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softlockup檢測(watchdog)原理(用于檢測系統(tǒng)調度是否正常)
摘自:http://blog.chinaunix.net/xmlrpc.php?r=blog/article&uid=14528823&id=4215888    
softlockup(watchdog)用于檢測系統(tǒng)調度是否正常，即軟鎖的情況，當發(fā)生softlockup時，內核不能調度，但還能響應中斷，對用戶的表現(xiàn)可能為：能ping通，但無法登陸系統(tǒng)，無法進行正常操作。
其基本原理為：為每個CPU啟動一個內核線程(watchdog/x)，此線程為優(yōu)先級最高的實時線程，在該線程得到調度時，會更新相應的計數(shù)(時間戳)，同時會啟動定時器，當定時器到期時檢查相應的時間戳，如果超過指定時間，都沒有更新，則說明這段時間內都沒有發(fā)生調度(因為此線程優(yōu)先級最高)，則打印相應告警或根據配置可以進入panic流程。
基本代碼分析(2.6.32)
rest_init->kernel_init->lockup_detector_init->cpu_callback->watchdog_prepare_cpu(初始化watchdog定時器):

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  1. static int watchdog_prepare_cpu(int cpu)
  2. {
  3.     struct hrtimer *hrtimer = &per_cpu(watchdog_hrtimer, cpu);

  5.     WARN_ON(per_cpu(softlockup_watchdog, cpu));
  6.     hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);//初始化高精度定時器
  7.     hrtimer->function = watchdog_timer_fn;//設置定時器處理函數(shù)

  9.     return 0;
  10. }
看門狗定時器處理函數(shù)：

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  1. static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
  2. {
  3. //獲取計數(shù)watchdog_touch_ts，該計數(shù)在watchdog內核線程被調度時更新
  4.     unsigned long touch_ts = __get_cpu_var(watchdog_touch_ts);
  5.     struct pt_regs *regs = get_irq_regs();
  6.     int duration;

  8.     /* kick the hardlockup detector */
  9. //增加中斷計數(shù)，證明沒有發(fā)生硬鎖(關中斷死鎖)
  10.     watchdog_interrupt_count();

  12.     /* kick the softlockup detector */
  13. //喚醒wathdog內核線程
  14.     wake_up_process(__get_cpu_var(softlockup_watchdog));

  16.     /* .. and repeat */
  17. //重啟定時器
  18.     hrtimer_forward_now(hrtimer, ns_to_ktime(get_sample_period()));
  19.     if (touch_ts == 0) {
  20.         if (unlikely(__get_cpu_var(softlockup_touch_sync))) {
  21.             /*
  22.              * If the time stamp was touched atomically
  23.              * make sure the scheduler tick is up to date.
  24.              */
  25.             __get_cpu_var(softlockup_touch_sync) = false;
  26.             sched_clock_tick();
  27.         }
  28.         __touch_watchdog();
  29.         return HRTIMER_RESTART;
  30.     }

  32.     /* check for a softlockup
  33.      * This is done by making sure a high priority task is
  34.      * being scheduled. The task touches the watchdog to
  35.      * indicate it is getting cpu time. If it hasn't then
  36.      * this is a good indication some task is hogging the cpu
  37.      */
  38. //判斷是否發(fā)生了軟鎖，原理是判斷touch_ts(時間戳)是否超過一定時間沒有更新
  39.     duration = is_softlockup(touch_ts);
  40.     if (unlikely(duration)) {
  41.         /* only warn once */
  42.         if (__get_cpu_var(soft_watchdog_warn) == true)
  43.             return HRTIMER_RESTART;
  44. //發(fā)生了軟鎖后，進行一些列的信息記錄和告警。
  45.         printk(KERN_EMERG "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
  46.             smp_processor_id(), duration,
  47.             current->comm, task_pid_nr(current));
  48.         print_modules();
  49.         print_irqtrace_events(current);
  50.         if (regs)
  51.             show_regs(regs);
  52.         else
  53.             dump_stack();
  54. //如果配置了softlockup_panic(proc中配置)，則panic
  55.         if (softlockup_panic)
  56.             panic("softlockup: hung tasks");
  57.         __get_cpu_var(soft_watchdog_warn) = true;
  58.     } else
  59.         __get_cpu_var(soft_watchdog_warn) = false;

  61.     return HRTIMER_RESTART;
  62. }

啟動看門狗，即創(chuàng)建watchdog內核線程。

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  1. static int watchdog_enable(int cpu)
  2. {
  3.     struct task_struct *p = per_cpu(softlockup_watchdog, cpu);
  4.     int err = 0;

  6.     /* enable the perf event */
  7.     err = watchdog_nmi_enable(cpu);

  9.     /* Regardless of err above, fall through and start softlockup */

  11.     /* create the watchdog thread */
  12.     if (!p) {
  13. //創(chuàng)建watchdog內核線程
  14.         p = kthread_create(watchdog, (void *)(unsigned long)cpu, "watchdog/%d", cpu);
  15.         if (IS_ERR(p)) {
  16.             printk(KERN_ERR "softlockup watchdog for %i failed\n", cpu);
  17.             if (!err)
  18.                 /* if hardlockup hasn't already set this */
  19.                 err = PTR_ERR(p);
  20.             goto out;
  21.         }
  22.         kthread_bind(p, cpu);
  23.         per_cpu(watchdog_touch_ts, cpu) = 0;
  24.         per_cpu(softlockup_watchdog, cpu) = p;
  25.         wake_up_process(p);
  26.     }

  28. out:
  29.     return err;
  30. }

watchdog內核線程執(zhí)行主函數(shù)，主要是要更新計數(shù)(時間戳)

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  1. static int watchdog(void *unused)
  2. {
  3. //設置為最高優(yōu)先級
  4.     struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
  5.     struct hrtimer *hrtimer = &__raw_get_cpu_var(watchdog_hrtimer);
  6. //設置為實時線程
  7.     sched_setscheduler(current, SCHED_FIFO, &param);

  9.     /* initialize timestamp */
  10. //初始化計數(shù)(時間戳)
  11.     __touch_watchdog();

  13.     /* kick off the timer for the hardlockup detector */
  14.     /* done here because hrtimer_start can only pin to smp_processor_id() */
  15. //啟動定時器，用于檢測是否發(fā)生軟鎖
  16.     hrtimer_start(hrtimer, ns_to_ktime(get_sample_period()),
  17.          HRTIMER_MODE_REL_PINNED);
  18. //睡眠
  19.     set_current_state(TASK_INTERRUPTIBLE);
  20.     /*
  21.      * Run briefly once per second to reset the softlockup timestamp.
  22.      * If this gets delayed for more than 60 seconds then the
  23.      * debug-printout triggers in watchdog_timer_fn().
  24.      */
  25.     while (!kthread_should_stop()) {
  26. //更新計數(shù)
  27.         __touch_watchdog();
  28.         schedule();

  30.         if (kthread_should_stop())
  31.             break;

  33.         set_current_state(TASK_INTERRUPTIBLE);
  34.     }
  35.     __set_current_state(TASK_RUNNING);

  37.     return 0;
  38. }

判斷是否發(fā)生軟鎖：is_softlockup

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  1. static int is_softlockup(unsigned long touch_ts)
  2. {
  3.     unsigned long now = get_timestamp(smp_processor_id());

  5.     /* Warn about unreasonable delays: */
  6. //檢測計數(shù)多久沒有更新了，如果超過了60s，則表示發(fā)生了軟鎖
  7.     if (time_after(now, touch_ts + softlockup_thresh))
  8.         return now - touch_ts;

  10.     return 0;
  11. }

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