hung 之 softlockup hardlockup 检测

1. softlockup & hardlockup 的含义

softlockup 指的是这样一种场景：由于内核程序设计问题，导致CPU长时间关闭抢占。

hardlockup 指的是这样一种场景：由于内核程序设计问题，导致CPU时钟中断长时间禁用。

softlockup 或 hardlockup 发生时，CPU被当前运行的任务独占，而其他任务得不到调度。

2. softlockup & hardlockup 检测机制

2.1 基本原理

内核检测 softlockup & hardlockup 的机制为 lockup detector。

lockup detector 本质上也是一个看门狗（watchdog）的思路，包括

1. 两条狗，即两个变量 watchdog_touch_ts（softlockup 狗）、hrtimer_interrupts（hardlockup 狗）。

2. 一个定时器，定时器的回调方法中更新 hrtimer_interrupts 的值加 1（喂 hardlockup 狗），同时在 CPU上添加一个喂狗任务，检查 softlockup 狗的状态。喂狗任务执行时将 watchdog_touch_ts 的值加1（喂 softlockup 狗）。

3. NMI计数器（PMU中断），回调方法中检查 hardlockup 狗的状态。

如果超过一定时间 hrtimer_interrupts 值没变化，则认为发生了 hardlockup；

如果超过一定时间 watchdog_touch_ts 值没变化，则认为发生了 softlockup。

2.2 功能开关

CONFIG_LOCKUP_DETECTOR 控制 lockup detector 功能是否使能。

CONFIG_SOFTLOCKUP_DETECTOR 控制 softlockup detector 功能是否使能。

CONFIG_HARDLOCKUP_DETECTOR_PERF 控制 hardlockup detector 功能是否使能。

下面是我本地的 Android 设备配置

yudi:/ # zcat /proc/config.gz |grep CONFIG_LOCKUP_DETECTOR
yudi:/ #yudi:/ # zcat /proc/config.gz |grep CONFIG_HARDLOCKUP_DETECTOR_PERF
yudi:/ #

2.3 功能配置

lockup detector 功能通过 /proc/sys/kernel/xxx 节点配置，包括

节点	功能
/proc/sys/kernel/watchdog	开/关 softlock watchdog，对应变量 watchdog_user_enabled
/proc/sys/kernel/nmi_watchdog	开/关 hardlock watchdog，对应变量 nmi_watchdog_user_enabled
/proc/sys/kernel/watchdog_thresh	表示 hardlockup 阈值（即时钟中断禁用时间超过阈值时，认为发生 hardlockup），对应变量 watchdog_thresh
/proc/sys/kernel/watchdog_cpumask	表示目标 CPU IDs 的掩码，对应变量 watchdog_allowed_mask

可以通过 sysctl 系统调用或者读写 /proc/sys/kernel/xxx 节点的方式获取、修改相关变量的值。

例如，通过写 /proc/sys/kernel/watchdog 节点可以修改 watchdog_user_enabled 的值，从而控制 softlockup watchdog 功能的开关。

//kernel/sysctl.c#if defined(CONFIG_LOCKUP_DETECTOR){.procname       = "watchdog",.data                = &watchdog_user_enabled,.maxlen                = sizeof(int),.mode                = 0644,.proc_handler   = proc_watchdog,.extra1                = SYSCTL_ZERO,.extra2                = SYSCTL_ONE,},{.procname        = "watchdog_thresh",.data                = &watchdog_thresh,.maxlen                = sizeof(int),.mode                = 0644,.proc_handler        = proc_watchdog_thresh,.extra1                = SYSCTL_ZERO,.extra2                = &sixty,},{.procname       = "nmi_watchdog",.data                = &nmi_watchdog_user_enabled,.maxlen                = sizeof(int),.mode                = NMI_WATCHDOG_SYSCTL_PERM,.proc_handler   = proc_nmi_watchdog,.extra1                = SYSCTL_ZERO,.extra2                = SYSCTL_ONE,},{.procname        = "watchdog_cpumask",.data                = &watchdog_cpumask_bits,.maxlen                = NR_CPUS,.mode                = 0644,.proc_handler        = proc_watchdog_cpumask,},
...

