tracing: Enable startup tracing on Android only after mojo is inited

We're refactoring startup tracing in Chrome, see design doc[1]. With
the new startup tracing architecture, each process needs to allocate a
shared memory buffer when enabling startup tracing. This is only
supported after mojo was initialized for the process.

On Android, we were previously enabling startup tracing right after the
native library is loaded. At this point, mojo is not ready yet. To
prepare for the startup tracing refactor, this patch moves startup
tracing initialization on android to ContentMainRunnerImpl::Initialize,
which is called after mojo was initialized by Main() in
//services/service_manager/embedder/main.cc.

We should only lose minimal tracing data due to this, since
ContentMainRunnerImpl::Initialize is still very early during startup,
and this doesn't affect recording of even earlier java events.

[1] https://docs.google.com/document/d/1FygJQbD29vMkfVfT7m0Lb1u1zZZU1VgXAPi_R0uBoTw/edit?usp=sharing

Bug: 1006753
Change-Id: I89aa292b3010bb501b42bd0bd735f8df75cea557
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036188Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738881}

content/app/android/library_loader_hooks.cc

@@ -23,9 +23,6 @@ bool LibraryLoaded(JNIEnv* env,
     base::android::InitReachedCodeProfilerAtStartup(library_process_type);
   }
 
-  // Enable startup tracing asap to avoid early TRACE_EVENT calls being ignored.
-  tracing::EnableStartupTracingIfNeeded();
-
   // Android's main browser loop is custom so we set the browser name here as
   // early as possible if this is the browser process or main webview process.
   if (library_process_type ==
@@ -37,8 +34,8 @@ bool LibraryLoaded(JNIEnv* env,
   base::trace_event::TraceLog::GetInstance()->SetProcessSortIndex(
       kTraceEventBrowserProcessSortIndex);
 
-  // Can only use event tracing after setting up the command line.
-  TRACE_EVENT0("jni", "JNI_OnLoad continuation");
+  // Tracing itself can only be enabled after mojo is initialized, we do so in
+  // ContentMainRunnerImpl::Initialize.
 
   logging::LoggingSettings settings;
   settings.logging_dest =

content/app/content_main_runner_impl.cc

@@ -602,9 +602,13 @@ int ContentMainRunnerImpl::Initialize(const ContentMainParams& params) {
 #endif  // defined(OS_MACOSX)
 
 #if defined(OS_ANDROID)
-  // See note at the initialization of ExitManager, below; basically,
-  // only Android builds have the ctor/dtor handlers set up to use
-  // TRACE_EVENT right away.
+  // Now that mojo's core is initialized (by service manager's Main()), we can
+  // enable tracing. Note that only Android builds have the ctor/dtor handlers
+  // set up to use trace events at this point (because AtExitManager is already
+  // set up when the library is loaded). Other platforms enable tracing below,
+  // after the initialization of AtExitManager.
+  tracing::EnableStartupTracingIfNeeded();
+
   TRACE_EVENT0("startup,benchmark,rail", "ContentMainRunnerImpl::Initialize");
 #endif  // OS_ANDROID
 
@@ -672,25 +676,23 @@ int ContentMainRunnerImpl::Initialize(const ContentMainParams& params) {
   ContentClientInitializer::Set(process_type, delegate_);
 
 #if !defined(OS_ANDROID)
-    // Enable startup tracing asap to avoid early TRACE_EVENT calls being
-    // ignored. For Android, startup tracing is enabled in an even earlier place
-    // content/app/android/library_loader_hooks.cc.
-    //
-    // Startup tracing flags are not (and should not) passed to Zygote
-    // processes. We will enable tracing when forked, if needed.
-    if (process_type != service_manager::switches::kZygoteProcess)
-      tracing::EnableStartupTracingIfNeeded();
-#endif  // !OS_ANDROID
+  // Enable startup tracing asap to avoid early TRACE_EVENT calls being
+  // ignored. For Android, startup tracing is enabled in an even earlier place
+  // above.
+  //
+  // Startup tracing flags are not (and should not be) passed to Zygote
+  // processes. We will enable tracing when forked, if needed.
+  if (process_type != service_manager::switches::kZygoteProcess)
+    tracing::EnableStartupTracingIfNeeded();
 
 #if defined(OS_WIN)
-    base::trace_event::TraceEventETWExport::EnableETWExport();
+  base::trace_event::TraceEventETWExport::EnableETWExport();
 #endif  // OS_WIN
 
-#if !defined(OS_ANDROID)
-    // Android tracing started at the beginning of the method.
-    // Other OSes have to wait till we get here in order for all the memory
-    // management setup to be completed.
-    TRACE_EVENT0("startup,benchmark,rail", "ContentMainRunnerImpl::Initialize");
+  // Android tracing started at the beginning of the method.
+  // Other OSes have to wait till we get here in order for all the memory
+  // management setup to be completed.
+  TRACE_EVENT0("startup,benchmark,rail", "ContentMainRunnerImpl::Initialize");
 #endif  // !OS_ANDROID
 
     // If we are on a platform where the default allocator is overridden (shim

content/browser/tracing/startup_tracing_browsertest.cc

@@ -51,14 +51,6 @@ class CommandlineStartupTracingTest : public ContentBrowserTest {
     command_line->AppendSwitchASCII(switches::kTraceStartupDuration, "3");
     command_line->AppendSwitchASCII(switches::kTraceStartupFile,
                                     temp_file_path_.AsUTF8Unsafe());
-
-#if defined(OS_ANDROID)
-    // On Android the startup tracing is initialized as soon as library load
-    // time, earlier than this point. So, reset the config and enable startup
-    // tracing here.
-    tracing::TraceStartupConfig::GetInstance()->EnableFromCommandLine();
-    tracing::EnableStartupTracingIfNeeded();
-#endif
   }
 
   void PreRunTestOnMainThread() override {
@@ -185,7 +177,6 @@ class BackgroundStartupTracingTest : public ContentBrowserTest {
     startup_config->enable_background_tracing_for_testing_ = true;
     startup_config->EnableFromBackgroundTracing();
     startup_config->startup_duration_in_seconds_ = 3;
-    tracing::EnableStartupTracingIfNeeded();
     command_line->AppendSwitchASCII(switches::kPerfettoOutputFile,
                                     temp_file_path_.AsUTF8Unsafe());
   }

content/public/test/browser_test_base.cc

@@ -404,6 +404,9 @@ void BrowserTestBase::SetUp() {
 
   InitializeMojo();
 
+  // We can only setup startup tracing after mojo is initialized above.
+  tracing::EnableStartupTracingIfNeeded();
+
   {
     SetBrowserClientForTesting(delegate->CreateContentBrowserClient());
     if (command_line->HasSwitch(switches::kSingleProcess))