在dmesg中，看到如下信息：
[424948.577401] ixgbe 0000:86:00.0 eth4: Fake Tx hang detected with timeout of 5 seconds [424949.535143] ixgbe 0000:86:00.1 eth5: Fake Tx hang detected with timeout of 5 seconds [424955.536045] ixgbe 0000:af:00.0 eth6: Fake Tx hang detected with timeout of 10 seconds [424955.567988] ixgbe 0000:af:00.1 eth7: Fake Tx hang detected with timeout of 10 seconds [424957.579250] ixgbe 0000:18:00.1 eth1: Fake Tx hang detected with timeout of 10 seconds [424957.579285] ixgbe 0000:3b:00.1 eth3: Fake Tx hang detected with timeout of 10 seconds [424958.568923] ixgbe 0000:86:00.0 eth4: Fake Tx hang detected with timeout of 10 seconds [424959.526676] ixgbe 0000:86:00.1 eth5: Fake Tx hang detected with timeout of 10 seconds [424975.489166] ixgbe 0000:af:00.0 eth6: Fake Tx hang detected with timeout of 20 seconds [424975.553019] ixgbe 0000:af:00.1 eth7: Fake Tx hang detected with timeout of 20 seconds [424977.532376] ixgbe 0000:18:00.1 eth1: Fake Tx hang detected with timeout of 20 seconds [424977.532409] ixgbe 0000:3b:00.1 eth3: Fake Tx hang detected with timeout of 20 seconds
检测超时的函数：
static void fm10k_tx_timeout(struct net_device *netdev) { struct fm10k_intfc *interface = netdev_priv(netdev); bool real_tx_hang = false; int i; #define TX_TIMEO_LIMIT 16000 for (i = 0; i < interface->num_tx_queues; i++) { struct fm10k_ring *tx_ring = interface->tx_ring[i]; if (check_for_tx_hang(tx_ring) && fm10k_check_tx_hang(tx_ring)) real_tx_hang = true; } if (real_tx_hang) { fm10k_tx_timeout_reset(interface); } else { netif_info(interface, drv, netdev, "Fake Tx hang detected with timeout of %d seconds ", netdev->watchdog_timeo / HZ); /* fake Tx hang - increase the kernel timeout */ if (netdev->watchdog_timeo < TX_TIMEO_LIMIT) netdev->watchdog_timeo *= 2;-----------按倍数递增，直到大于16s，本文就是5-10-20递增， } }
网卡检测是否hung的关键函数是 fm10k_tx_timeout，如果 if (check_for_tx_hang(tx_ring) && fm10k_check_tx_hang(tx_ring)) 条件满足，则会属于real hung，否则是fake hung。
check_for_tx_hang(tx_ring)肯定都是满足的，一般在probe的时候就会设置，fm10k_check_tx_hang 的代码如下：
bool fm10k_check_tx_hang(struct fm10k_ring *tx_ring) { u32 tx_done = fm10k_get_tx_completed(tx_ring); u32 tx_done_old = tx_ring->tx_stats.tx_done_old; u32 tx_pending = fm10k_get_tx_pending(tx_ring, true); clear_check_for_tx_hang(tx_ring); /* Check for a hung queue, but be thorough. This verifies * that a transmit has been completed since the previous * check AND there is at least one packet pending. By * requiring this to fail twice we avoid races with * clearing the ARMED bit and conditions where we * run the check_tx_hang logic with a transmit completion * pending but without time to complete it yet. */ if (!tx_pending || (tx_done_old != tx_done)) {-----------------没有pending的报文，或者pending的值没变过 /* update completed stats and continue */ tx_ring->tx_stats.tx_done_old = tx_done; /* reset the countdown */ clear_bit(__FM10K_HANG_CHECK_ARMED, &tx_ring->state); return false; } /* make sure it is true for two checks in a row */ return test_and_set_bit(__FM10K_HANG_CHECK_ARMED, &tx_ring->state);----------------两次alarm，则肯定返回true }