例如，写 /proc/sys/kernel/watchdog 节点，设置 watchdog_user_enabled 变量的值，如果值改变则调用 proc_watchdog_update 方法更新 watchdog action。

// kernel/watchdog.c/** common function for watchdog, nmi_watchdog and soft_watchdog parameter** caller             | table->data points to      | 'which'* -------------------|----------------------------|--------------------------* proc_watchdog      | watchdog_user_enabled      | NMI_WATCHDOG_ENABLED |*                    |                            | SOFT_WATCHDOG_ENABLED* -------------------|----------------------------|--------------------------* proc_nmi_watchdog  | nmi_watchdog_user_enabled  | NMI_WATCHDOG_ENABLED* -------------------|----------------------------|--------------------------* proc_soft_watchdog | soft_watchdog_user_enabled | SOFT_WATCHDOG_ENABLED*/
static int proc_watchdog_common(int which, struct ctl_table *table, int write,void *buffer, size_t *lenp, loff_t *ppos)
{int err, old, *param = table->data;mutex_lock(&watchdog_mutex);if (!write) {/** On read synchronize the userspace interface. This is a* racy snapshot.*/*param = (watchdog_enabled & which) != 0;err = proc_dointvec_minmax(table, write, buffer, lenp, ppos);} else {old = READ_ONCE(*param);// 3err = proc_dointvec_minmax(table, write, buffer, lenp, ppos);if (!err && old != READ_ONCE(*param))proc_watchdog_update(); // 4}mutex_unlock(&watchdog_mutex);return err;
}/** /proc/sys/kernel/watchdog*/
// 1
int proc_watchdog(struct ctl_table *table, int write,void *buffer, size_t *lenp, loff_t *ppos)
{// 2return proc_watchdog_common(NMI_WATCHDOG_ENABLED|SOFT_WATCHDOG_ENABLED,table, write, buffer, lenp, ppos);
}

2.4 代码实现

2.4.1 初始化

2.4.1.1 初始化场景

lockup detector 功能的初始化方法是 __lockup_detector_reconfigure。

内核启动或 lockup detector 参数发生变化，调用初始化方法。

内核启动时，在 lockup_detector_setup 方法中调用初始化方法。

/** Create the watchdog thread infrastructure and configure the detector(s).** The threads are not unparked as watchdog_allowed_mask is empty.  When* the threads are successfully initialized, take the proper locks and* unpark the threads in the watchdog_cpumask if the watchdog is enabled.*/
static __init void lockup_detector_setup(void)
{/** If sysctl is off and watchdog got disabled on the command line,* nothing to do here.*/lockup_detector_update_enable();if (!IS_ENABLED(CONFIG_SYSCTL) &&!(watchdog_enabled && watchdog_thresh))return;mutex_lock(&watchdog_mutex);__lockup_detector_reconfigure();softlockup_initialized = true;mutex_unlock(&watchdog_mutex);
}

lockup detector 参数（例如 watchdog_user_enabled）变化时，在 proc_watchdog_update 方法中调用初始化方法。

/* Propagate any changes to the watchdog threads */
static void proc_watchdog_update(void)
{/* Remove impossible cpus to keep sysctl output clean. */cpumask_and(&watchdog_cpumask, &watchdog_cpumask, cpu_possible_mask);__lockup_detector_reconfigure();
}

在内核启动时，初始化 lockup detector 的方法中，会检查 CONFIG_SYSCTL 是否使能。如果CONFIG_SYSCTL 未使能，且 watchdog_enabled 或 watchdog_thresh 中有至少一个变量没有有效值（即为0），那么 lockup detector 初始化失败，直接返回。

不过，虽然 watchdog_enabled 的缺省值是 0，但是由于 CONFIG_SYSCTL 一般都是使能的，所以通常来说 lockup detector 初始化不会失败。

下面是 watchdog_enabled 和 watchdog_thresh 变量的定义。

unsigned long __read_mostly watchdog_enabled;
...
int __read_mostly watchdog_thresh = 10;

