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Watchdog机制及实例分析

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1. 概述

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2. Watchdog机制解析

2.1 Watchdog的初始化

Watchdog是在System Server中初始化的：

/frameworks/base/services/java/com/android/server/SystemServer.java

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public final class SystemServer {
  ... ...
    /**
     * Starts a miscellaneous grab bag of stuff that has yet to be refactored
     * and organized.
     */
    private void startOtherServices() {
    ... ...
        try {
          ... ...
            traceBeginAndSlog("InitWatchdog");
            final Watchdog watchdog = Watchdog.getInstance(); // 获取Watchdog对象初始化
            watchdog.init(context, mActivityManagerService); // 注册receiver以接收系统重启广播
            Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
          ... ...
        }
    ... ...
        mActivityManagerService.systemReady(new Runnable() {
            @Override
            public void run() {
              ... ...
                Watchdog.getInstance().start();
              ... ...
             }
        });
    }

Watchdog在初始化时构造了很多HandlerChecker，大致可以分为两类：

  • Monitor Checker：用于检查是Monitor对象可能发生的死锁, AMS, PKMS, WMS等核心的系统服务都是Monitor对象。在log中会有watchdog: Blocked in monitor xxx foreground thread(android.fg)，所以其实就是android.fg线程。
  • Looper Checker：用于检查线程的消息队列是否长时间处于工作状态。Watchdog自身的消息队列，UI, IO, Display这些全局的消息队列都是被检查的对象。此外，一些重要的线程的消息队列，也会加入到Looper Checker中，譬如AMS, PKMS，这些是在对应的对象初始化时加入的。

/frameworks/base/services/core/java/com/android/server/Watchdog.java

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/** This class calls its monitor every minute. Killing this process if they don't return **/
public class Watchdog extends Thread {
  ... ...

    /* This handler will be used to post message back onto the main thread */
    final ArrayList<HandlerChecker> mHandlerCheckers = new ArrayList<>();
    final HandlerChecker mMonitorChecker;
  ... ...      
    public static Watchdog getInstance() {
        if (sWatchdog == null) {
            sWatchdog = new Watchdog(); // new一个Watchdog对象
        }

        return sWatchdog;
    }
... ...
    private Watchdog() {
        super("watchdog");
        // Initialize handler checkers for each common thread we want to check.  Note
        // that we are not currently checking the background thread, since it can
        // potentially hold longer running operations with no guarantees about the timeliness
        // of operations there.

        // The shared foreground thread is the main checker.  It is where we
        // will also dispatch monitor checks and do other work.
  		// 可以看一下FgThread的构造函数，super("android.fg", xxx, true)，所以其实就是android.fg线程
        mMonitorChecker = new HandlerChecker(FgThread.getHandler(),
                "foreground thread", DEFAULT_TIMEOUT);
        mHandlerCheckers.add(mMonitorChecker); //添加MonitorChecker
        // Add checker for main thread.  We only do a quick check since there
        // can be UI running on the thread.
  		// 添加Looper Checker
        mHandlerCheckers.add(new HandlerChecker(new Handler(Looper.getMainLooper()),
                "main thread", DEFAULT_TIMEOUT));
        // Add checker for shared UI thread.
        mHandlerCheckers.add(new HandlerChecker(UiThread.getHandler(),
                "ui thread", DEFAULT_TIMEOUT));
        // And also check IO thread.
        mHandlerCheckers.add(new HandlerChecker(IoThread.getHandler(),
                "i/o thread", DEFAULT_TIMEOUT));
        // And the display thread.
        mHandlerCheckers.add(new HandlerChecker(DisplayThread.getHandler(),
                "display thread", DEFAULT_TIMEOUT));

        // Initialize monitor for Binder threads.
        addMonitor(new BinderThreadMonitor());
    }

