interrupt instead of NMI and the hardlockup detector would be broken.


thanks for all the comment

BRs,
Lecopzer