Re: [PATCH v2] [watchdog] combine nmi_watchdog and softlockup

[Don Zickus]

On Fri, Apr 16, 2010 at 03:47:14AM +0200, Frederic Weisbecker wrote:
> On Thu, Apr 15, 2010 at 05:25:10PM -0400, Don Zickus wrote:
> > The new nmi_watchdog (which uses the perf event subsystem) is very
> > similar in structure to the softlockup detector.  Using Ingo's suggestion,
> > I combined the two functionalities into one file, kernel/watchdog.c.
> > 
> > Now both the nmi_watchdog (or hardlockup detector) and softlockup detector
> > sit on top of the perf event subsystem, which is run every 60 seconds or so
> > to see if there are any lockups.
> > 
> > To detect hardlockups, cpus not responding to interrupts, I implemented an
> > hrtimer that runs 5 times for every perf event overflow event.  If that stops
> > counting on a cpu, then the cpu is most likely in trouble.
> > 
> > To detect softlockups, tasks not yielding to the scheduler, I used the
> > previous kthread idea that now gets kicked every time the hrtimer fires.
> > If the kthread isn't being scheduled neither is anyone else and the
> > warning is printed to the console.
> > 
> > I tested this on x86_64 and both the softlockup and hardlockup paths work.
> > 
> > V2:
> > - cleaned up the Kconfig and softlockup combination
> > - surrounded hardlockup cases with #ifdef CONFIG_PERF_EVENTS_NMI
> > - seperated out the softlockup case from perf event subsystem
> > - re-arranged the enabling/disabling nmi watchdog from proc space
> > - added cpumasks for hardlockup failure cases
> > - removed fallback to soft events if no PMU exists for hard events
> > 
> > TODO:
> > - figure out how to make an arch-agnostic clock2cycles call (if possible)
> >   to feed into perf events as a sample period
> 
> 
> 
> In fact we also have the sample_freq thing that let you run
> against a frequency (events per sec) rather than a sample
> period.
> 
> We do some calculations that recompute the actual sample
> period on top of this frequency in a regular base as the
> events come.
> 
> It's rather unfortunate we can't have 0.xxxx frequencies
> but may be we can work that out by adding a freq_unscale
> field, which would basically produce the frequency like
> this:
> 
> 	freq = event->attr.sample_freq * (10 ^ -event->attr.freq_unscale)
> 
> This is not trivial though, so let's rather focus on the
> real matter for now :)

Interesting thanks.

> 
> 
> > diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> > index e8b78a0..79425f9 100644
> > --- a/arch/x86/kernel/apic/hw_nmi.c
> > +++ b/arch/x86/kernel/apic/hw_nmi.c
> > @@ -89,7 +89,7 @@ int hw_nmi_is_cpu_stuck(struct pt_regs *regs)
> >  
> >  u64 hw_nmi_get_sample_period(void)
> >  {
> > -	return cpu_khz * 1000;
> > +	return (u64)(cpu_khz) * 1000 * 60;
> >  }
> >  
> >  #ifdef ARCH_HAS_NMI_WATCHDOG
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 6f7bba9..83be6d7 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -338,6 +338,12 @@ extern int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
> >  					 size_t *lenp, loff_t *ppos);
> >  #endif
> >  
> > +#ifdef CONFIG_NMI_WATCHDOG
> > +extern int proc_dowatchdog_thresh(struct ctl_table *table, int write,
> > +				  void __user *buffer,
> > +				  size_t *lenp, loff_t *ppos);
> > +#endif
> > +
> >  /* Attach to any functions which should be ignored in wchan output. */
> >  #define __sched		__attribute__((__section__(".sched.text")))
> >  
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 7331a16..0b83612 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -948,6 +948,7 @@ config PERF_USE_VMALLOC
> >  
> >  config PERF_EVENTS_NMI
> >  	bool
> > +	depends on PERF_EVENTS
> >  	help
> >  	  Arch has support for nmi_watchdog
> 
> 
> 
> That looks too general. It's more about the fact the arch supports
> cpu cycle events and generates NMIs on overflow.