在 Linux 内核中，定义全局变量时，它们的默认值取决于变量的存储类别和位置。对于未显式初始化的全局变量，它们的默认值为 0。这是因为全局变量会被存储在 BSS（Block Started by Symbol）段中，BSS 段在程序启动时会被初始化为 0。因此 watchdog_enabled 的默认值是 0。

另外 watchdog_enabled 还设置了 __read_mostly 属性，这是一个内核特定的属性，用于提示编译器将变量放置在一个更适合频繁读取的内存区域，但这不会改变变量的初始化规则。

2.4.1.2 初始化方法

在 softlockup detector 使能时（CONFIG_SOFTLOCKUP_DETECTOR 定义）先调用 softlockup_start_all 使能 softlockup detector，然后再调用 watchdog_nmi_start（实际上是一个空函数，真正起作用的是 watchdog_nmi_enable 方法）使能 hardlockup detector。

否则只调用 watchdog_nmi_start 使能 hardlockup detector。

#ifdef CONFIG_SOFTLOCKUP_DETECTOR
...
static void __lockup_detector_reconfigure(void)
{cpus_read_lock();watchdog_nmi_stop();softlockup_stop_all();set_sample_period();lockup_detector_update_enable();if (watchdog_enabled && watchdog_thresh)softlockup_start_all();watchdog_nmi_start();cpus_read_unlock();/** Must be called outside the cpus locked section to prevent* recursive locking in the perf code.*/__lockup_detector_cleanup();
}
#else /* CONFIG_SOFTLOCKUP_DETECTOR */
static void __lockup_detector_reconfigure(void)
{cpus_read_lock();watchdog_nmi_stop();lockup_detector_update_enable();watchdog_nmi_start();cpus_read_unlock();
}
...

2.4.2 softlockup 检测

softlockup detector 初始化时，调用 smp_call_on_cpu 在 watchdog_allowed_mask 标记的所有 CPU（一般是对所有CPU）上执行一次 softlockup_start_fn 方法。

// kernel/watchdog.c
static void softlockup_start_all(void)
{int cpu;cpumask_copy(&watchdog_allowed_mask, &watchdog_cpumask);for_each_cpu(cpu, &watchdog_allowed_mask)smp_call_on_cpu(cpu, softlockup_start_fn, NULL, false);
}

smp_call_on_cpu 本质上是向目标 CPU 发送一个跨处理器中断（Inter-Processor Interrupt, IPI），并在目标 CPU 上的中断处理程序中执行目标函数。这个函数会在目标 CPU 上的中断上下文中执行，通常是在中断处理程序中或者在中断处理完成后立即执行。这确保了函数能够在指定的 CPU 上执行，而无需创建新的线程。

发送 IPI: 调用 smp_call_on_cpu 会导致当前 CPU 向目标 CPU 发送一个 IPI 请求。
处理 IPI: 目标 CPU 收到 IPI 后，会触发中断，进入中断处理程序。
执行目标函数: 在中断处理程序中，目标函数会被调用，传递给 smp_call_on_cpu 的参数也会传递给目标函数。
中断退出: 目标函数执行完毕后，中断处理程序退出，目标 CPU 恢复正常的执行流。

softlockup_start_fn 方法在当前 CPU 上设置并启动定时器 watchdog_hrtimer，然后更新 watchdog_touch_ts 变量的值（watchdog_touch_ts 变量就是被喂的狗），最后调用 watchdog_nmi_enable 使能 hardlockup detector。

定时器到达时的处理函数设置为 watchdog_timer_fn；

定时器周期设置为 sample_period；

static int softlockup_start_fn(void *data)
{watchdog_enable(smp_processor_id());return 0;
}

static void watchdog_enable(unsigned int cpu)
{struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);struct completion *done = this_cpu_ptr(&softlockup_completion);WARN_ON_ONCE(cpu != smp_processor_id());init_completion(done);complete(done);/** Start the timer first to prevent the NMI watchdog triggering* before the timer has a chance to fire.*/// 设置并启动定时器hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);hrtimer->function = watchdog_timer_fn;hrtimer_start(hrtimer, ns_to_ktime(sample_period),HRTIMER_MODE_REL_PINNED_HARD);/* Initialize timestamp */// 更新一下 watchdog_touch_ts（设置为当前的时间戳）update_touch_ts();/* Enable the perf event */// 使能 hard lockupif (watchdog_enabled & NMI_WATCHDOG_ENABLED)watchdog_nmi_enable(cpu);
}