伴随网卡hung打印的，一般都有cpu的softlock，如果cpu 是softlock，而且tx做了cpu绑定的话，那么该cpu对应的tx则会没有pending报文，从而触发hung。如果没有做绑定，则这个tx可能被多个cpu来使用，如果再出现hung，则要查看对应的tx的锁，是否被拿了没有释放。
阶段性总结一下：
内核中检测hung有不同的对象，不同的级别。
1.本文说的网卡的hung，针对的是某个设备，级别是网卡的队列，原理是检测是否有pending的tx包超时没有处理。它依赖于网卡设备正常。
2.还有一种检测某个调度进程的hung的机制，就是hung_task.c文件中的khungtaskd内核线程，该内核线程检测处于uninterrupt状态的进程持续的时间，如果大于一个阈值，则认为该进程hung住了，这个检测的方法是遍历task，然后看task的调度次数是否变化了，这个是单个进程级别。对象是处于uninterrupt状态的进程如果时间长了，则认为hung，它依赖于调度。
3.一种是检测softlock导致的hung，主要是检测某个cpu级别进程调度是否正常，是watchdog内核线程来做的，因为它是实时进程，如果前后两次它没有获取到调度，则说明调度出了问题，这个前后是指通过hrtimer的硬中断来触发的wakeup来判断。这个对象是某个cpu核（到超线程级别）。它依赖于硬中断，关抢占时间长了没有让出cpu，则会出softlock。
4.一种是检测hardlock的hung，它依赖于nmi，原理就是利用3里面那个hrtimer，每次3里面的hrtimer来了，则增长 当前cpu的 hrtimer_interrupts ，如果前后两次nmi的回调检测这个计数没有增长，则认为cpu遇到了hardlock，也就是关中断时间长了，则会出hardlock。
下面详细描述：
[root@centos7 WakeTest]# ps -ef |grep -i khungtaskd |grep -v grep root 93 2 0 9月04 ? 00:00:00 [khungtaskd]----------------------检测处于D状态的进程是否长时间未被调度
名称是khungtaskd，和watchdog注意区分：
static int __init hung_task_init(void) { atomic_notifier_chain_register(&panic_notifier_list, &panic_block); watchdog_task = kthread_run(watchdog, NULL, "khungtaskd");--------虽然内核线程的函数是watchdog，但是线程名字却是khungtaskd return 0; }
另外一个名称为watchdog内核线程：
ps |grep -i watchdog 6 2 0 ffff880c11980080 IN 0.0 0 0 [watchdog/0] 10 2 1 ffff880c11a2b580 IN 0.0 0 0 [watchdog/1] 14 2 2 ffff880c11a56a80 IN 0.0 0 0 [watchdog/2] 18 2 3 ffff880c11a62080 IN 0.0 0 0 [watchdog/3] 22 2 4 ffff880c11a9f580 IN 0.0 0 0 [watchdog/4] 26 2 5 ffff880c11aa8a80 IN 0.0 0 0 [watchdog/5] 30 2 6 ffff880c11ab4080 IN 0.0 0 0 [watchdog/6] 34 2 7 ffff880c11acd580 IN 0.0 0 0 [watchdog/7] 38 2 8 ffff880c11ad6a80 IN 0.0 0 0 [watchdog/8] 42 2 9 ffff880c11b04080 IN 0.0 0 0 [watchdog/9] 46 2 10 ffff880c11b45580 IN 0.0 0 0 [watchdog/10] 50 2 11 ffff880c11b4ea80 IN 0.0 0 0 [watchdog/11] 54 2 12 ffff880c11b5e080 IN 0.0 0 0 [watchdog/12] 58 2 13 ffff880c11b77580 IN 0.0 0 0 [watchdog/13] 62 2 14 ffff880c11b80a80 IN 0.0 0 0 [watchdog/14] 66 2 15 ffff880c11baa080 IN 0.0 0 0 [watchdog/15]
这个是由watchdog.c中，每个cpu一个：
static struct smp_hotplug_thread watchdog_threads = { .store = &softlockup_watchdog, .thread_should_run = watchdog_should_run, .thread_fn = watchdog, .thread_comm = "watchdog/%u", .setup = watchdog_enable, .cleanup = watchdog_cleanup, .park = watchdog_disable, .unpark = watchdog_enable, };
使能的一些函数以及回调：