I was trying to figure out a way to add the PERF_EVENTS dependency as I
didn't want to impose it on the CONFIG_NMI_WATCHDOG if that config
supported softlockup (which doesn't need the PERF_EVENTS).

> 
> 
>   
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index 8a5abe5..56ba99d 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -75,9 +75,11 @@ obj-$(CONFIG_GCOV_KERNEL) += gcov/
> >  obj-$(CONFIG_AUDIT_TREE) += audit_tree.o
> >  obj-$(CONFIG_KPROBES) += kprobes.o
> >  obj-$(CONFIG_KGDB) += kgdb.o
> > +ifneq ($(CONFIG_NMI_WATCHDOG),y)
> >  obj-$(CONFIG_DETECT_SOFTLOCKUP) += softlockup.o
> > -obj-$(CONFIG_NMI_WATCHDOG) += nmi_watchdog.o
> > +endif
> 
> 
> 
> I'm confused, do we have two versions of the softlockup
> detector now? You should drop the older one.

Originally Ingo talked about a migration path, so I was going to support
the older one in case the new one was having issues, sort of like what he
suggested about moving the nmi code from arch/x86/kernel/apic/nmi.c to
kernel/watchdog.c.  But I can probably drop the softlockup case as the
migration isn't as tricky as the nmi case.

> 
> 
> 
> >  obj-$(CONFIG_DETECT_HUNG_TASK) += hung_task.o
> > +obj-$(CONFIG_NMI_WATCHDOG) += watchdog.o
> >  obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
> >  obj-$(CONFIG_SECCOMP) += seccomp.o
> >  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index ac72c9e..2165b22 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -704,6 +704,15 @@ static struct ctl_table kern_table[] = {
> >  		.mode           = 0644,
> >  		.proc_handler   = proc_nmi_enabled,
> >  	},
> > +	{
> > +		.procname	= "watchdog_thresh",
> > +		.data		= &softlockup_thresh,
> > +		.maxlen		= sizeof(int),
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_dowatchdog_thresh,
> > +		.extra1		= &neg_one,
> > +		.extra2		= &sixty,
> > +	},
> >  #endif
> >  #if defined(CONFIG_X86_LOCAL_APIC) && defined(CONFIG_X86) && !defined(CONFIG_NMI_WATCHDOG)
> >  	{
> > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > new file mode 100644
> > index 0000000..fad5b1a
> > --- /dev/null
> > +++ b/kernel/watchdog.c
> > @@ -0,0 +1,570 @@
> > +/*
> > + * Detect Hard/Soft Lockups using the NMI
> 
> 
> 
> Well, softlockups detection doesn't use NMI.

Yeah, sorry.  I dropped part of the code per your suggestion and forget to
update the comments.

> 
> 
> 
> > +/*
> > + * Returns seconds, approximately.  We don't need nanosecond
> > + * resolution, and we don't need to waste time with a big divide when
> > + * 2^30ns == 1.074s.
> > + */
> > +static unsigned long get_timestamp(int this_cpu)
> > +{
> > +	return cpu_clock(this_cpu) >> 30LL;  /* 2^30 ~= 10^9 */
> > +}
> > +
> > +static unsigned long get_sample_period(void)
> > +{
> > +	/*
> > +	 * convert softlockup_thresh from seconds to ns
> > +	 * the divide by 5 is to give hrtimer 5 chances to
> > +	 * increment before the hardlockup detector generates
> > +	 * a warning
> > +	 */
> > +	return softlockup_thresh / 5 * NSEC_PER_SEC;
> > +}
> > +
> > +/* Commands for resetting the watchdog */
> > +static void __touch_watchdog(void)
> > +{
> > +	int this_cpu = raw_smp_processor_id();
> > +
> > +	__raw_get_cpu_var(watchdog_touch_ts) = get_timestamp(this_cpu);
> 
> 
> 
> Please use __get_cpu_var() instead so that we keep the preempt disabled
> check from smp_processor_id()

ok.

