gecko-dev/toolkit/xre/EventTracer.cpp

/* 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/. */

/*
 * Event loop instrumentation. This code attempts to measure the
 * latency of the UI-thread event loop by firing native events at it from
 * a background thread, and measuring how long it takes for them
 * to be serviced. The measurement interval (kMeasureInterval, below)
 * is also used as the upper bound of acceptable response time.
 * When an event takes longer than that interval to be serviced,
 * a sample will be written to the log.
 *
 * Usage:
 *
 * Set MOZ_INSTRUMENT_EVENT_LOOP=1 in the environment to enable
 * this instrumentation. Currently only the UI process is instrumented.
 *
 * Set MOZ_INSTRUMENT_EVENT_LOOP_OUTPUT in the environment to a
 * file path to contain the log output, the default is to log to stdout.
 *
 * Set MOZ_INSTRUMENT_EVENT_LOOP_THRESHOLD in the environment to an
 * integer number of milliseconds to change the threshold for reporting.
 * The default is 20 milliseconds. Unresponsive periods shorter than this
 * threshold will not be reported.
 *
 * Set MOZ_INSTRUMENT_EVENT_LOOP_INTERVAL in the environment to an
 * integer number of milliseconds to change the maximum sampling frequency.
 * This variable controls how often events will be sent to the main
 * thread's event loop to sample responsiveness. The sampler will not
 * send events twice within LOOP_INTERVAL milliseconds.
 * The default is 10 milliseconds.
 *
 * All logged output lines start with MOZ_EVENT_TRACE. All timestamps
 * output are milliseconds since the epoch (PRTime / 1000).
 *
 * On startup, a line of the form:
 *   MOZ_EVENT_TRACE start <timestamp>
 * will be output.
 *
 * On shutdown, a line of the form:
 *   MOZ_EVENT_TRACE stop <timestamp>
 * will be output.
 *
 * When an event servicing time exceeds the threshold, a line of the form:
 *   MOZ_EVENT_TRACE sample <timestamp> <duration>
 * will be output, where <duration> is the number of milliseconds that
 * it took for the event to be serviced. Duration may contain a fractional
 * component.
 */

#include "GeckoProfiler.h"

#include "EventTracer.h"

#include <stdio.h>

#include "mozilla/Preferences.h"
#include "mozilla/TimeStamp.h"
#include "mozilla/WidgetTraceEvent.h"
#include "nsDebug.h"
#include <limits.h>
#include <prenv.h>
#include <prinrval.h>
#include <prthread.h>
#include <prtime.h>

#include "nsThreadUtils.h"

using mozilla::FireAndWaitForTracerEvent;
using mozilla::TimeDuration;
using mozilla::TimeStamp;

namespace {

PRThread* sTracerThread = nullptr;
bool sExit = false;

struct TracerStartClosure {
  bool mLogTracing;
  int32_t mThresholdInterval;
};

/*
 * The tracer thread fires events at the native event loop roughly
 * every kMeasureInterval. It will sleep to attempt not to send them
 * more quickly, but if the response time is longer than kMeasureInterval
 * it will not send another event until the previous response is received.
 *
 * The output defaults to stdout, but can be redirected to a file by
 * settting the environment variable MOZ_INSTRUMENT_EVENT_LOOP_OUTPUT
 * to the name of a file to use.
 */
void TracerThread(void* arg) {
  AUTO_PROFILER_REGISTER_THREAD("Event Tracer");
  NS_SetCurrentThreadName("Event Tracer");

  TracerStartClosure* threadArgs = static_cast<TracerStartClosure*>(arg);

