gecko/xpcom/threads/BackgroundHangMonitor.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "mozilla/BackgroundHangMonitor.h"
#include "mozilla/LinkedList.h"
#include "mozilla/Monitor.h"
#include "mozilla/Move.h"
#include "mozilla/StaticPtr.h"
#include "mozilla/Telemetry.h"
#include "mozilla/ThreadHangStats.h"
#include "mozilla/ThreadLocal.h"

#include "prinrval.h"
#include "prthread.h"
#include "ThreadStackHelper.h"

#include <algorithm>

namespace mozilla {

/**
 * BackgroundHangManager is the global object that
 * manages all instances of BackgroundHangThread.
 */
class BackgroundHangManager : public AtomicRefCounted<BackgroundHangManager>
{
private:
  // Background hang monitor thread function
  static void MonitorThread(void* aData)
  {
    PR_SetCurrentThreadName("BgHangManager");
    /* We do not hold a reference to BackgroundHangManager here
       because the monitor thread only exists as long as the
       BackgroundHangManager instance exists. We stop the monitor
       thread in the BackgroundHangManager destructor, and we can
       only get to the destructor if we don't hold a reference here. */
    static_cast<BackgroundHangManager*>(aData)->RunMonitorThread();
  }

  // Hang monitor thread
  PRThread* mHangMonitorThread;
  // Stop hang monitoring
  bool mShutdown;

  BackgroundHangManager(const BackgroundHangManager&);
  BackgroundHangManager& operator=(const BackgroundHangManager&);
  void RunMonitorThread();

public:
  static StaticRefPtr<BackgroundHangManager> sInstance;

  // Lock for access to members of this class
  Monitor mLock;
  // Current time as seen by hang monitors
  PRIntervalTime mIntervalNow;
  // List of BackgroundHangThread instances associated with each thread
  LinkedList<BackgroundHangThread> mHangThreads;

  void Shutdown()
  {
    MonitorAutoLock autoLock(mLock);
    mShutdown = true;
    autoLock.Notify();
  }

  void Wakeup()
  {
    // PR_CreateThread could have failed earlier
    if (mHangMonitorThread) {
      // Use PR_Interrupt to avoid potentially taking a lock
      PR_Interrupt(mHangMonitorThread);
    }
  }

  BackgroundHangManager();
  ~BackgroundHangManager();
};

/**
 * BackgroundHangThread is a per-thread object that is used
 * by all instances of BackgroundHangMonitor to monitor hangs.
 */
class BackgroundHangThread : public RefCounted<BackgroundHangThread>
                           , public LinkedListElement<BackgroundHangThread>
{
private:
  static ThreadLocal<BackgroundHangThread*> sTlsKey;

  BackgroundHangThread(const BackgroundHangThread&);
  BackgroundHangThread& operator=(const BackgroundHangThread&);

  /* Keep a reference to the manager, so we can keep going even
     after BackgroundHangManager::Shutdown is called. */
  const RefPtr<BackgroundHangManager> mManager;
  // Unique thread ID for identification
  const PRThread* mThreadID;

public:
  static BackgroundHangThread* FindThread();

  static void Startup()
  {
    /* We can tolerate init() failing.
       The if block turns off warn_unused_result. */
    if (!sTlsKey.init()) {}
  }

  // Hang timeout in ticks
  const PRIntervalTime mTimeout;
  // PermaHang timeout in ticks
  const PRIntervalTime mMaxTimeout;
  // Time at last activity
  PRIntervalTime mInterval;
  // Time when a hang started
  PRIntervalTime mHangStart;
  // Is the thread in a hang
  bool mHanging;
  // Is the thread in a waiting state
  bool mWaiting;
  // Platform-specific helper to get hang stacks
  ThreadStackHelper mStackHelper;
  // Stack of current hang
  Telemetry::HangHistogram::Stack mHangStack;
  // Statistics for telemetry
  Telemetry::ThreadHangStats mStats;

  BackgroundHangThread(const char* aName,
                       uint32_t aTimeoutMs,
                       uint32_t aMaxTimeoutMs);
  ~BackgroundHangThread();