> 
> 
> 
> > +}
> > +
> > +void touch_watchdog(void)
> > +{
> > +	__raw_get_cpu_var(watchdog_touch_ts) = 0;
> 
> 
> Same here.
> 
> 
> 
> > +}
> > +EXPORT_SYMBOL(touch_watchdog);
> > +
> > +void touch_all_watchdog(void)
> > +{
> > +	int cpu;
> > +
> > +	for_each_online_cpu(cpu)
> > +		per_cpu(watchdog_touch_ts, cpu) = 0;
> > +}
> > +
> > +/* deprecated functions */
> > +void touch_nmi_watchdog(void)
> > +{
> > +	touch_watchdog();
> > +}
> > +EXPORT_SYMBOL(touch_nmi_watchdog);
> > +
> > +void touch_all_nmi_watchdog(void)
> > +{
> > +	touch_all_watchdog();
> > +}
> > +
> > +void touch_softlockup_watchdog(void)
> > +{
> > +	touch_watchdog();
> > +}
> > +
> > +void touch_all_softlockup_watchdogs(void)
> > +{
> > +	touch_all_watchdog();
> > +}
> > +
> > +void softlockup_tick(void)
> > +{
> > +}
> > +/* end of deprecated functions */
> 
> 
> 
> You should replace the call sites directly.

ok.

> 
> 
> 
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +/* watchdog detector functions */
> > +static int is_hardlockup(int cpu)
> > +{
> > +	unsigned long hrint = per_cpu(hrtimer_interrupts, cpu);
> > +
> > +	if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
> > +		return 1;
> > +
> > +	per_cpu(hrtimer_interrupts_saved, cpu) = hrint;
> > +	return 0;
> > +}
> > +#endif
> > +
> > +static int is_softlockup(unsigned long touch_ts, int cpu)
> > +{
> > +	unsigned long now = get_timestamp(cpu);
> > +
> > +	/* Warn about unreasonable delays: */
> > +	if (now > (touch_ts + softlockup_thresh))
> > +		return now - touch_ts;
> > +
> > +	return 0;
> > +}
> > +
> > +static int
> > +watchdog_panic(struct notifier_block *this, unsigned long event, void *ptr)
> > +{
> > +	did_panic = 1;
> > +
> > +	return NOTIFY_DONE;
> > +}
> > +
> > +static struct notifier_block panic_block = {
> > +	.notifier_call = watchdog_panic,
> > +};
> > +
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +struct perf_event_attr wd_hw_attr = {
> > +	.type		= PERF_TYPE_HARDWARE,
> > +	.config		= PERF_COUNT_HW_CPU_CYCLES,
> > +	.size		= sizeof(struct perf_event_attr),
> > +	.pinned		= 1,
> > +	.disabled	= 1,
> > +};
> > +
> > +struct perf_event_attr wd_sw_attr = {
> > +	.type		= PERF_TYPE_SOFTWARE,
> > +	.config		= PERF_COUNT_SW_CPU_CLOCK,
> > +	.size		= sizeof(struct perf_event_attr),
> > +	.pinned		= 1,
> > +	.disabled	= 1,
> > +};
> 
> 
> 
> 
> Why do you keep the software clock, I don't see how it can be
> useful to detect hardlockups. It triggers in a regular irq
> not an NMI.

sorry, I dropped part of the code and forgot to drop this too.

