简介

android系统中SystemServer WatchDog的主要作用是监控SystemServer进程的运行状态,防止其卡住或者死锁。

具体来说,watchDog线程会定期去检查SystemServer线程的运行情况。如果发现SystemServer线程超过一定时间未有响应,watchDog会认为SystemServer进程发生了问题,这时它会采取以下行动:

1. 打印出SystemServer线程当前的堆栈信息,以帮助定位问题。

日志格式如下"Blocked in monitor(monitor 不为空)|Blocked in handler on（monitor为空）"

2. 重启SystemServer进程。watchDog线程会先杀死已卡住的SystemServer进程,然后重新fork出一个新的SystemServer进程。

通过这种机制,watchDog线程可以像一只“看门狗”一样时刻监视SystemServer的状态,一旦发现SystemServer发生故障,就可以及时采取行动重启它,从而提高系统的健壮性和稳定性。watchDog线程在系统启动时由Init进程 fork 出,它需要持续运行以保护 SystemServer 不会发生故障时无人管控的情况

实现方式：

通过监听system_server进程中时间敏感线程的调度时间来判断进程当前是否卡顿，或者长时间持锁

WatchDog初始化：

       //system_server前台线程mMonitorChecker = new HandlerChecker(FgThread.getHandler(),"foreground thread");mHandlerCheckers.add(withDefaultTimeout(mMonitorChecker));// Add checker for main thread.  We only do a quick check since there// can be UI running on the thread.//system_server主线程mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(new Handler(Looper.getMainLooper()), "main thread")));// Add checker for shared UI thread. system_server ui线程mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(UiThread.getHandler(), "ui thread")));// And also check IO thread.mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(IoThread.getHandler(), "i/o thread")));// And the display thread.mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(DisplayThread.getHandler(), "display thread")));// And the animation thread. system_server 动画执行线程mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(AnimationThread.getHandler(), "animation thread")));// And the surface animation thread.mHandlerCheckers.add(withDefaultTimeout(new HandlerChecker(SurfaceAnimationThread.getHandler(),"surface animation thread")));//检测是否binder线程池耗尽addMonitor(new BinderThreadMonitor());

  WatchDog中循环：

    public class Watchdog implements Dumpable {private void run() {boolean waitedHalf = false;while (true) {for (int i=0; i<mHandlerCheckers.size(); i++) {HandlerCheckerAndTimeout hc = mHandlerCheckers.get(i);//向handler中插入空消息或者monitor检测消息hc.checker().scheduleCheckLocked(hc.customTimeoutMillis().orElse(watchdogTimeoutMillis * Build.HW_TIMEOUT_MULTIPLIER));}long start = SystemClock.uptimeMillis();while (timeout > 0) {//睡眠半个检测周期，后检测消息是否得到及时处理mLock.wait(timeout);}final int waitState = evaluateCheckerCompletionLocked();if (waitState == COMPLETED) {// The monitors have returned; resetwaitedHalf = false;continue;} else if (waitState == WAITING) {continue;} else if (waitState == WAITED_HALF) {if (!waitedHalf) {Slog.i(TAG, "WAITED_HALF");waitedHalf = true;blockedCheckers = getCheckersWithStateLocked(WAITED_HALF);subject = describeCheckersLocked(blockedCheckers);pids = new ArrayList<>(mInterestingJavaPids);doWaitedHalfDump = true;} else {continue;}} else {//所有超时的handlerblockedCheckers = getCheckersWithStateLocked(OVERDUE);subject = describeCheckersLocked(blockedCheckers);allowRestart = mAllowRestart;pids = new ArrayList<>(mInterestingJavaPids);}} //打印handler消息logWatchog(doWaitedHalfDump, subject, pids);//杀掉system_server进程Process.killProcess(Process.myPid());System.exit(10);}public final class HandlerChecker implements Runnable {public void scheduleCheckLocked(long handlerCheckerTimeoutMillis) {mWaitMax = handlerCheckerTimeoutMillis;if (mCompleted) {// Safe to update monitors in queue, Handler is not in the middle of workmMonitors.addAll(mMonitorQueue);mMonitorQueue.clear();}//如果当前monitors为空并且消息队列中无消息if ((mMonitors.size() == 0 && mHandler.getLooper().getQueue().isPolling())|| (mPauseCount > 0)) {mCompleted = true;return;}if (!mCompleted) {// we already have a check in flight, so no needreturn;}mCompleted = false;mCurrentMonitor = null;mStartTime = SystemClock.uptimeMillis();//把自身post到队列中，检测mMonitors耗时，如果mMonitors为空则仅检测handler中是否有阻塞消息，mMonitors中大多是检测锁对象是否及时释放mHandler.postAtFrontOfQueue(this);}@Overridepublic void run() {final int size = mMonitors.size();for (int i = 0 ; i < size ; i++) {synchronized (mLock) {mCurrentMonitor = mMonitors.get(i);}mCurrentMonitor.monitor();}synchronized (mLock) {mCompleted = true;mCurrentMonitor = null;}}}}

 WatchDog addMonitor：

//frameworks/base/services/core/java/com/android/server/Watchdog.javapublic void addMonitor(Monitor monitor) {synchronized (mLock) {mMonitorChecker.addMonitorLocked(monitor);}
}//frameworks/base/services/core/java/com/android/server/Watchdog$HandlerChecker.javavoid addMonitorLocked(Monitor monitor) {mMonitorQueue.add(monitor);}//frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java//单纯检测是否有方法长时间持有锁 public void monitor() {synchronized (this) { }}
//frameworks/base/services/core/java/com/android/server/input/InputManagerService.java
//检测是否持有一系列锁
public void monitor() {synchronized (mInputFilterLock) { }synchronized (mAssociationsLock) { /* Test if blocked by associations lock. */}synchronized (mLidSwitchLock) { /* Test if blocked by lid switch lock. */ }synchronized (mInputMonitors) { /* Test if blocked by input monitor lock. */ }synchronized (mAdditionalDisplayInputPropertiesLock) { /* Test if blocked by props lock */ }mBatteryController.monitor();mNativeInputManger.monitor();}//frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp
//分别检测reader writer线程是否有阻塞任务
static void nativeMonitor(JNIEnv* env, jobject nativeImplObj) {NativeInputManager* im = getNativeInputManager(env, nativeImplObj);im->getInputManager()->getReader().monitor();im->getInputManager()->getDispatcher().monitor();
}
//frameworks/native/services/inputflinger/reader/InputReader.cpp
//inputReader Thread是否有长时间未读取的消息
void InputReader::monitor() {std::unique_lock<std::mutex> lock(mLock);mEventHub->wake();mReaderIsAliveCondition.wait(lock);// Check the EventHubmEventHub->monitor();
}