  // Report a hang; aManager->mLock IS locked
  void ReportHang(PRIntervalTime aHangTime);
  // Report a permanent hang; aManager->mLock IS locked
  void ReportPermaHang() const;
  // Called by BackgroundHangMonitor::NotifyActivity
  void NotifyActivity();
  // Called by BackgroundHangMonitor::NotifyWait
  void NotifyWait()
  {
    NotifyActivity();
    mWaiting = true;
  }
};


StaticRefPtr<BackgroundHangManager> BackgroundHangManager::sInstance;

ThreadLocal<BackgroundHangThread*> BackgroundHangThread::sTlsKey;


BackgroundHangManager::BackgroundHangManager()
  : mShutdown(false)
  , mLock("BackgroundHangManager")
  , mIntervalNow(0)
{
  // Lock so we don't race against the new monitor thread
  MonitorAutoLock autoLock(mLock);
  mHangMonitorThread = PR_CreateThread(
    PR_USER_THREAD, MonitorThread, this,
    PR_PRIORITY_LOW, PR_GLOBAL_THREAD, PR_JOINABLE_THREAD, 0);

  MOZ_ASSERT(mHangMonitorThread,
    "Failed to create monitor thread");
}

BackgroundHangManager::~BackgroundHangManager()
{
  MOZ_ASSERT(mShutdown,
    "Destruction without Shutdown call");
  MOZ_ASSERT(mHangThreads.isEmpty(),
    "Destruction with outstanding monitors");
  MOZ_ASSERT(mHangMonitorThread,
    "No monitor thread");

  // PR_CreateThread could have failed above due to resource limitation
  if (mHangMonitorThread) {
    // The monitor thread can only live as long as the instance lives
    PR_JoinThread(mHangMonitorThread);
  }
}

void
BackgroundHangManager::RunMonitorThread()
{
  // Keep us locked except when waiting
  MonitorAutoLock autoLock(mLock);

  /* mIntervalNow is updated at various intervals determined by waitTime.
     However, if an update latency is too long (due to CPU scheduling, system
     sleep, etc.), we don't update mIntervalNow at all. This is done so that
     long latencies in our timing are not detected as hangs. systemTime is
     used to track PR_IntervalNow() and determine our latency. */

  PRIntervalTime systemTime = PR_IntervalNow();
  // Default values for the first iteration of thread loop
  PRIntervalTime waitTime = PR_INTERVAL_NO_WAIT;
  PRIntervalTime recheckTimeout = PR_INTERVAL_NO_WAIT;

  while (!mShutdown) {

    PR_ClearInterrupt();
    nsresult rv = autoLock.Wait(waitTime);

    PRIntervalTime newTime = PR_IntervalNow();
    PRIntervalTime systemInterval = newTime - systemTime;
    systemTime = newTime;

    /* waitTime is a quarter of the shortest timeout value; If our timing
       latency is low enough (less than half the shortest timeout value),
       we can update mIntervalNow. */
    if (MOZ_LIKELY(waitTime != PR_INTERVAL_NO_TIMEOUT &&
                   systemInterval < 2 * waitTime)) {
      mIntervalNow += systemInterval;
    }

    /* If it's before the next recheck timeout, and our wait did not
       get interrupted (either through Notify or PR_Interrupt), we can
       keep the current waitTime and skip iterating through hang monitors. */
    if (MOZ_LIKELY(systemInterval < recheckTimeout &&
                   systemInterval >= waitTime &&
                   rv == NS_OK)) {
      recheckTimeout -= systemInterval;
      continue;
    }

    /* We are in one of the following scenarios,
     - Hang or permahang recheck timeout
     - Thread added/removed
     - Thread wait or hang ended
       In all cases, we want to go through our list of hang
       monitors and update waitTime and recheckTimeout. */
    waitTime = PR_INTERVAL_NO_TIMEOUT;
    recheckTimeout = PR_INTERVAL_NO_TIMEOUT;

    // Locally hold mIntervalNow
    PRIntervalTime intervalNow = mIntervalNow;