更新 watchdog_touch_ts，即将 watchdog_touch_ts 变量修改为当前的 timestamp。

/* Commands for resetting the watchdog */
static void update_touch_ts(void)
{__this_cpu_write(watchdog_touch_ts, get_timestamp());
}

定时器周期 sample_period

sample_period 缺省值是 0，但是在初始化时，会调用 set_sample_period 设置 sample_period 的值。设置为 20s/5 = 4s。

static void set_sample_period(void)
{/** convert watchdog_thresh from seconds to ns* the divide by 5 is to give hrtimer several chances (two* or three with the current relation between the soft* and hard thresholds) to increment before the* hardlockup detector generates a warning*/sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 5);watchdog_update_hrtimer_threshold(sample_period);
}

int __read_mostly watchdog_thresh = 10;/** Hard-lockup warnings should be triggered after just a few seconds. Soft-* lockups can have false positives under extreme conditions. So we generally* want a higher threshold for soft lockups than for hard lockups. So we couple* the thresholds with a factor: we make the soft threshold twice the amount of* time the hard threshold is.*/
static int get_softlockup_thresh(void)
{return watchdog_thresh * 2;
}

定时器到达时的处理函数 watchdog_timer_fn。

定时器周期默认是 5s，softlockup 阈值默认是 20s。

定时器每次到达时，在目标 cpu 上执行一个（异步）喂狗任务，喂狗任务将变量 watchdog_touch_ts 值加 1。

如果经过 4 个定时器周期，watchdog_touch_ts 值都没有变化，则说明喂狗任务经过 20s 都没有得到执行，判断发生了 softlockup。

/* watchdog kicker functions */
static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
{// watchdog_touch_ts 的值unsigned long touch_ts = __this_cpu_read(watchdog_touch_ts);struct pt_regs *regs = get_irq_regs();int duration;int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;if (!watchdog_enabled)return HRTIMER_NORESTART;/* kick the hardlockup detector */watchdog_interrupt_count();/* kick the softlockup detector */// completion_done 检查上一次的喂狗是否完成，如果上一次喂狗还没完成，这里就不会再喂狗了。// 否则，设置喂狗状态为未完成状态，然后调用 stop_one_cpu_nowait 在目标 CPU 上添加一个喂狗任务（softlockup_fn）。// 注意，喂狗任务不会 block 当前线程的执行if (completion_done(this_cpu_ptr(&softlockup_completion))) {reinit_completion(this_cpu_ptr(&softlockup_completion));stop_one_cpu_nowait(smp_processor_id(),softlockup_fn, NULL,this_cpu_ptr(&softlockup_stop_work));}/* .. and repeat */// repeat 定时器hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));if (touch_ts == SOFTLOCKUP_RESET) {if (unlikely(__this_cpu_read(softlockup_touch_sync))) {/** If the time stamp was touched atomically* make sure the scheduler tick is up to date.*/__this_cpu_write(softlockup_touch_sync, false);sched_clock_tick();}/* Clear the guest paused flag on watchdog reset */kvm_check_and_clear_guest_paused();update_touch_ts();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*/// is_softlockup 方法检查 watchdog_touch_ts 值与当前时间戳的差值，// 如果差值超过阈值，说明 watchdog_touch_ts 长时间未更新，即喂狗方法长时间未得到执行duration = is_softlockup(touch_ts);// 如果喂狗方法长时间未得到执行，dump stack 甚至 panicif (unlikely(duration)) {/** If a virtual machine is stopped by the host it can look to* the watchdog like a soft lockup, check to see if the host* stopped the vm before we issue the warning*/if (kvm_check_and_clear_guest_paused())return HRTIMER_RESTART;/** Prevent multiple soft-lockup reports if one cpu is already* engaged in dumping all cpu back traces.*/if (softlockup_all_cpu_backtrace) {if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn))return HRTIMER_RESTART;}/* Start period for the next softlockup warning. */update_touch_ts();pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",smp_processor_id(), duration,current->comm, task_pid_nr(current));print_modules();print_irqtrace_events(current);if (regs)show_regs(regs);elsedump_stack();if (softlockup_all_cpu_backtrace) {trigger_allbutself_cpu_backtrace();clear_bit_unlock(0, &soft_lockup_nmi_warn);}add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);if (softlockup_panic)panic("softlockup: hung tasks");}return HRTIMER_RESTART;
}