> 
> 
> 
> > +
> > +/* Callback function for perf event subsystem */
> > +void watchdog_overflow_callback(struct perf_event *event, int nmi,
> > +		 struct perf_sample_data *data,
> > +		 struct pt_regs *regs)
> > +{
> > +	int this_cpu = smp_processor_id();
> > +	unsigned long touch_ts = per_cpu(watchdog_touch_ts, this_cpu);
> > +
> > +	if (touch_ts == 0) {
> > +		__touch_watchdog();
> > +		return;
> > +	}
> > +
> > +	/* check for a hardlockup
> > +	 * This is done by making sure our timer interrupt
> > +	 * is incrementing.  The timer interrupt should have
> > +	 * fired multiple times before we overflow'd.  If it hasn't
> > +	 * then this is a good indication the cpu is stuck
> > +	 */
> > +	if (is_hardlockup(this_cpu)) {
> > +		/* only print hardlockups once */
> > +		if (cpumask_test_cpu(this_cpu, to_cpumask(hardlockup_mask)))
> > +			return;
> > +
> > +		if (hardlockup_panic)
> > +			panic("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > +		else
> > +			WARN(1, "Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > +
> > +		cpumask_set_cpu(this_cpu, to_cpumask(hardlockup_mask));
> 
> 
> 
> May be have an arch spin lock there to update your cpu mask safely.

ok.

> 
> 
> 
> > +		return;
> > +	}
> > +
> > +	cpumask_clear_cpu(this_cpu, to_cpumask(hardlockup_mask));
> 
> 
> 
> Hmm...this is probably not necessary.

I was just thinking of the case where dispite the WARN above, the cpu
actually recovered and then failed again separately.  But I probably won't
spend anymore time defending it. :-)

> 
> 
> 
> > +	return;
> > +}
> > +#endif /* CONFIG_PERF_EVENTS_NMI */
> > +
> > +/* watchdog kicker functions */
> > +static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
> > +{
> > +	int this_cpu = smp_processor_id();
> > +	unsigned long touch_ts = per_cpu(watchdog_touch_ts, this_cpu);
> > +	struct pt_regs *regs = get_irq_regs();
> > +	int duration;
> > +
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +	/* kick the hardlockup detector */
> > +	__get_cpu_var(hrtimer_interrupts)++;
> > +#endif
> 
> 
> 
> Please avoid such ifdefs in the middle of the code.
> 
> It's better to gather hardlockup matters in a single ifdef block:
> 
> #ifdef CONFIG_PERF_EVENTS_NMI
> define your functions here
> #else
> define (if needed) your off case functions here (static inline stubs)
> #endif

ok.

> 
> 
> 
> > +
> > +	/* kick the softlockup detector */
> > +	wake_up_process(__get_cpu_var(softlockup_watchdog));
> > +
> > +	/* .. and repeat */
> > +	hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
> > +
> > +	if (touch_ts == 0) {
> > +		__touch_watchdog();
> > +		return HRTIMER_RESTART;
> > +	}
> > +
> > +	/* check for a softlockup
> > +	 * This is done by making sure a high priority task is
> > +	 * being scheduled.  The task touches the watchdog to
> > +	 * indicate it is getting cpu time.  If it hasn't then
> > +	 * this is a good indication some task is hogging the cpu
> > +	 */
> > +	duration = is_softlockup(touch_ts, this_cpu);
> > +	if (duration) {
> > +		printk(KERN_ERR "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
> > +			this_cpu, duration,
> > +			current->comm, task_pid_nr(current));
> > +		print_modules();
> > +		print_irqtrace_events(current);
> > +		if (regs)
> > +			show_regs(regs);
> > +		else
> > +			dump_stack();
> > +
> > +		if (softlockup_panic)
> > +			panic("softlockup: hung tasks");
> 
> 
> 
> You probably want a backtrace cpu mask here as well
> (but better don't use the same than the hardlockup thing)

yup.

