[ThreadPool] Add a feature to disable priority inheritance

Details @ https://docs.google.com/document/d/13PIBPuSPJbrgHAgyRbY22EWAfH2narnxpa_CgBmZbSY/edit

R=fdoray@chromium.org

Bug: 1022972
Change-Id: I89b0404b28c92fc144b40352eed95a847f1a19ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906637
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714441}

base/task/task_features.cc

@@ -31,4 +31,7 @@ const Feature kUseNativeThreadPool = {"UseNativeThreadPool",
 const Feature kUseFiveMinutesThreadReclaimTime = {
     "UseFiveMinutesThreadReclaimTime", base::FEATURE_DISABLED_BY_DEFAULT};
 
+const Feature kNoPriorityInheritanceFromThreadPool{
+    "NoPriorityInheritanceFromThreadPool", base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace base

base/task/task_features.h

@@ -41,6 +41,12 @@ extern const BASE_EXPORT Feature kUseNativeThreadPool;
 // minutes, instead of 30 seconds.
 extern const BASE_EXPORT Feature kUseFiveMinutesThreadReclaimTime;
 
+// Under this feature, the current default of inheriting priority when posting
+// from the ThreadPool is disabled.
+// Details @
+// https://docs.google.com/document/d/13PIBPuSPJbrgHAgyRbY22EWAfH2narnxpa_CgBmZbSY
+extern const BASE_EXPORT Feature kNoPriorityInheritanceFromThreadPool;
+
 }  // namespace base
 
 #endif  // BASE_TASK_TASK_FEATURES_H_

base/task/task_traits.cc

@@ -12,6 +12,17 @@
 
 namespace base {
 
+// The state of |FeatureList::IsEnabled(kNoPriorityInheritanceFromThreadPool)|
+// as controlled by intenal::SetNoPriorityInheritanceFromThreadPool().
+bool g_no_priority_inheritance_from_thread_pool = false;
+
+void TaskTraits::InheritPriority(TaskPriority priority) {
+  if (priority_set_explicitly() || g_no_priority_inheritance_from_thread_pool)
+    return;
+
+  priority_ = static_cast<uint8_t>(priority);
+}
+
 const char* TaskPriorityToString(TaskPriority task_priority) {
   switch (task_priority) {
     case TaskPriority::BEST_EFFORT:
@@ -50,4 +61,10 @@ std::ostream& operator<<(std::ostream& os,
   return os;
 }
 
+namespace internal {
+void SetNoPriorityInheritanceFromThreadPool() {
+  g_no_priority_inheritance_from_thread_pool = true;
+}
+}  // namespace internal
+
 }  // namespace base

base/task/task_traits.h

@@ -304,11 +304,7 @@ class BASE_EXPORT TaskTraits {
   }
 
   // Sets the priority to |priority| if it wasn't explicitly set before.
-  void InheritPriority(TaskPriority priority) {
-    if (priority_set_explicitly())
-      return;
-    priority_ = static_cast<uint8_t>(priority);
-  }
+  void InheritPriority(TaskPriority priority);
 
   // Returns true if the priority was set explicitly.
   constexpr bool priority_set_explicitly() const {
@@ -418,6 +414,13 @@ BASE_EXPORT std::ostream& operator<<(
     std::ostream& os,
     const TaskShutdownBehavior& shutdown_behavior);
 
+namespace internal {
+// Enables the kNoPriorityInheritanceFromThreadPool experimental feature. Must
+// be done statically when the ThreadPoolImpl is initialized because it's racy
+// to check the state later.
+void SetNoPriorityInheritanceFromThreadPool();
+}  // namespace internal
+
 }  // namespace base
 
 #endif  // BASE_TASK_TASK_TRAITS_H_

base/task/thread_pool/thread_pool_impl.cc

@@ -123,6 +123,9 @@ void ThreadPoolImpl::Start(const ThreadPoolInstance::InitParams& init_params,
 
   internal::InitializeThreadPrioritiesFeature();
 
+  if (FeatureList::IsEnabled(kNoPriorityInheritanceFromThreadPool))
+    internal::SetNoPriorityInheritanceFromThreadPool();
+
   // The max number of concurrent BEST_EFFORT tasks is |kMaxBestEffortTasks|,
   // unless the max number of foreground threads is lower.
   const int max_best_effort_tasks =