喂狗函数 softlockup_fn，执行

更新 watchdog_touch_ts 的值
设置喂狗状态为完成

/** The watchdog thread function - touches the timestamp.** It only runs once every sample_period seconds (4 seconds by* default) to reset the softlockup timestamp. If this gets delayed* for more than 2*watchdog_thresh seconds then the debug-printout* triggers in watchdog_timer_fn().*/
static int softlockup_fn(void *data)
{update_touch_ts();complete(this_cpu_ptr(&softlockup_completion));return 0;
}

为什么喂狗任务 softlockup_fn 要通过 stop_one_cpu_nowait 方法放到目标 CPU 上执行，而不是用 smp_call_on_cpu。

1. stop_one_cpu_nowait 的工作机制

stop_one_cpu_nowait 使用的是工作队列机制，将一个工作项安排到目标 CPU 上执行。如果目标 CPU 禁止了抢占，工作队列项将会被延迟执行，直到抢占再次被允许。

2. CPU 禁止抢占

如果目标 CPU 禁止了抢占，stop_one_cpu_nowait 安排的任务可能会受到影响。在 Linux 内核中，抢占（preemption）是指允许高优先级任务中断低优先级任务的执行。当抢占被禁止时，内核不会切换到其他任务，即使有高优先级的任务需要执行。这可能导致安排在目标 CPU 上的任务无法及时执行，直到抢占被允许。

当目标 CPU 禁止了抢占时，内核会保持当前任务的执行，直到当前任务显式地放弃 CPU 或抢占再次被允许。这意味着：

a. 当前任务的独占执行：当前任务将独占 CPU，任何其他任务，包括通过 stop_one_cpu_nowait 安排的任务，都不会得到执行。

b. 延迟执行：stop_one_cpu_nowait 安排的任务将在目标 CPU 恢复抢占后才有机会执行。

而 smp_call_on_cpu 是通过核间中断的方式在目标CPU上执行函数，即使CPU禁用抢占也依然会执行。

由于 softlockup 的目的就是为了发现（软件设计问题导致）CPU长时间关闭抢占的情况，因此用 smp_call_on_cpu 的方式来喂狗是不行的。

2.4.3 hardlockup 检测

使能 hardlockup 的方法是 watchdog_nmi_enable。

/** These functions can be overridden if an architecture implements its* own hardlockup detector.** watchdog_nmi_enable/disable can be implemented to start and stop when* softlockup watchdog threads start and stop. The arch must select the* SOFTLOCKUP_DETECTOR Kconfig.*/
int __weak watchdog_nmi_enable(unsigned int cpu)
{hardlockup_detector_perf_enable();return 0;
}

hardlockup_detector_perf_enable 方法由 watchdog_hld 模块实现，并且由宏 CONFIG_HARDLOCKUP_DETECTOR_PERF 控制使能。

// include/linux/nmi.h#if defined(CONFIG_HARDLOCKUP_DETECTOR_PERF)
extern void arch_touch_nmi_watchdog(void);
extern void hardlockup_detector_perf_stop(void);
extern void hardlockup_detector_perf_restart(void);
extern void hardlockup_detector_perf_disable(void);
extern void hardlockup_detector_perf_enable(void);
extern void hardlockup_detector_perf_cleanup(void);
extern int hardlockup_detector_perf_init(void);
#else
static inline void hardlockup_detector_perf_stop(void) { }
static inline void hardlockup_detector_perf_restart(void) { }
static inline void hardlockup_detector_perf_disable(void) { }
static inline void hardlockup_detector_perf_enable(void) { }
static inline void hardlockup_detector_perf_cleanup(void) { }
# if !defined(CONFIG_HAVE_NMI_WATCHDOG)
static inline int hardlockup_detector_perf_init(void) { return -ENODEV; }
static inline void arch_touch_nmi_watchdog(void) {}
# else
static inline int hardlockup_detector_perf_init(void) { return 0; }
# endif
#endif