Monitor Checker和Looper Checker的侧重点不一样，前者预警我们不能长时间持有核心系统服务的对象锁，否则会阻塞很多函数的运行；后者预警我们不能长时间的霸占消息队列，否则其他消息将得不到处理。这两类都会导致系统卡住(System Not Responding)。

init注册了一个广播接收器用来接收重启系统的广播：

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    final class RebootRequestReceiver extends BroadcastReceiver {
        @Override
        public void onReceive(Context c, Intent intent) {
            if (intent.getIntExtra("nowait", 0) != 0) {
                rebootSystem("Received ACTION_REBOOT broadcast");
                return;
            }
            Slog.w(TAG, "Unsupported ACTION_REBOOT broadcast: " + intent);
        }
    }
... ...
    public void init(Context context, ActivityManagerService activity) {
        mResolver = context.getContentResolver();
        mActivity = activity;

        context.registerReceiver(new RebootRequestReceiver(),
                new IntentFilter(Intent.ACTION_REBOOT),
                android.Manifest.permission.REBOOT, null);
    }

2.2 触发Watchdog

初始化Watchdog后，在SystemServer中start，作为SystemServer进程中的一个单独的线程运行，但是想要触发Watchdog的运行还需要AMS、PMS等系统服务加入到Watchdog的监测集，也就是需要Watchdog关注的对象，Watchdog只关注一些核心的系统服务。

需要Watchdog检测的对象，需要将自己添加到Watchdog的监测集中，以AMS为例：

/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

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public ActivityManagerService(Context systemContext) {
  ... ...
    Watchdog.getInstance().addMonitor(this);
    Watchdog.getInstance().addThread(mHandler);
}

/** In this method we try to acquire our lock to make sure that we have not deadlocked */
public void monitor() {
    synchronized (this) { }
}

/frameworks/base/services/core/java/com/android/server/Watchdog.java

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public class Watchdog extends Thread {
  ... ...
    // 1 addMonitor
	public void addMonitor(Monitor monitor) {
        synchronized (this) {
            if (isAlive()) {
                throw new RuntimeException("Monitors can't be added once the Watchdog is running");
            }
            // 实际调用Watchdog内部类HandlerChecker的addMonitor
            mMonitorChecker.addMonitor(monitor);
        }
    }

    // 2 addThread
    public void addThread(Handler thread) {
        addThread(thread, DEFAULT_TIMEOUT);
    }

    public void addThread(Handler thread, long timeoutMillis) {
        synchronized (this) {
            if (isAlive()) {
                throw new RuntimeException("Threads can't be added once the Watchdog is running");
            }
            final String name = thread.getLooper().getThread().getName(); // 线程名
            mHandlerCheckers.add(new HandlerChecker(thread, name, timeoutMillis));
        }
    }
  ... ...
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public class Watchdog extends Thread {
  ... ...
	public final class HandlerChecker implements Runnable {
      ... ...
        HandlerChecker(Handler handler, String name, long waitMaxMillis) {
            mHandler = handler;
            mName = name;
            mWaitMax = waitMaxMillis;
            mCompleted = true;
        }
        public void addMonitor(Monitor monitor) {
        	// mMonitors是一个ArrayList<Monitor> mMonitors = new ArrayList<Monitor>
            // 将monitor对象添加到MonitorChecker中，在AMS中则是把AMS对象添加到此list
            mMonitors.add(monitor);
        }
      ... ...

AMS在构造函数中分别通过addMonitor和 addThread方法把自己添加到Monitor Checker对象中，把自己的Handler添加到Looper Checker对象中。

2.3 Watchdog监测机制

Watchdog继承了Thread，也就是说本身就是一个线程，来看一下它的run()方法实现：

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@Override
public void run() {
    boolean waitedHalf = false;
    while (true) {
        final ArrayList<HandlerChecker> blockedCheckers;
        final String subject;
        final boolean allowRestart;
        int debuggerWasConnected = 0;
        synchronized (this) {
            long timeout = CHECK_INTERVAL; // 值为30s
            // Make sure we (re)spin the checkers that have become idle within
            // this wait-and-check interval
            // 2.3.1 调度所有的HandlerChecker，给所有受监控的线程发送消息
            for(int i=0; i<mHandlerCheckers.size(); i++) {
                HandlerChecker hc = mHandlerCheckers.get(i);
                hc.scheduleCheckLocked();
            }

            if (debuggerWasConnected > 0) {
                debuggerWasConnected--;
            }