> 
> 
> 
> > +	}
> > +
> > +	return HRTIMER_RESTART;
> > +}
> > +
> > +
> > +/*
> > + * The watchdog thread - touches the timestamp.
> > + */
> > +static int watchdog(void *__bind_cpu)
> > +{
> > +	struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
> > +	struct hrtimer *hrtimer = &per_cpu(watchdog_hrtimer, (unsigned long)__bind_cpu);
> > +
> > +	sched_setscheduler(current, SCHED_FIFO, &param);
> > +
> > +	/* initialize timestamp */
> > +	__touch_watchdog();
> > +
> > +	/* kick off the timer for the hardlockup detector */
> > +	/* done here because hrtimer_start can only pin to smp_processor_id() */
> > +	hrtimer_start(hrtimer, ns_to_ktime(get_sample_period()),
> > +		      HRTIMER_MODE_REL_PINNED);
> > +
> > +	set_current_state(TASK_INTERRUPTIBLE);
> > +	/*
> > +	 * Run briefly once per second to reset the softlockup timestamp.
> > +	 * If this gets delayed for more than 60 seconds then the
> > +	 * debug-printout triggers in softlockup_tick().
> > +	 */
> > +	while (!kthread_should_stop()) {
> > +		__touch_watchdog();
> > +		schedule();
> > +
> > +		if (kthread_should_stop())
> > +			break;
> > +
> > +		set_current_state(TASK_INTERRUPTIBLE);
> > +	}
> > +	__set_current_state(TASK_RUNNING);
> > +
> > +	return 0;
> > +}
> > +
> > +
> > +/* prepare/enable/disable routines */
> > +static int watchdog_prepare_cpu(int cpu)
> > +{
> > +	struct hrtimer *hrtimer = &per_cpu(watchdog_hrtimer, cpu);
> > +
> > +	BUG_ON(per_cpu(softlockup_watchdog, cpu));
> 
> 
> 
> Please warn here instead, nothing seriously dangerous is going to
> happen.
> 
> 
> 
> > +	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> > +	hrtimer->function = watchdog_timer_fn;
> > +
> > +	return 0;
> > +}
> > +
> > +static int watchdog_enable(int cpu)
> > +{
> > +	struct task_struct *p = per_cpu(softlockup_watchdog, cpu);
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +	struct perf_event_attr *wd_attr;
> > +	struct perf_event *event = per_cpu(watchdog_ev, cpu);
> > +
> > +	/* is it already setup and enabled? */
> > +	if (event && event->state > PERF_EVENT_STATE_OFF)
> > +		goto out;
> > +
> > +	/* it is setup but not enabled */
> > +	if (event != NULL)
> > +		goto out_enable;
> > +
> > +	/* Try to register using hardware perf events */
> > +	wd_attr = &wd_hw_attr;
> > +	wd_attr->sample_period = hw_nmi_get_sample_period();
> > +	event = perf_event_create_kernel_counter(wd_attr, cpu, -1, watchdog_overflow_callback);
> > +	if (!IS_ERR(event)) {
> > +		printk(KERN_INFO "NMI watchdog enabled, takes one hw-pmu counter.\n");
> > +		goto out_save;
> > +	}
> > +
> > +	printk(KERN_ERR "NMI watchdog failed to create perf event on cpu%i: %p\n", cpu, event);
> > +	return -1;
> > +
> > +	/* success path */
> > +out_save:
> > +	per_cpu(watchdog_ev, cpu) = event;
> > +out_enable:
> > +	perf_event_enable(per_cpu(watchdog_ev, cpu));
> > +out:
> > +#endif /* CONFIG_PERF_EVENTS_NMI */
> 
> 
> 
> Especially such kind of idef in a function plus goto in the middle, we really
> want to avoid that.
> 
> You want a watchdog_nmi_enable() call instead that does nothing
> if !CONFIG_PERF_EVENTS_NMI.

ok.