hardlockup_detector_perf_enable 方法定义在 watchdog_hld.c 中。

调用 hardlockup_detector_event_create 方法。

// kernel/watchdog_hld.c/*** hardlockup_detector_perf_enable - Enable the local event*/
void hardlockup_detector_perf_enable(void)
{if (hardlockup_detector_event_create())return;/* use original value for check */if (!atomic_fetch_inc(&watchdog_cpus))pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");perf_event_enable(this_cpu_read(watchdog_ev));
}

hardlockup_detector_perf_enable 方法。

static int hardlockup_detector_event_create(void)
{unsigned int cpu = smp_processor_id();struct perf_event_attr *wd_attr;struct perf_event *evt;wd_attr = &wd_hw_attr;wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);/* Try to register using hardware perf events */evt = perf_event_create_kernel_counter(wd_attr, cpu, NULL,watchdog_overflow_callback, NULL);if (IS_ERR(evt)) {pr_debug("Perf event create on CPU %d failed with %ld\n", cpu,PTR_ERR(evt));return PTR_ERR(evt);}this_cpu_write(watchdog_ev, evt);return 0;
}

这里的 watchdog_thresh 是 watchdog.c 中定义的变量，默认值是 10s。

将 watchdog_thresh 转换成 CPU周期数，然后调用 perf_event_create_kernel_counter 方法向 PMU（Performance Monitor Unit）注册一个回调，当 PMU 计数超过目标CPU周期数时执行回调方法 watchdog_overflow_callback。

watchdog_overflow_callback 调用 is_hardlockup（定义在 watchdog.c 中）检查定时器中断数（即变量 hrtimer_interrupts，也定义在 watchdog.c 中）是否有变化。

如果没有变化就认为定时器中断被禁用了很长时间，触发 dump stack 甚至 panic。

/* 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 (__this_cpu_read(watchdog_nmi_touch) == true) {__this_cpu_write(watchdog_nmi_touch, false);return;}if (!watchdog_check_timestamp())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()) {int this_cpu = smp_processor_id();/* only print hardlockups once */if (__this_cpu_read(hard_watchdog_warn) == true)return;pr_emerg("Watchdog detected hard LOCKUP on cpu %d\n",this_cpu);print_modules();print_irqtrace_events(current);if (regs)show_regs(regs);elsedump_stack();/** 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))trigger_allbutself_cpu_backtrace();if (hardlockup_panic)nmi_panic(regs, "Hard LOCKUP");__this_cpu_write(hard_watchdog_warn, true);return;}__this_cpu_write(hard_watchdog_warn, false);return;
}

hrtimer_interrupts 变量以及 is_hardlockup 方法的定义。

// kernel/watchdog.cstatic DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);/* watchdog detector functions */
bool is_hardlockup(void)
{unsigned long hrint = __this_cpu_read(hrtimer_interrupts);if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)return true;__this_cpu_write(hrtimer_interrupts_saved, hrint);return false;
}

hrtimer_interrupts 变量值的更新。

在 softlockup 定时器的回调方法 watchdog_timer_fn 中，更新 hrtimer_interrupts 的值（+1）。

// kernel/watchdog.c/* watchdog kicker functions */
static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
{unsigned long touch_ts = __this_cpu_read(watchdog_touch_ts);struct pt_regs *regs = get_irq_regs();int duration;int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;if (!watchdog_enabled)return HRTIMER_NORESTART;/* kick the hardlockup detector */watchdog_interrupt_count();
...