  // These are the defaults. They can be overridden by environment vars.
  // This should be set to the maximum latency we'd like to allow
  // for responsiveness.
  int32_t thresholdInterval = threadArgs->mThresholdInterval;
  PRIntervalTime threshold = PR_MillisecondsToInterval(thresholdInterval);
  // This is the sampling interval.
  PRIntervalTime interval = PR_MillisecondsToInterval(thresholdInterval / 2);

  sExit = false;
  FILE* log = nullptr;
  char* envfile = PR_GetEnv("MOZ_INSTRUMENT_EVENT_LOOP_OUTPUT");
  if (envfile) {
    log = fopen(envfile, "w");
  }
  if (log == nullptr) log = stdout;

  char* thresholdenv = PR_GetEnv("MOZ_INSTRUMENT_EVENT_LOOP_THRESHOLD");
  if (thresholdenv && *thresholdenv) {
    int val = atoi(thresholdenv);
    if (val != 0 && val != INT_MAX && val != INT_MIN) {
      threshold = PR_MillisecondsToInterval(val);
    }
  }

  char* intervalenv = PR_GetEnv("MOZ_INSTRUMENT_EVENT_LOOP_INTERVAL");
  if (intervalenv && *intervalenv) {
    int val = atoi(intervalenv);
    if (val != 0 && val != INT_MAX && val != INT_MIN) {
      interval = PR_MillisecondsToInterval(val);
    }
  }

  if (threadArgs->mLogTracing) {
    long long now = PR_Now() / PR_USEC_PER_MSEC;
    fprintf(log, "MOZ_EVENT_TRACE start %llu\n", now);
  }

  while (!sExit) {
    TimeStamp start(TimeStamp::Now());
    PRIntervalTime next_sleep = interval;

    // TODO: only wait up to a maximum of interval; return
    // early if that threshold is exceeded and dump a stack trace
    // or do something else useful.
    if (FireAndWaitForTracerEvent()) {
      TimeDuration duration = TimeStamp::Now() - start;
      // Only report samples that exceed our measurement threshold.
      long long now = PR_Now() / PR_USEC_PER_MSEC;
      if (threadArgs->mLogTracing && duration.ToMilliseconds() > threshold) {
        fprintf(log, "MOZ_EVENT_TRACE sample %llu %lf\n", now,
                duration.ToMilliseconds());
      }

      if (next_sleep > duration.ToMilliseconds()) {
        next_sleep -= int(duration.ToMilliseconds());
      } else {
        // Don't sleep at all if this event took longer than the measure
        // interval to deliver.
        next_sleep = 0;
      }
    }

    if (next_sleep != 0 && !sExit) {
      PR_Sleep(next_sleep);
    }
  }

  if (threadArgs->mLogTracing) {
    long long now = PR_Now() / PR_USEC_PER_MSEC;
    fprintf(log, "MOZ_EVENT_TRACE stop %llu\n", now);
  }

  if (log != stdout) fclose(log);

  delete threadArgs;
}

}  // namespace

namespace mozilla {

bool InitEventTracing(bool aLog) {
  if (sTracerThread) return true;

  // Initialize the widget backend.
  if (!InitWidgetTracing()) return false;

  // The tracer thread owns the object and will delete it.
  TracerStartClosure* args = new TracerStartClosure();
  args->mLogTracing = aLog;

  // Pass the default threshold interval.
  int32_t thresholdInterval = 20;
  Preferences::GetInt("devtools.eventlooplag.threshold", &thresholdInterval);
  args->mThresholdInterval = thresholdInterval;

  // Create a thread that will fire events back at the
  // main thread to measure responsiveness.
  MOZ_ASSERT(!sTracerThread, "Event tracing already initialized!");
  sTracerThread =
      PR_CreateThread(PR_USER_THREAD, TracerThread, args, PR_PRIORITY_NORMAL,
                      PR_GLOBAL_THREAD, PR_JOINABLE_THREAD, 0);
  return sTracerThread != nullptr;
}

void ShutdownEventTracing() {
  if (!sTracerThread) return;

  sExit = true;
  // Ensure that the tracer thread doesn't hang.
  SignalTracerThread();

  if (sTracerThread) PR_JoinThread(sTracerThread);
  sTracerThread = nullptr;

  // Allow the widget backend to clean up.
  CleanUpWidgetTracing();
}

}  // namespace mozilla