Re: [PATCH v5 10/18] watchdog/hardlockup: Add a "cpu" param to watchdog_hardlockup_check()

[Petr Mladek]

On Fri 2023-05-19 10:18:34, Douglas Anderson wrote:
> In preparation for the buddy hardlockup detector where the CPU
> checking for lockup might not be the currently running CPU, add a
> "cpu" parameter to watchdog_hardlockup_check().
> 
> As part of this change, make hrtimer_interrupts an atomic_t since now
> the CPU incrementing the value and the CPU reading the value might be
> different. Technially this could also be done with just READ_ONCE and
> WRITE_ONCE, but atomic_t feels a little cleaner in this case.
> 
> While hrtimer_interrupts is made atomic_t, we change
> hrtimer_interrupts_saved from "unsigned long" to "int". The "int" is
> needed to match the data type backing atomic_t for hrtimer_interrupts.
> Even if this changes us from 64-bits to 32-bits (which I don't think
> is true for most compilers), it doesn't really matter. All we ever do
> is increment it every few seconds and compare it to an old value so
> 32-bits is fine (even 16-bits would be). The "signed" vs "unsigned"
> also doesn't matter for simple equality comparisons.
> 
> hrtimer_interrupts_saved is _not_ switched to atomic_t nor even
> accessed with READ_ONCE / WRITE_ONCE. The hrtimer_interrupts_saved is
> always consistently accessed with the same CPU. NOTE: with the
> upcoming "buddy" detector there is one special case. When a CPU goes
> offline/online then we can change which CPU is the one to consistently
> access a given instance of hrtimer_interrupts_saved. We still can't
> end up with a partially updated hrtimer_interrupts_saved, however,
> because we end up petting all affected CPUs to make sure the new and
> old CPU can't end up somehow read/write hrtimer_interrupts_saved at
> the same time.
> 
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -87,29 +87,34 @@ __setup("nmi_watchdog=", hardlockup_panic_setup);
>  
>  #if defined(CONFIG_HARDLOCKUP_DETECTOR_PERF)
>  
> -static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
> -static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
> +static DEFINE_PER_CPU(atomic_t, hrtimer_interrupts);
> +static DEFINE_PER_CPU(int, hrtimer_interrupts_saved);
>  static DEFINE_PER_CPU(bool, watchdog_hardlockup_warned);
>  static unsigned long watchdog_hardlockup_all_cpu_dumped;
>  
> -static bool is_hardlockup(void)
> +static bool is_hardlockup(unsigned int cpu)
>  {
> -	unsigned long hrint = __this_cpu_read(hrtimer_interrupts);
> +	int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));
>  
> -	if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
> +	if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
>  		return true;
>  
> -	__this_cpu_write(hrtimer_interrupts_saved, hrint);
> +	/*
> +	 * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
> +	 * for hrtimer_interrupts_saved. hrtimer_interrupts_saved is
> +	 * written/read by a single CPU.
> +	 */
> +	per_cpu(hrtimer_interrupts_saved, cpu) = hrint;
>  
>  	return false;
>  }
>  
>  static void watchdog_hardlockup_kick(void)
>  {
> -	__this_cpu_inc(hrtimer_interrupts);
> +	atomic_inc(raw_cpu_ptr(&hrtimer_interrupts));

Is there any particular reason why raw_*() is needed, please?

My expectation is that the raw_ API should be used only when
there is a good reason for it. Where a good reason might be
when the checks might fail but the consistency is guaranteed
another way.

IMHO, we should use:

	atomic_inc(this_cpu_ptr(&hrtimer_interrupts));

To be honest, I am a bit lost in the per_cpu API definitions.

But this_cpu_ptr() seems to be implemented the same way as
per_cpu_ptr() when CONFIG_DEBUG_PREEMPT is enabled.
And we use per_cpu_ptr() in is_hardlockup().

Also this_cpu_ptr() is used more commonly:

$> git grep this_cpu_ptr | wc -l
1385
$> git grep raw_cpu_ptr | wc -l
114

>  }
>  
> -void watchdog_hardlockup_check(struct pt_regs *regs)
> +void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
>  {
>  	/*
>  	 * Check for a hardlockup by making sure the CPU's timer
> @@ -117,35 +122,42 @@ void watchdog_hardlockup_check(struct pt_regs *regs)
>  	 * fired multiple times before we overflow'd. If it hasn't
>  	 * then this is a good indication the cpu is stuck
>  	 */
> -	if (is_hardlockup()) {
> +	if (is_hardlockup(cpu)) {
>  		unsigned int this_cpu = smp_processor_id();
> +		struct cpumask backtrace_mask = *cpu_online_mask;

Does this work, please?

IMHO, we should use cpumask_copy().

>  
>  		/* Only print hardlockups once. */
> -		if (__this_cpu_read(watchdog_hardlockup_warned))
> +		if (per_cpu(watchdog_hardlockup_warned, cpu))
>  			return;
>  

Otherwise, it looks good to me.

Best Regards,
Petr