> 
> 
> 
> > +
> > +	/* create the watchdog thread */
> > +	if (!p) {
> > +		p = kthread_create(watchdog, (void *)(unsigned long)cpu, "watchdog/%d", cpu);
> > +		if (IS_ERR(p)) {
> > +			printk(KERN_ERR "softlockup watchdog for %i failed\n", cpu);
> > +			return -1;
> > +		}
> > +		kthread_bind(p, cpu);
> > +		per_cpu(watchdog_touch_ts, cpu) = 0;
> > +		per_cpu(softlockup_watchdog, cpu) = p;
> > +		wake_up_process(p);
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static void watchdog_disable(int cpu)
> > +{
> > +	struct task_struct *p = per_cpu(softlockup_watchdog, cpu);
> > +	struct hrtimer *hrtimer = &per_cpu(watchdog_hrtimer, cpu);
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +	struct perf_event *event = per_cpu(watchdog_ev, cpu);
> > +#endif
> > +
> > +	/*
> > +	 * cancel the timer first to stop incrementing the stats
> > +	 * and waking up the kthread
> > +	 */
> > +	hrtimer_cancel(hrtimer);
> > +
> > +#ifdef CONFIG_PERF_EVENTS_NMI
> > +	if (event) {
> > +		perf_event_disable(event);
> > +		per_cpu(watchdog_ev, cpu) = NULL;
> > +
> > +		/* should be in cleanup, but blocks oprofile */
> > +		perf_event_release_kernel(event);
> > +	}
> > +#endif
> > +
> > +	if (p) {
> > +		per_cpu(softlockup_watchdog, cpu) = NULL;
> > +		kthread_stop(p);
> > +	}
> > +
> > +	/* if any cpu succeeds, watchdog is considered enabled for the system */
> > +	nmi_watchdog_enabled = 1;
> > +}
> > +
> > +static void watchdog_cleanup(int cpu)
> > +{
> > +
> > +}
> > +
> > +static void watchdog_enable_all_cpus(void)
> > +{
> > +	int cpu;
> > +	int result;
> > +
> > +	for_each_online_cpu(cpu)
> > +		result += watchdog_enable(cpu);
> > +
> > +	if (result)
> > +		printk(KERN_ERR "watchdog: failed to be enabled on some cpus\n");
> > +
> > +}
> > +
> > +static void watchdog_disable_all_cpus(void)
> > +{
> > +	int cpu;
> > +
> > +	for_each_online_cpu(cpu)
> > +		watchdog_disable(cpu);
> > +
> > +	/* if all watchdogs are disabled, then they are disabled for the system */
> > +	nmi_watchdog_enabled = 0;
> > +}
> > +
> > +
> > +/* sysctl functions */
> > +#ifdef CONFIG_SYSCTL
> > +/*
> > + * proc handler for /proc/sys/kernel/nmi_watchdog
> > + */
> > +
> > +int proc_nmi_enabled(struct ctl_table *table, int write,
> > +		     void __user *buffer, size_t *length, loff_t *ppos)
> > +{
> > +	touch_all_watchdog();
> 
> 
> 
> Why do you need to touch watchdogs here?

just legacy stuff I copied over.  I can remove.

