在由单片机构成的微型计算机系统中,由于单片机的工作常常会受到来自外界电磁场的干扰,造成程序的跑飞,而陷入死循环,程序的正常运行被打断,由单片机控制的系统无法继续工作,会造成整个系统的陷入停滞状态,发生不可预料的后果,所以出于对单片机运行状态进行实时监测的考虑,便产生了一种专门用于监测单片机程序运行状态的芯片,俗称”看门狗”。
在Android系统中也需要看好几个重要的Service门，用于发现出了问题的Service杀掉SystemServer进程，所以有必要了解并分析其系统问题。
那么被监控的有哪些Service呢？
ActivityManagerService.java :frameworks\base\services\java\com\android\server\am
PowerManagerService.java :frameworks\base\services\java\com\android\server
WindowManagerService.java :frameworks\base\services\java\com\android\server
下面就依次分析一下其整个处理流程：
1、初始化
run @ SysemServer.java
Slog.i(TAG, “Init Watchdog”);
Watchdog.getInstance().init(context, battery, power, alarm,
ActivityManagerService.self());
这里使用单例模式创建：
public static Watchdog getInstance() {
if (sWatchdog == null) {
sWatchdog = new Watchdog();
}
return sWatchdog;
}
public void init(Context context, BatteryService battery,
PowerManagerService power, AlarmManagerService alarm,
ActivityManagerService activity) {
// 上下文环境变量
mResolver = context.getContentResolver();
mBattery = battery;
mPower = power;
mAlarm = alarm;
mActivity = activity;
// 登记 RebootReceiver() 接收，用于reboot广播接收使用
context.registerReceiver(new RebootReceiver(),
new IntentFilter(REBOOT_ACTION));
…
// 系统启动时间
mBootTime = System.currentTimeMillis();
}
ok,调用init函数启动完毕
2、运行中
run @ SysemServer.java
调用 Watchdog.getInstance().start(); 启动看门狗
首先看下 Watchdog 类定义：
/** This class calls its monitor every minute. Killing this process if they don’t return **/
public class Watchdog extends Thread {
}
从线程类中继承，即会在一个单独线程中运行，调用thrrad.start()即调用 Watchdog.java 中的 run() 函数
public void run() {
boolean waitedHalf = false;
while (true) {
mCompleted = false;
// 1、给mHandler发送 MONITOR 消息，用于请求检查 Service是否工作正常
mHandler.sendEmptyMessage(MONITOR);
synchronized (this) {
// 2、进行 wait 等待 timeout 时间确认是否退出循环
long timeout = TIME_TO_WAIT;
// NOTE: We use uptimeMillis() here because we do not want to increment the time we
// wait while asleep. If the device is asleep then the thing that we are waiting
// to timeout on is asleep as well and won’t have a chance to run, causing a false
// positive on when to kill things.
long start = SystemClock.uptimeMillis();
while (timeout > 0 && !mForceKillSystem) {
try {
wait(timeout); // notifyAll() is called when mForceKillSystem is set
} catch (InterruptedException e) {
Log.wtf(TAG, e);
}
timeout = TIME_TO_WAIT – (SystemClock.uptimeMillis() – start);
}
// 3、如果 mCompleted 为真表示service一切正常，后面会再讲到
if (mCompleted && !mForceKillSystem) {
// The monitors have returned.
waitedHalf = false;
continue;
}
// 4、表明检测到了有 deadlock-detection 条件发生，利用 dumpStackTraces 打印堆栈依信息
if (!waitedHalf) {
// We’ve waited half the deadlock-detection interval. Pull a stack
// trace and wait another half.
ArrayList<Integer> pids = new ArrayList<Integer>();
pids.add(Process.myPid());
ActivityManagerService.dumpStackTraces(true, pids, null, null);
waitedHalf = true;
continue; // 不过这里会再次检测一次
}
}
SystemClock.sleep(2000);
// 5、打印内核栈调用关系
// Pull our own kernel thread stacks as well if we’re configured for that
if (RECORD_KERNEL_THREADS) {
dumpKernelStackTraces();
}
// 6、ok,系统出问题了，检测到某个 Service 出现死锁情况，杀死SystemServer进程
// Only kill the process if the debugger is not attached.
if (!Debug.isDebuggerConnected()) {
Slog.w(TAG, “*** WATCHDOG KILLING SYSTEM PROCESS: ” + name);
Process.killProcess(Process.myPid());
System.exit(10);
} else {
Slog.w(TAG, “Debugger connected: Watchdog is *not* killing the system process”);
}
waitedHalf = false;
}
}
主要工作逻辑：监控线程每隔一段时间发送一条 MONITOR 线另外一个线程，另个一个线程会检查各个 Service 是否正常运行，看门狗就不停的检查并等待结果，失败则杀死SystemServer.
3、Service 检查线程
/**
* Used for scheduling monitor callbacks and checking memory usage.
*/
final class HeartbeatHandler extends Handler {
@Override
public void handleMessage(Message msg) { // Looper 消息处理函数
switch (msg.what) {
case MONITOR: {
// 依次检测各个服务，即调用 monitor() 函数
final int size = mMonitors.size();
for (int i = 0 ; i < size ; i++) {
mCurrentMonitor = mMonitors.get(i);
mCurrentMonitor.monitor();
}
// 检测成功则设置 mCompleted 变量为 true
synchronized (Watchdog.this) {
mCompleted = true;
mCurrentMonitor = null;
}
下面我们来看一下各个Service如何确定自已运行ok呢？以 ActivityManagerService 为例：
首先加入检查队列：
private ActivityManagerService() {
// Add ourself to the Watchdog monitors.
Watchdog.getInstance().addMonitor(this);
}
然后实现 monitor() 函数：
/** In this method we try to acquire our lock to make sure that we have not deadlocked */
public void monitor() {
synchronized (this) { }
}
明白了吧，其实就是检查这个 Service 是否发生了死锁，对于此情况就只能kill SystemServer系统了。对于死锁的产生原因非常多，但有个情况需要注意：java层死锁可能发生在调用native函数，而native函数可能与硬件交互导致时间过长而没有返回，从而导致长时间占用导致问题。
4、内存使用检测
消息发送
final class GlobalPssCollected implements Runnable {
public void run() {
mHandler.sendEmptyMessage(GLOBAL_PSS);
}
}
检测内存处理函数：
final class HeartbeatHandler extends Handler {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case GLOBAL_PSS: {
if (mHaveGlobalPss) {
// During the last pass we collected pss information, so
// now it is time to report it.
mHaveGlobalPss = false;
if (localLOGV) Slog.v(TAG, “Received global pss, logging.”);
logGlobalMemory();
}
} break;
其主要功能如下,统计pSS状况及读取相关linux内核中内存信息：
void logGlobalMemory() {
mActivity.collectPss(stats);
Process.readProcLines(“/proc/meminfo”, mMemInfoFields, mMemInfoSizes);
Process.readProcLines(“/proc/vmstat”, mVMStatFields, mVMStatSizes);
}
