Bug 1708042, add control priority to the main thread, r=bas

Differential Revision: https://phabricator.services.mozilla.com/D115404

modules/libpref/init/StaticPrefList.yaml

@@ -10651,6 +10651,12 @@
   value: true
   mirror: always
 
+# If control tasks aren't enabled, they get medium high priority.
+- name: threads.control_event_queue.enabled
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "timer."
 #---------------------------------------------------------------------------

xpcom/threads/EventQueue.cpp

@@ -33,6 +33,8 @@ void EventQueueInternal<ItemsPerPage>::PutEvent(
       static_cast<uint32_t>(EventQueuePriority::InputHigh));
   static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_VSYNC) ==
                 static_cast<uint32_t>(EventQueuePriority::Vsync));
+  static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_CONTROL) ==
+                static_cast<uint32_t>(EventQueuePriority::Control));
 
   if (mForwardToTC) {
     TaskController* tc = TaskController::Get();

xpcom/threads/EventQueue.h

@@ -25,6 +25,7 @@ enum class EventQueuePriority {
   InputHigh,
   Vsync,
   InputHighest,
+  Control,
 
   Count
 };

xpcom/threads/ThreadEventQueue.cpp

@@ -15,6 +15,7 @@
 #include "ThreadEventTarget.h"
 #include "mozilla/ProfilerLabels.h"
 #include "mozilla/TaskController.h"
+#include "mozilla/StaticPrefs_threads.h"
 
 using namespace mozilla;
 
@@ -81,7 +82,9 @@ bool ThreadEventQueue::PutEventInternal(already_AddRefed<nsIRunnable>&& aEvent,
       if (nsCOMPtr<nsIRunnablePriority> runnablePrio = do_QueryInterface(e)) {
         uint32_t prio = nsIRunnablePriority::PRIORITY_NORMAL;
         runnablePrio->GetPriority(&prio);
-        if (prio == nsIRunnablePriority::PRIORITY_VSYNC) {
+        if (prio == nsIRunnablePriority::PRIORITY_CONTROL) {
+          aPriority = EventQueuePriority::Control;
+        } else if (prio == nsIRunnablePriority::PRIORITY_VSYNC) {
           aPriority = EventQueuePriority::Vsync;
         } else if (prio == nsIRunnablePriority::PRIORITY_INPUT_HIGH) {
           aPriority = EventQueuePriority::InputHigh;
@@ -93,6 +96,11 @@ bool ThreadEventQueue::PutEventInternal(already_AddRefed<nsIRunnable>&& aEvent,
           aPriority = EventQueuePriority::Idle;
         }
       }
+
+      if (aPriority == EventQueuePriority::Control &&
+          !StaticPrefs::threads_control_event_queue_enabled()) {
+        aPriority = EventQueuePriority::MediumHigh;
+      }
     }
 
     MutexAutoLock lock(mLock);

xpcom/threads/nsIRunnable.idl

@@ -29,6 +29,10 @@ interface nsIRunnablePriority : nsISupports
     const unsigned short PRIORITY_MEDIUMHIGH = 4;
     const unsigned short PRIORITY_INPUT_HIGH = 5;
     const unsigned short PRIORITY_VSYNC = 6;
+    // INPUT_HIGHEST is InputTaskManager's internal priority
+    //const unsigned short PRIORITY_INPUT_HIGHEST = 7;
+    const unsigned short PRIORITY_CONTROL = 8;
+
     readonly attribute unsigned long priority;
 };