> 
> 
> 
> > +	proc_dointvec(table, write, buffer, length, ppos);
> > +
> > +	if (nmi_watchdog_enabled)
> > +		watchdog_enable_all_cpus();
> > +	else
> > +		watchdog_disable_all_cpus();
> > +	return 0;
> > +}
> > +
> > +int proc_dowatchdog_thresh(struct ctl_table *table, int write,
> > +			     void __user *buffer,
> > +			     size_t *lenp, loff_t *ppos)
> > +{
> > +	touch_all_watchdog();
> 
> 
> 
> Same here?
> 
> 
> > +	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> > +}
> > +
> > +/* stub functions */
> > +int proc_dosoftlockup_thresh(struct ctl_table *table, int write,
> > +			     void __user *buffer,
> > +			     size_t *lenp, loff_t *ppos)
> > +{
> > +	return proc_dowatchdog_thresh(table, write, buffer, lenp, ppos);
> > +}
> > +/* end of stub functions */
> > +#endif /* CONFIG_SYSCTL */
> > +
> > +
> > +/*
> > + * Create/destroy watchdog threads as CPUs come and go:
> > + */
> > +static int __cpuinit
> > +cpu_callback(struct notifier_block *nfb, unsigned long action, void *hcpu)
> > +{
> > +	int hotcpu = (unsigned long)hcpu;
> > +
> > +	switch (action) {
> > +	case CPU_UP_PREPARE:
> > +	case CPU_UP_PREPARE_FROZEN:
> > +		if (watchdog_prepare_cpu(hotcpu))
> > +			return NOTIFY_BAD;
> > +		break;
> > +	case CPU_ONLINE:
> > +	case CPU_ONLINE_FROZEN:
> > +		if (watchdog_enable(hotcpu))
> > +			return NOTIFY_BAD;
> > +		break;
> > +#ifdef CONFIG_HOTPLUG_CPU
> > +	case CPU_UP_CANCELED:
> > +	case CPU_UP_CANCELED_FROZEN:
> > +		watchdog_disable(hotcpu);
> > +		break;
> > +	case CPU_DEAD:
> > +	case CPU_DEAD_FROZEN:
> > +		watchdog_disable(hotcpu);
> > +		watchdog_cleanup(hotcpu);
> > +		break;
> > +#endif /* CONFIG_HOTPLUG_CPU */
> > +	}
> > +	return NOTIFY_OK;
> > +}
> > +
> > +static struct notifier_block __cpuinitdata cpu_nfb = {
> > +	.notifier_call = cpu_callback
> > +};
> > +
> > +static int __init spawn_watchdog_task(void)
> > +{
> > +	void *cpu = (void *)(long)smp_processor_id();
> > +	int err;
> > +
> > +	if (no_watchdog)
> > +		return 0;
> > +
> > +	err = cpu_callback(&cpu_nfb, CPU_UP_PREPARE, cpu);
> > +	if (err == NOTIFY_BAD) {
> > +		BUG();
> 
> 
> 
> Please warn instead, there is nothing fatal here.
> 
> 
> 
> > +		return 1;
> > +	}
> > +	cpu_callback(&cpu_nfb, CPU_ONLINE, cpu);
> > +	register_cpu_notifier(&cpu_nfb);
> > +
> > +	atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
> > +
> > +	return 0;
> > +}
> > +early_initcall(spawn_watchdog_task);
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index e2e73cc..280794a 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -171,15 +171,24 @@ config DETECT_SOFTLOCKUP
> >  	   support it.)
> >  
> >  config NMI_WATCHDOG
> > -	bool "Detect Hard Lockups with an NMI Watchdog"
> > -	depends on DEBUG_KERNEL && PERF_EVENTS && PERF_EVENTS_NMI
> > +	bool "Detect Hard and Soft Lockups"
> > +	depends on DEBUG_KERNEL
> > +	default DETECT_SOFTLOCKUP
> 
> 
> 
> I'd suggest to drop the NMI prefix, this is not anymore
> about detecting hard lockups only.
> 
> May be config WATCHDOG or config LOCKUP_DETECTOR if it's taken
> already.

yeah, I thought about that too.  Will work on it.

> 
> 
> Also you should half-drop the DETECT_SOFTLOCKUP thing:
> keep it's definition but drop the ability to choose it from
> the prompt:
> 
> config DETECT_SOFTLOCKUP
> 	bool
> 	depends on DEBUG_KERNEL && !S390
> 	default y
> 
> This way we keep it for compatibility with def_configs, it will
> enable the WATCHDOG by default if it is "y", we can schedule
> its removal later.

I understand the general idea but not quite the implementation idea.  I will work
on it and see what I come up with.

Thanks for the review!

Cheers,
Don
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