定时器周期默认是 5s，hardlockup 阈值默认是 10s。

定时器每次到达时，更新 hrtimer_interrupts 的值（+1）。

如果经过 2 个定时器周期，hrtimer_interrupts 值都没有变化，则说明经过 10s 没有产生时钟中断，判断发生了 hardlockup。

3. 忽略 lockup 检查

内核模块可以通过调用 touch_nmi_watchdog 或 touch_all_softlockup_watchdogs 方法影响 softlockup、hardlockup 的 check。

touch_nmi_watchdog 可以让忽略当前一次的 hardlockup 检查；

touch_all_softlockup_watchdogs 可以让忽略当前一次的 softlockup 检查。

3.1 touch_nmi_watchdog

touch_nmi_watchdog 方法。

将变量 watchdog_nmi_touch 置为 true！

// include/linux/nmi.h/*** touch_nmi_watchdog - restart NMI watchdog timeout.** If the architecture supports the NMI watchdog, touch_nmi_watchdog()* may be used to reset the timeout - for code which intentionally* disables interrupts for a long time. This call is stateless.*/
static inline void touch_nmi_watchdog(void)
{arch_touch_nmi_watchdog();touch_softlockup_watchdog();
}

static DEFINE_PER_CPU(bool, watchdog_nmi_touch);notrace 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);
}
EXPORT_SYMBOL(arch_touch_nmi_watchdog);

hardlockup 的 PMU nmi 中断回调方法中，首先就会检查一下 watchdog_nmi_touch 的值。

如果 watchdog_nmi_touch 被设置成了 true，就只是将 watchdog_nmi_touch 重新设为 false 就返回了，不继续检查是否发生了 hardlockup（等下一次 PMU nmi 中断再检查）。

// kernel/watchdog_hld.c/* 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 (__this_cpu_read(watchdog_nmi_touch) == true) {__this_cpu_write(watchdog_nmi_touch, false);return;}...

3.2 touch_all_softlockup_watchdogs

touch_all_softlockup_watchdogs 方法。

将每个 CPU 的 watchdog_touch_ts 变量置为 SOFTLOCKUP_RESET（LONG_MAX）
调用 wq_watchdog_touch(-1) 将 workqueue 模块的变量 wq_watchdog_touched 置为当前的 jiffies（表示当前时间的 jiffies 数量）

注意. wq_watchdog_touch(-1) 实际上与 lockup 无关，它影响的是 workqueue 的 watchdog（检测 workqueue 卡死的机制，跟 lockup watchdog 不是一回事）。

// kernel/watchdog.cvoid touch_all_softlockup_watchdogs(void)
{int cpu;/** watchdog_mutex cannpt be taken here, as this might be called* from (soft)interrupt context, so the access to* watchdog_allowed_cpumask might race with a concurrent update.** The watchdog time stamp can race against a concurrent real* update as well, the only side effect might be a cycle delay for* the softlockup check.*/for_each_cpu(cpu, &watchdog_allowed_mask)per_cpu(watchdog_touch_ts, cpu) = SOFTLOCKUP_RESET;wq_watchdog_touch(-1);
}

在 softlockup 检测定时器的回调函数 watchdog_timer_fn 中，如果检查到 watchdog_touch_ts 的值是 SOFTLOCKUP_RESET，不会认为是发生了 softlockup，而是直接返回。

// kernel/watchdog.c/* watchdog kicker functions */
static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
{unsigned long touch_ts = __this_cpu_read(watchdog_touch_ts);struct pt_regs *regs = get_irq_regs();int duration;int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;if (!watchdog_enabled)return HRTIMER_NORESTART;/* kick the hardlockup detector */watchdog_interrupt_count();/* kick the softlockup detector */if (completion_done(this_cpu_ptr(&softlockup_completion))) {reinit_completion(this_cpu_ptr(&softlockup_completion));stop_one_cpu_nowait(smp_processor_id(),softlockup_fn, NULL,this_cpu_ptr(&softlockup_stop_work));}/* .. and repeat */hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));if (touch_ts == SOFTLOCKUP_RESET) {if (unlikely(__this_cpu_read(softlockup_touch_sync))) {/** If the time stamp was touched atomically* make sure the scheduler tick is up to date.*/__this_cpu_write(softlockup_touch_sync, false);sched_clock_tick();}/* Clear the guest paused flag on watchdog reset */kvm_check_and_clear_guest_paused();update_touch_ts();return HRTIMER_RESTART;}... ...