    // iterate through hang monitors
    for (BackgroundHangThread* currentThread = mHangThreads.getFirst();
         currentThread; currentThread = currentThread->getNext()) {

      if (currentThread->mWaiting) {
        // Thread is waiting, not hanging
        continue;
      }
      PRIntervalTime interval = currentThread->mInterval;
      PRIntervalTime hangTime = intervalNow - interval;
      if (MOZ_UNLIKELY(hangTime >= currentThread->mMaxTimeout)) {
        // A permahang started
        // Skip subsequent iterations and tolerate a race on mWaiting here
        currentThread->mWaiting = true;
        currentThread->ReportPermaHang();
        continue;
      }

      if (MOZ_LIKELY(!currentThread->mHanging)) {
        if (MOZ_UNLIKELY(hangTime >= currentThread->mTimeout)) {
          // A hang started
          currentThread->mStackHelper.GetStack(currentThread->mHangStack);
          currentThread->mHangStart = interval;
          currentThread->mHanging = true;
        }
      } else {
        if (MOZ_LIKELY(interval != currentThread->mHangStart)) {
          // A hang ended
          currentThread->ReportHang(intervalNow - currentThread->mHangStart);
          currentThread->mHanging = false;
        }
      }

      /* If we are hanging, the next time we check for hang status is when
         the hang turns into a permahang. If we're not hanging, the next
         recheck timeout is when we may be entering a hang. */
      PRIntervalTime nextRecheck;
      if (currentThread->mHanging) {
        nextRecheck = currentThread->mMaxTimeout;
      } else {
        nextRecheck = currentThread->mTimeout;
      }
      recheckTimeout = std::min(recheckTimeout, nextRecheck - hangTime);

      /* We wait for a quarter of the shortest timeout
         value to give mIntervalNow enough granularity. */
      waitTime = std::min(waitTime, currentThread->mTimeout / 4);
    }
  }

  /* We are shutting down now.
     Wait for all outstanding monitors to unregister. */
  while (!mHangThreads.isEmpty()) {
    autoLock.Wait(PR_INTERVAL_NO_TIMEOUT);
  }
}


BackgroundHangThread::BackgroundHangThread(const char* aName,
                                           uint32_t aTimeoutMs,
                                           uint32_t aMaxTimeoutMs)
  : mManager(BackgroundHangManager::sInstance)
  , mThreadID(PR_GetCurrentThread())
  , mTimeout(aTimeoutMs == BackgroundHangMonitor::kNoTimeout
             ? PR_INTERVAL_NO_TIMEOUT
             : PR_MillisecondsToInterval(aTimeoutMs))
  , mMaxTimeout(aMaxTimeoutMs == BackgroundHangMonitor::kNoTimeout
                ? PR_INTERVAL_NO_TIMEOUT
                : PR_MillisecondsToInterval(aMaxTimeoutMs))
  , mInterval(mManager->mIntervalNow)
  , mHangStart(mInterval)
  , mHanging(false)
  , mWaiting(true)
  , mStats(aName)
{
  if (sTlsKey.initialized()) {
    sTlsKey.set(this);
  }
  // Lock here because LinkedList is not thread-safe
  MonitorAutoLock autoLock(mManager->mLock);
  // Add to thread list
  mManager->mHangThreads.insertBack(this);
  // Wake up monitor thread to process new thread
  autoLock.Notify();
}

BackgroundHangThread::~BackgroundHangThread()
{
  // Lock here because LinkedList is not thread-safe
  MonitorAutoLock autoLock(mManager->mLock);
  // Remove from thread list
  remove();
  // Wake up monitor thread to process removed thread
  autoLock.Notify();

  // We no longer have a thread
  if (sTlsKey.initialized()) {
    sTlsKey.set(nullptr);
  }

  // Move our copy of ThreadHangStats to Telemetry storage
  Telemetry::RecordThreadHangStats(mStats);
}

void
BackgroundHangThread::ReportHang(PRIntervalTime aHangTime)
{
  // Recovered from a hang; called on the monitor thread
  // mManager->mLock IS locked

  Telemetry::HangHistogram newHistogram(Move(mHangStack));
  for (Telemetry::HangHistogram* oldHistogram = mStats.mHangs.begin();
       oldHistogram != mStats.mHangs.end(); oldHistogram++) {
    if (newHistogram == *oldHistogram) {
      // New histogram matches old one
      oldHistogram->Add(aHangTime);
      return;
    }
  }
  // Add new histogram
  newHistogram.Add(aHangTime);
  mStats.mHangs.append(Move(newHistogram));
}

void
BackgroundHangThread::ReportPermaHang() const
{
  // Permanently hanged; called on the monitor thread
  // mManager->mLock IS locked

