Date:   Wed, 13 Apr 2022 12:25:57 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Lecopzer Chen <lecopzer.chen@...iatek.com>
Cc:     acme@...nel.org, akpm@...ux-foundation.org,
        alexander.shishkin@...ux.intel.com, catalin.marinas@....com,
        davem@...emloft.net, jolsa@...hat.com, jthierry@...hat.com,
        keescook@...omium.org, kernelfans@...il.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, mark.rutland@....com,
        masahiroy@...nel.org, matthias.bgg@...il.com, maz@...nel.org,
        mcgrof@...nel.org, mingo@...hat.com, namhyung@...nel.org,
        nixiaoming@...wei.com, peterz@...radead.org,
        sparclinux@...r.kernel.org, sumit.garg@...aro.org,
        wangqing@...o.com, will@...nel.org, yj.chiang@...iatek.com
Subject: Re: [PATCH v3 5/5] arm64: Enable perf events based hard lockup
 detector

On Fri 2022-04-08 00:59:49, Lecopzer Chen wrote:
> 
> > On Tue 2022-04-05 20:53:04, Lecopzer Chen wrote:
> > >  
> > > > 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@linaro.org
> > > > > 
> > > > > --- /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@linaro.org
> > > 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.
> > 
> > I see.
> > 
> > > 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.
> > 
> > Or you could split it in two patches and add
> > hw_nmi_get_sample_period() in the earlier patch.
> > 
> > 
> > > But if you still think this should put into a separate patch, I'll do it:)
> > 
> > It is always better to split the changes whenever possible. It makes
> > the review easier. And it also helps to find the real culprit of
> > a regression using bisection.
> 
> Okay, I'll split this part into another change, thanks.
> 
> 
> > > > > +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
> > 
> > Yes, it is far from obvious.
> > 
> > > 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;
> > 
> > No, please, remove this optimization. It just makes problems:
> > 
> >    + it requires a comment here because the logic is far from obvious.
> > 
> >    + it is the reason why we need another variable to avoid the race in
> >      lockup_detector_check(), see the discussion about the 4th patch.
> 
> After some days studying, if I remove this if-condition which means the
> following hardlockup_detector_perf_init() needs to return -EBUSY.
> However, the default return value that if pmu is not ready is -ENOENT.

I see.

> 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.

Best Regards,
Petr

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