Message-ID: <20220421163035.26402-1-lecopzer.chen@mediatek.com>
Date:   Fri, 22 Apr 2022 00:30:35 +0800
From:   Lecopzer Chen <lecopzer.chen@...iatek.com>
To:     <pmladek@...e.com>
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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.
> 

Sorry for late reply.

It's really a good idea.

Since I have already had lots things to revise in v3, I'm now preparing the V4.
I'll send it in these few days.

Thanks a lots for your great idea.


BRs,
Lecopzer

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