  // TODO: Add telemetry reporting for perma-hangs
}

MOZ_ALWAYS_INLINE void
BackgroundHangThread::NotifyActivity()
{
  PRIntervalTime intervalNow = mManager->mIntervalNow;
  if (mWaiting) {
    mInterval = intervalNow;
    mWaiting = false;
    /* We have to wake up the manager thread because when all threads
       are waiting, the manager thread waits indefinitely as well. */
    mManager->Wakeup();
  } else {
    PRIntervalTime duration = intervalNow - mInterval;
    mStats.mActivity.Add(duration);
    if (MOZ_UNLIKELY(duration >= mTimeout)) {
      /* Wake up the manager thread to tell it that a hang ended */
      mManager->Wakeup();
    }
    mInterval = intervalNow;
  }
}

BackgroundHangThread*
BackgroundHangThread::FindThread()
{
  if (sTlsKey.initialized()) {
    // Use TLS if available
    return sTlsKey.get();
  }
  // If TLS is unavailable, we can search through the thread list
  RefPtr<BackgroundHangManager> manager(BackgroundHangManager::sInstance);
  MOZ_ASSERT(manager, "Creating BackgroundHangMonitor after shutdown");

  PRThread* threadID = PR_GetCurrentThread();
  // Lock thread list for traversal
  MonitorAutoLock autoLock(manager->mLock);
  for (BackgroundHangThread* thread = manager->mHangThreads.getFirst();
       thread; thread = thread->getNext()) {
    if (thread->mThreadID == threadID) {
      return thread;
    }
  }
  // Current thread is not initialized
  return nullptr;
}


void
BackgroundHangMonitor::Startup()
{
  MOZ_ASSERT(!BackgroundHangManager::sInstance, "Already initialized");
  ThreadStackHelper::Startup();
  BackgroundHangThread::Startup();
  BackgroundHangManager::sInstance = new BackgroundHangManager();
}

void
BackgroundHangMonitor::Shutdown()
{
  MOZ_ASSERT(BackgroundHangManager::sInstance, "Not initialized");
  /* Scope our lock inside Shutdown() because the sInstance object can
     be destroyed as soon as we set sInstance to nullptr below, and
     we don't want to hold the lock when it's being destroyed. */
  BackgroundHangManager::sInstance->Shutdown();
  BackgroundHangManager::sInstance = nullptr;
  ThreadStackHelper::Shutdown();
}

BackgroundHangMonitor::BackgroundHangMonitor(const char* aName,
                                             uint32_t aTimeoutMs,
                                             uint32_t aMaxTimeoutMs)
  : mThread(BackgroundHangThread::FindThread())
{
  if (!mThread) {
    mThread = new BackgroundHangThread(aName, aTimeoutMs, aMaxTimeoutMs);
  }
}

BackgroundHangMonitor::BackgroundHangMonitor()
  : mThread(BackgroundHangThread::FindThread())
{
  MOZ_ASSERT(mThread, "Thread not initialized for hang monitoring");
}

BackgroundHangMonitor::~BackgroundHangMonitor()
{
}

void
BackgroundHangMonitor::NotifyActivity()
{
  mThread->NotifyActivity();
}

void
BackgroundHangMonitor::NotifyWait()
{
  mThread->NotifyWait();
}


/* Because we are iterating through the BackgroundHangThread linked list,
   we need to take a lock. Using MonitorAutoLock as a base class makes
   sure all of that is taken care of for us. */
BackgroundHangMonitor::ThreadHangStatsIterator::ThreadHangStatsIterator()
  : MonitorAutoLock(BackgroundHangManager::sInstance->mLock)
  , mThread(BackgroundHangManager::sInstance->mHangThreads.getFirst())
{
}

Telemetry::ThreadHangStats*
BackgroundHangMonitor::ThreadHangStatsIterator::GetNext()
{
  if (!mThread) {
    return nullptr;
  }
  Telemetry::ThreadHangStats* stats = &mThread->mStats;
  mThread = mThread->getNext();
  return stats;
}

} // namespace mozilla