            // 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.
            // 2.3.2 睡眠一段时间
            long start = SystemClock.uptimeMillis();
            while (timeout > 0) {
                if (Debug.isDebuggerConnected()) {
                    debuggerWasConnected = 2;
                }
                try {
                    wait(timeout); // 线程休眠且释放锁
                } catch (InterruptedException e) {
                    Log.wtf(TAG, e);
                }
                if (Debug.isDebuggerConnected()) {
                    debuggerWasConnected = 2;
                }
                timeout = CHECK_INTERVAL - (SystemClock.uptimeMillis() - start);
            }

            // 2.3.3 检查是否有线程或者服务出了问题
            final int waitState = evaluateCheckerCompletionLocked();
            if (waitState == COMPLETED) {
                // The monitors have returned; reset
                waitedHalf = false;
                continue;
            } else if (waitState == WAITING) {
                // still waiting but within their configured intervals; back off and recheck
                continue;
            } else if (waitState == WAITED_HALF) {
                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,
                            NATIVE_STACKS_OF_INTEREST);
                    waitedHalf = true;
                }
                continue;
            }

            // something is overdue!
            // 2.3.4 存在超时的HandlerChecker
            blockedCheckers = getBlockedCheckersLocked();
            subject = describeCheckersLocked(blockedCheckers);
            allowRestart = mAllowRestart;
        }

        // If we got here, that means that the system is most likely hung.
        // First collect stack traces from all threads of the system process.
        // Then kill this process so that the system will restart.
        // 走到这里意味着系统hung了，首先收集系统进程所有线程的stack trace，然后kill进程以重启。
        EventLog.writeEvent(EventLogTags.WATCHDOG, subject);

        ArrayList<Integer> pids = new ArrayList<Integer>();
        pids.add(Process.myPid());
        if (mPhonePid > 0) pids.add(mPhonePid);
        // Pass !waitedHalf so that just in case we somehow wind up here without having
        // dumped the halfway stacks, we properly re-initialize the trace file.
        final File stack = ActivityManagerService.dumpStackTraces(
                !waitedHalf, pids, null, null, NATIVE_STACKS_OF_INTEREST);

        // Give some extra time to make sure the stack traces get written.
        // The system's been hanging for a minute, another second or two won't hurt much.
        // 线程休眠2秒钟以确保trace输出完毕
        SystemClock.sleep(2000);

        // 输出kernel log（dump和backtrace），尝试把error添加到dropbox
        ... ...

        // Only kill the process if the debugger is not attached.
        if (Debug.isDebuggerConnected()) {
            debuggerWasConnected = 2;
        }
        if (debuggerWasConnected >= 2) {
            Slog.w(TAG, "Debugger connected: Watchdog is *not* killing the system process");
        } else if (debuggerWasConnected > 0) {
            Slog.w(TAG, "Debugger was connected: Watchdog is *not* killing the system process");
        } else if (!allowRestart) {
            Slog.w(TAG, "Restart not allowed: Watchdog is *not* killing the system process");
        } else {
            Slog.w(TAG, "*** WATCHDOG KILLING SYSTEM PROCESS: " + subject);
            for (int i=0; i<blockedCheckers.size(); i++) {
                Slog.w(TAG, blockedCheckers.get(i).getName() + " stack trace:");
                StackTraceElement[] stackTrace
                        = blockedCheckers.get(i).getThread().getStackTrace();
                for (StackTraceElement element: stackTrace) {
                    Slog.w(TAG, "    at " + element);
                }
            }
            // 2.3.5 保存日志，判断是否需要杀掉系统进程
            Slog.w(TAG, "*** GOODBYE!");
            // 杀死SystemServer
            Process.killProcess(Process.myPid());
            System.exit(10);
        }

