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

[Petr Mladek]

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.

> +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.

> +
> +	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.

Best Regards,
Petr