Trace event micro-optimization: use finer grained lock.

base/debug/trace_event_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/debug/trace_event_impl.h"
 
 #include <algorithm>
 
 #include "base/base_switches.h"
 #include "base/bind.h"
@@ skipping 82 matching lines @@
   "trace_event_overhead"};
 
 // The enabled flag is char instead of bool so that the API can be used from C.
 unsigned char g_category_group_enabled[MAX_CATEGORY_GROUPS] = { 0 };
 // Indexes here have to match the g_category_groups array indexes above.
 const int g_category_already_shutdown = 1;
 const int g_category_categories_exhausted = 2;
 const int g_category_metadata = 3;
 const int g_category_trace_event_overhead = 4;
 const int g_num_builtin_categories = 5;
-int g_category_index = g_num_builtin_categories; // Skip default categories.
+// Skip default categories.
+base::subtle::AtomicWord g_category_index = g_num_builtin_categories;
 
 // The name of the current thread. This is used to decide if the current
 // thread name has changed. We combine all the seen thread names into the
 // output name for the thread.
 LazyInstance<ThreadLocalPointer<const char> >::Leaky
     g_current_thread_name = LAZY_INSTANCE_INITIALIZER;
 
 TimeTicks ThreadNow() {
   return TimeTicks::IsThreadNowSupported() ?
       TimeTicks::ThreadNow() : TimeTicks();
@@ skipping 1138 matching lines @@
         delay->SetMode(TraceEventSyntheticDelay::ALTERNATING);
       }
     }
   }
 }
 
 const unsigned char* TraceLog::GetCategoryGroupEnabledInternal(
     const char* category_group) {
   DCHECK(!strchr(category_group, '"')) <<
       "Category groups may not contain double quote";
-  AutoLock lock(lock_);
+  // The g_category_groups is append only, avoid using a lock for the fast path.
+  int current_category_index = base::subtle::NoBarrier_Load(&g_category_index);
 
-  unsigned char* category_group_enabled = NULL;
   // Search for pre-existing category group.
-  for (int i = 0; i < g_category_index; i++) {
+  for (int i = 0; i < current_category_index; ++i) {
     if (strcmp(g_category_groups[i], category_group) == 0) {
-      category_group_enabled = &g_category_group_enabled[i];
-      break;
+      return &g_category_group_enabled[i];
     }
   }
 
-  if (!category_group_enabled) {
-    // Create a new category group
-    DCHECK(g_category_index < MAX_CATEGORY_GROUPS) <<
-        "must increase MAX_CATEGORY_GROUPS";
-    if (g_category_index < MAX_CATEGORY_GROUPS) {
-      int new_index = g_category_index++;
-      // Don't hold on to the category_group pointer, so that we can create
-      // category groups with strings not known at compile time (this is
-      // required by SetWatchEvent).
-      const char* new_group = strdup(category_group);
-      ANNOTATE_LEAKING_OBJECT_PTR(new_group);
-      g_category_groups[new_index] = new_group;
-      DCHECK(!g_category_group_enabled[new_index]);
-      // Note that if both included and excluded patterns in the
-      // CategoryFilter are empty, we exclude nothing,
-      // thereby enabling this category group.
-      UpdateCategoryGroupEnabledFlag(new_index);
-      category_group_enabled = &g_category_group_enabled[new_index];
-    } else {
-      category_group_enabled =
-          &g_category_group_enabled[g_category_categories_exhausted];
+  unsigned char* category_group_enabled = NULL;
+  // This is the slow path: the lock is not held in the case above, so more
+  // than one thread could have reached here trying to add the same category.
+  // Only hold to lock when actually appending a new category, and
+  // check the categories groups again.
+  AutoLock lock(lock_);
+  for (int i = 0; i < g_category_index; ++i) {
+    if (strcmp(g_category_groups[i], category_group) == 0) {
+      return &g_category_group_enabled[i];
     }
   }
+
+  // Create a new category group.
+  DCHECK(g_category_index < MAX_CATEGORY_GROUPS) <<
+      "must increase MAX_CATEGORY_GROUPS";
+  if (g_category_index < MAX_CATEGORY_GROUPS) {
+    int new_index = base::subtle::Acquire_Load(&g_category_index);

[bulach, 2014/02/13 17:55:37]

the + 1 was here.. :(

+    // Don't hold on to the category_group pointer, so that we can create
+    // category groups with strings not known at compile time (this is
+    // required by SetWatchEvent).
+    const char* new_group = strdup(category_group);
+    ANNOTATE_LEAKING_OBJECT_PTR(new_group);
+    g_category_groups[new_index] = new_group;
+    DCHECK(!g_category_group_enabled[new_index]);
+    // Note that if both included and excluded patterns in the
+    // CategoryFilter are empty, we exclude nothing,
+    // thereby enabling this category group.
+    UpdateCategoryGroupEnabledFlag(new_index);
+    category_group_enabled = &g_category_group_enabled[new_index];
+    // Update the max index now.
+    base::subtle::Release_Store(&g_category_index, new_index + 1);
+  } else {
+    category_group_enabled =
+        &g_category_group_enabled[g_category_categories_exhausted];
+  }
   return category_group_enabled;
 }
 
 void TraceLog::GetKnownCategoryGroups(
     std::vector<std::string>* category_groups) {
   AutoLock lock(lock_);
   category_groups->push_back(
       g_category_groups[g_category_trace_event_overhead]);
   for (int i = g_num_builtin_categories; i < g_category_index; i++)
     category_groups->push_back(g_category_groups[i]);
@@ skipping 1099 matching lines @@
 }
 
 ScopedTraceBinaryEfficient::~ScopedTraceBinaryEfficient() {
   if (*category_group_enabled_) {
     TRACE_EVENT_API_UPDATE_TRACE_EVENT_DURATION(category_group_enabled_,
                                                 name_, event_handle_);
   }
 }
 
 }  // namespace trace_event_internal