gecko-dev/toolkit/components/backgroundhangmonitor/BackgroundHangMonitor.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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/. */

#ifndef mozilla_BackgroundHangMonitor_h
#define mozilla_BackgroundHangMonitor_h

#include "mozilla/CPUUsageWatcher.h"
#include "mozilla/HangAnnotations.h"
#include "mozilla/Monitor.h"
#include "mozilla/RefPtr.h"

#include "nsString.h"

#include <stdint.h>

namespace mozilla {

class BackgroundHangThread;
class BackgroundHangManager;

/**
 * The background hang monitor is responsible for detecting and reporting
 * hangs in main and background threads. A thread registers itself using
 * the BackgroundHangMonitor object and periodically calls its methods to
 * inform the hang monitor of the thread's activity. Each thread is given
 * a thread name, a timeout, and a maximum timeout. If one of the thread's
 * tasks runs for longer than the timeout duration but shorter than the
 * maximum timeout, a (transient) hang is reported. On the other hand, if
 * a task runs for longer than the maximum timeout duration or never
 * finishes (e.g. in a deadlock), a permahang is reported.
 *
 * Tasks are defined arbitrarily, but are typically represented by events
 * in an event loop -- processing one event is equivalent to running one
 * task. To ensure responsiveness, tasks in a thread often have a target
 * running time. This is a good starting point for determining the timeout
 * and maximum timeout values. For example, the Compositor thread has a
 * responsiveness goal of 60Hz or 17ms, so a starting timeout could be
 * 100ms. Considering some platforms (e.g. Android) can terminate the app
 * when a critical thread hangs for longer than a few seconds, a good
 * starting maximum timeout is 4 or 5 seconds.
 *
 * A thread registers itself through the BackgroundHangMonitor constructor.
 * Multiple BackgroundHangMonitor objects can be used in one thread. The
 * constructor without arguments can be used when it is known that the thread
 * already has a BackgroundHangMonitor registered. When all instances of
 * BackgroundHangMonitor are destroyed, the thread is unregistered.
 *
 * The thread then uses two methods to inform BackgroundHangMonitor of the
 * thread's activity:
 *
 *  > BackgroundHangMonitor::NotifyActivity should be called *before*
 *    starting a task. The task run time is determined by the interval
 *    between this call and the next NotifyActivity call.
 *
 *  > BackgroundHangMonitor::NotifyWait should be called *before* the
 *    thread enters a wait state (e.g. to wait for a new event). This
 *    prevents a waiting thread from being detected as hanging. The wait
 *    state is automatically cleared at the next NotifyActivity call.
 *
 * The following example shows hang monitoring in a simple event loop:
 *
 *  void thread_main()
 *  {
 *    mozilla::BackgroundHangMonitor hangMonitor("example1", 100, 1000);
 *    while (!exiting) {
 *      hangMonitor.NotifyActivity();
 *      process_next_event();
 *      hangMonitor.NotifyWait();
 *      wait_for_next_event();
 *    }
 *  }
 *
 * The following example shows reentrancy in nested event loops:
 *
 *  void thread_main()
 *  {
 *    mozilla::BackgroundHangMonitor hangMonitor("example2", 100, 1000);
 *    while (!exiting) {
 *      hangMonitor.NotifyActivity();
 *      process_next_event();
 *      hangMonitor.NotifyWait();
 *      wait_for_next_event();
 *    }
 *  }
 *
 *  void process_next_event()
 *  {
 *    mozilla::BackgroundHangMonitor hangMonitor();
 *    if (is_sync_event) {
 *      while (!finished_event) {
 *        hangMonitor.NotifyActivity();
 *        process_next_event();
 *        hangMonitor.NotifyWait();
 *        wait_for_next_event();
 *      }
 *    } else {
 *      process_nonsync_event();
 *    }
 *  }
 */
class BackgroundHangMonitor {
 private:
  friend BackgroundHangManager;

  RefPtr<BackgroundHangThread> mThread;

  static bool ShouldDisableOnBeta(const nsCString&);
  static bool DisableOnBeta();

 public:
  static const uint32_t kNoTimeout = 0;
  enum ThreadType {
    // For a new BackgroundHangMonitor for thread T, only create a new
    // monitoring thread for T if one doesn't already exist. If one does,
    // share that pre-existing monitoring thread.
    THREAD_SHARED,
    // For a new BackgroundHangMonitor for thread T, create a new
    // monitoring thread for T even if there are other, pre-existing
    // monitoring threads for T.
    THREAD_PRIVATE
  };

  /**
   * Enable hang monitoring.
   * Must return before using BackgroundHangMonitor.
   */
  static void Startup();

  /**
   * Disable hang monitoring.
   * Can be called without destroying all BackgroundHangMonitors first.
   */
  static void Shutdown();

  /**
   * Returns true if BHR is disabled.
   */
  static bool IsDisabled();

  /**
   * Start monitoring hangs for the current thread.
   *
   * @param aName Name to identify the thread with
   * @param aTimeoutMs Amount of time in milliseconds without
   *  activity before registering a hang
   * @param aMaxTimeoutMs Amount of time in milliseconds without
   *  activity before registering a permanent hang
   * @param aThreadType
   *  The ThreadType type of monitoring thread that should be created
   *  for this monitor. See the documentation for ThreadType.
   */
  BackgroundHangMonitor(const char* aName, uint32_t aTimeoutMs,
                        uint32_t aMaxTimeoutMs,
                        ThreadType aThreadType = THREAD_SHARED);

  /**
   * Monitor hangs using an existing monitor
   * associated with the current thread.
   */
  BackgroundHangMonitor();

  /**
   * Destroys the hang monitor; hang monitoring for a thread stops
   * when all monitors associated with the thread are destroyed.
   */
  ~BackgroundHangMonitor();

  /**
   * Notify the hang monitor of pending current thread activity.
   * Call this method before starting an "activity" or after
   * exiting from a wait state.
   */
  void NotifyActivity();

  /**
   * Notify the hang monitor of current thread wait.
   * Call this method before entering a wait state; call
   * NotifyActivity when subsequently exiting the wait state.
   */
  void NotifyWait();

  /**
   * Register an annotator with BHR for the current thread.
   * @param aAnnotator annotator to register
   * @return true if the annotator was registered, otherwise false.
   */
  static bool RegisterAnnotator(BackgroundHangAnnotator& aAnnotator);

  /**
   * Unregister an annotator that was previously registered via
   * RegisterAnnotator.
   * @param aAnnotator annotator to unregister
   * @return true if there are still remaining annotators registered
   */
  static bool UnregisterAnnotator(BackgroundHangAnnotator& aAnnotator);
};

}  // namespace mozilla

#endif  // mozilla_BackgroundHangMonitor_h