        waitedHalf = false;
    }
}
2.3.1 scheduleCheckLocked()方法

Watchdog运行后开始循环，调用每一个HandlerChecker的scheduleCheckLocked()方法：

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public final class HandlerChecker implements Runnable {
  ... ...
    public void scheduleCheckLocked() {
        if (mMonitors.size() == 0 && mHandler.getLooper().getQueue().isPolling()) {
            // ... ...
            mCompleted = true;
            return;
        }

        if (!mCompleted) {
            // we already have a check in flight, so no need
            return;
        }

        mCompleted = false;
        mCurrentMonitor = null;
        mStartTime = SystemClock.uptimeMillis();
        mHandler.postAtFrontOfQueue(this); // 给监控的线程发送消息
    }
  ... ...

HandlerChecker对象既要监控服务，也要监控某个线程，所以代码中要先判断mMonitors.size是否为0，如果为0则说明这个HandlerChecker没有监控服务，这时如果被监控线程的消息队列处于空闲状态（调用isIdling()检查），则说明线程运行良好，把mCompleted设为true后就可以返回了。否则先把mCompleted设为false，然后记录消息开始发送的时间到变量mStartTime中，最后调用postAtFrontOfQueue()方法给被监控的线程发送一个消息。

2.3.2 定期检查

调度完HandlerChecker给受监控的线程发送完消息后，开始定期检查是否超时，每一次的检查的间隔由常量CHECK_INTERVAL设定，为30s，调用wait()方法（线程休眠且释放锁）让WatchDog线程睡眠一段时间。

2.3.3 检查线程或服务是否有问题

调用evaluateCheckerCompletionLocked()方法来检查是否有问题：

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private int evaluateCheckerCompletionLocked() {
    int state = COMPLETED;
    for (int i=0; i<mHandlerCheckers.size(); i++) {
        HandlerChecker hc = mHandlerCheckers.get(i);
        state = Math.max(state, hc.getCompletionStateLocked());
    }
    return state;
}
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public int getCompletionStateLocked() {
    if (mCompleted) {
        return COMPLETED;
    } else {
        long latency = SystemClock.uptimeMillis() - mStartTime;
        if (latency < mWaitMax/2) {
            return WAITING;
        } else if (latency < mWaitMax) {
            return WAITED_HALF;
        }
    }
    return OVERDUE;
}

根据等待时间来确认返回HandlerChecker对象的状态，

  • COMPLETED表示已经完成

  • WAITING和WAITED_HALF表示还在等待，但未超时

  • OVERDUE表示已经超时，默认情况下timeout是1分钟，但监测对象可以通过传参自行设定，譬如PKMS的Handler Checker的超时是10分钟:

  • ``` java
    private static final long WATCHDOG_TIMEOUT = 10006010; // ten minutes
    Watchdog.getInstance().addThread(mHandler, WATCHDOG_TIMEOUT);

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    ##### 2.3.4 getBlockedCheckersLocked

    ``` java
        private ArrayList<HandlerChecker> getBlockedCheckersLocked() {
            ArrayList<HandlerChecker> checkers = new ArrayList<HandlerChecker>();
            for (int i=0; i<mHandlerCheckers.size(); i++) {
                HandlerChecker hc = mHandlerCheckers.get(i);
                if (hc.isOverdueLocked()) {
                    checkers.add(hc);
                }
            }
            return checkers;
        }

如果超时时间到了，还有HandlerChecker处于未完成的状态(OVERDUE)，则通过getBlockedCheckersLocked()方法，获取阻塞的HandlerChecker，生成一些描述信息

2.3.5 保存日志

保存日志，包括一些运行时的堆栈信息，这些日志是我们解决Watchdog问题的重要依据。如果判断需要杀掉system_server进程，则给当前进程(system_server)发送signal。

3. 实例分析

4. 总结

Donate comment here.