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 * Detect hard lockups on a system
 * started by Don Zickus, Copyright (C) 2010 Red Hat, Inc.
 * Note: Most of this code is borrowed heavily from the original softlockup
 * detector, so thanks to Ingo for the initial implementation.
 * Some chunks also taken from the old x86-specific nmi watchdog code, thanks
 * to those contributors as well.

#define pr_fmt(fmt) "NMI watchdog: " fmt

#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/sched/debug.h>

#include <asm/irq_regs.h>
#include <linux/perf_event.h>

static DEFINE_PER_CPU(bool, hard_watchdog_warn);
static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
static DEFINE_PER_CPU(struct perf_event *, watchdog_ev);

static unsigned long hardlockup_allcpu_dumped;

void arch_touch_nmi_watchdog(void)
	 * Using __raw here because some code paths have
	 * preemption enabled.  If preemption is enabled
	 * then interrupts should be enabled too, in which
	 * case we shouldn't have to worry about the watchdog
	 * going off.
	raw_cpu_write(watchdog_nmi_touch, true);

static DEFINE_PER_CPU(ktime_t, last_timestamp);
static DEFINE_PER_CPU(unsigned int, nmi_rearmed);
static ktime_t watchdog_hrtimer_sample_threshold __read_mostly;

void watchdog_update_hrtimer_threshold(u64 period)
	 * The hrtimer runs with a period of (watchdog_threshold * 2) / 5
	 * So it runs effectively with 2.5 times the rate of the NMI
	 * watchdog. That means the hrtimer should fire 2-3 times before
	 * the NMI watchdog expires. The NMI watchdog on x86 is based on
	 * unhalted CPU cycles, so if Turbo-Mode is enabled the CPU cycles
	 * might run way faster than expected and the NMI fires in a
	 * smaller period than the one deduced from the nominal CPU
	 * frequency. Depending on the Turbo-Mode factor this might be fast
	 * enough to get the NMI period smaller than the hrtimer watchdog
	 * period and trigger false positives.
	 * The sample threshold is used to check in the NMI handler whether
	 * the minimum time between two NMI samples has elapsed. That
	 * prevents false positives.
	 * Set this to 4/5 of the actual watchdog threshold period so the
	 * hrtimer is guaranteed to fire at least once within the real
	 * watchdog threshold.
	watchdog_hrtimer_sample_threshold = period * 2;

static bool watchdog_check_timestamp(void)
	ktime_t delta, now = ktime_get_mono_fast_ns();

	delta = now - __this_cpu_read(last_timestamp);
	if (delta < watchdog_hrtimer_sample_threshold) {
		 * If ktime is jiffies based, a stalled timer would prevent
		 * jiffies from being incremented and the filter would look
		 * at a stale timestamp and never trigger.
		if (__this_cpu_inc_return(nmi_rearmed) < 10)
			return false;
	__this_cpu_write(nmi_rearmed, 0);
	__this_cpu_write(last_timestamp, now);
	return true;
static inline bool watchdog_check_timestamp(void)
	return true;

static struct perf_event_attr wd_hw_attr = {
	.size		= sizeof(struct perf_event_attr),
	.pinned		= 1,
	.disabled	= 1,

/* Callback function for perf event subsystem */
static void watchdog_overflow_callback(struct perf_event *event,
		 struct perf_sample_data *data,
		 struct pt_regs *regs)
	/* Ensure the watchdog never gets throttled */
	event->hw.interrupts = 0;

	if (atomic_read(&watchdog_park_in_progress) != 0)

	if (__this_cpu_read(watchdog_nmi_touch) == true) {
		__this_cpu_write(watchdog_nmi_touch, false);

	if (!watchdog_check_timestamp())

	/* 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()) {
		int this_cpu = smp_processor_id();

		/* only print hardlockups once */
		if (__this_cpu_read(hard_watchdog_warn) == true)

		pr_emerg("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
		if (regs)

		 * Perform all-CPU dump only once to avoid multiple hardlockups
		 * generating interleaving traces
		if (sysctl_hardlockup_all_cpu_backtrace &&
				!test_and_set_bit(0, &hardlockup_allcpu_dumped))

		if (hardlockup_panic)
			nmi_panic(regs, "Hard LOCKUP");

		__this_cpu_write(hard_watchdog_warn, true);

	__this_cpu_write(hard_watchdog_warn, false);

 * People like the simple clean cpu node info on boot.
 * Reduce the watchdog noise by only printing messages
 * that are different from what cpu0 displayed.
static unsigned long firstcpu_err;
static atomic_t watchdog_cpus;

int watchdog_nmi_enable(unsigned int cpu)
	struct perf_event_attr *wd_attr;
	struct perf_event *event = per_cpu(watchdog_ev, cpu);
	int firstcpu = 0;

	/* nothing to do if the hard lockup detector is disabled */
	if (!(watchdog_enabled & NMI_WATCHDOG_ENABLED))
		goto out;

	/* 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;

	if (atomic_inc_return(&watchdog_cpus) == 1)
		firstcpu = 1;

	wd_attr = &wd_hw_attr;
	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);

	/* Try to register using hardware perf events */
	event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, watchdog_overflow_callback, NULL);

	/* save the first cpu's error for future comparision */
	if (firstcpu && IS_ERR(event))
		firstcpu_err = PTR_ERR(event);

	if (!IS_ERR(event)) {
		/* only print for the first cpu initialized */
		if (firstcpu || firstcpu_err)
			pr_info("enabled on all CPUs, permanently consumes one hw-PMU counter.\n");
		goto out_save;

	 * Disable the hard lockup detector if _any_ CPU fails to set up
	 * set up the hardware perf event. The watchdog() function checks
	 * the NMI_WATCHDOG_ENABLED bit periodically.
	 * The barriers are for syncing up watchdog_enabled across all the
	 * cpus, as clear_bit() does not use barriers.
	clear_bit(NMI_WATCHDOG_ENABLED_BIT, &watchdog_enabled);

	/* skip displaying the same error again */
	if (!firstcpu && (PTR_ERR(event) == firstcpu_err))
		return PTR_ERR(event);

	/* vary the KERN level based on the returned errno */
	if (PTR_ERR(event) == -EOPNOTSUPP)
		pr_info("disabled (cpu%i): not supported (no LAPIC?)\n", cpu);
	else if (PTR_ERR(event) == -ENOENT)
		pr_warn("disabled (cpu%i): hardware events not enabled\n",
		pr_err("disabled (cpu%i): unable to create perf event: %ld\n",
			cpu, PTR_ERR(event));

	pr_info("Shutting down hard lockup detector on all cpus\n");

	return PTR_ERR(event);

	/* success path */
	per_cpu(watchdog_ev, cpu) = event;
	perf_event_enable(per_cpu(watchdog_ev, cpu));
	return 0;

void watchdog_nmi_disable(unsigned int cpu)
	struct perf_event *event = per_cpu(watchdog_ev, cpu);

	if (event) {
		per_cpu(watchdog_ev, cpu) = NULL;

		/* should be in cleanup, but blocks oprofile */

		/* watchdog_nmi_enable() expects this to be zero initially. */
		if (atomic_dec_and_test(&watchdog_cpus))
			firstcpu_err = 0;