/* * common function for watchdog, nmi_watchdog and soft_watchdog parameter * * caller | table->data points to | 'which' contains the flag(s) * -------------------|-----------------------|----------------------------- * proc_watchdog | watchdog_user_enabled | NMI_WATCHDOG_ENABLED or'ed * | | with SOFT_WATCHDOG_ENABLED * -------------------|-----------------------|----------------------------- * proc_nmi_watchdog | nmi_watchdog_enabled | NMI_WATCHDOG_ENABLED * -------------------|-----------------------|----------------------------- * proc_soft_watchdog | soft_watchdog_enabled | SOFT_WATCHDOG_ENABLED */
要关闭这些内核线程，使用：
[root@centos7 WakeTest]# echo 0 > /proc/sys/kernel/watchdog [root@centos7 WakeTest]# ps -ef |grep -w watchdog |grep -v grep [root@centos7 WakeTest]# [root@centos7 WakeTest]# [root@centos7 WakeTest]# echo 1 > /proc/sys/kernel/watchdog [root@centos7 WakeTest]# ps -ef |grep -w watchdog |grep -v grep root 13496 2 0 11:34 ? 00:00:00 [watchdog/0] root 13497 2 0 11:34 ? 00:00:00 [watchdog/1] root 13498 2 0 11:34 ? 00:00:00 [watchdog/2] root 13499 2 0 11:34 ? 00:00:00 [watchdog/3] root 13500 2 0 11:34 ? 00:00:00 [watchdog/4] root 13501 2 0 11:34 ? 00:00:00 [watchdog/5] root 13502 2 0 11:34 ? 00:00:00 [watchdog/6] root 13503 2 0 11:34 ? 00:00:00 [watchdog/7] root 13504 2 0 11:34 ? 00:00:00 [watchdog/8] root 13505 2 0 11:34 ? 00:00:00 [watchdog/9] root 13506 2 0 11:34 ? 00:00:00 [watchdog/10] root 13507 2 0 11:34 ? 00:00:00 [watchdog/11] root 13508 2 0 11:34 ? 00:00:00 [watchdog/12] root 13509 2 0 11:34 ? 00:00:00 [watchdog/13] root 13510 2 0 11:34 ? 00:00:00 [watchdog/14] root 13511 2 0 11:34 ? 00:00:00 [watchdog/15]
他们都是实时进程：
top - 11:07:00 up 20:49, 10 users, load average: 41.97, 45.49, 48.37 Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie %Cpu(s): 7.1 us, 14.7 sy, 0.0 ni, 54.7 id, 4.2 wa, 2.5 hi, 16.8 si, 0.0 st, 57.3 id_exact, 2.9 hi_exact, 20.0 irq_exact KiB Mem : 36231846+total, 50661748 free, 11323638+used, 19842035+buff/cache KiB Swap: 0 total, 0 free, 0 used. 16986171+avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 403 root rt 0 0 0 0 S 0.0 0.0 0:00.10 watchdog/3
watchdog检测的原理是：
watchdog函数负责根据当前时间戳来更新一个自己保存的时间戳percpu变量watchdog_touch_ts （取到s级别）
，然后另外的一个hrtimer负责比较当前时间与watchdog_touch_ts 这个变量的差值，如果这个差值大于某个阈值watchdog，则认为异常。 hrtimer同时负责wakeup watchdog线程，
hrtimer 中用 is_softlockup 用来确定是否已经软锁，按道理唤醒watchdog之后，watchdog应该要调度，同时更新时间戳，如果没有更新，说明没有获得调度，由于watchdog内核线程是
绑定cpu核的实时线程，实时线程未能调度，则代表这个cpu出现了软锁。
static int is_softlockup(unsigned long touch_ts)-----------------------touch_ts就是watchdog线程write的时间 { unsigned long now = get_timestamp(); if ((watchdog_enabled & SOFT_WATCHDOG_ENABLED) && watchdog_thresh){ /* Warn about unreasonable delays. */ if (time_after(now, touch_ts + get_softlockup_thresh())) return now - touch_ts; } return 0; }
这个检测机制，大家可以看到，明显依赖于硬中断的到来，假设某个cpu关闭硬中断很长的时间，那显然就没办法保证watchdog的运行了，所以又必要检测一下，这个hardlock登上舞台。
static 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; }