[base] Experiment with only invoking ::PeekMessage when there are pending messages

::PeekMessage can process sent-messages and internal events. It seems
the kernel also uses it as a heuristic to deschedule the main thread when
it thinks it's out of messages. In MessagePumpForUI's design, it's
possible to have application tasks without pending native tasks
(kMsgHaveWork is dropped when running out of native tasks).

This CL uses ::GetQueueStatus() to probe the presence of messages before
invoking ::PeekMessage().

Bug: 1075960
Change-Id: I4a93b22adf225ae2d62aabeb1a7a734232cf7fb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2258456
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781498}

base/message_loop/message_pump_win.cc

@@ -463,10 +463,6 @@ void MessagePumpForUI::KillNativeTimer() {
 bool MessagePumpForUI::ProcessNextWindowsMessage() {
   DCHECK_CALLED_ON_VALID_THREAD(bound_thread_);
 
-  // If there are sent messages in the queue then PeekMessage internally
-  // dispatches the message and returns false. We return true in this
-  // case to ensure that the message loop peeks again instead of calling
-  // MsgWaitForMultipleObjectsEx.
   bool more_work_is_plausible = false;
   {
     // Individually trace ::GetQueueStatus and ::PeekMessage because sampling
@@ -477,7 +473,22 @@ bool MessagePumpForUI::ProcessNextWindowsMessage() {
     // profiler's thread while the sampled thread is swapped out on this frame).
     TRACE_EVENT0("base",
                  "MessagePumpForUI::ProcessNextWindowsMessage GetQueueStatus");
-    DWORD queue_status = ::GetQueueStatus(QS_SENDMESSAGE);
+
+    // Experiment with querying all messages and not invoking PeekMessage at all
+    // if there's nothing there.
+    static const auto get_queue_status_flag =
+        FeatureList::IsEnabled(kPreventMessagePumpHangs) ? QS_ALLINPUT
+                                                         : QS_SENDMESSAGE;
+
+    const DWORD queue_status = ::GetQueueStatus(get_queue_status_flag);
+
+    if (get_queue_status_flag == QS_ALLINPUT && !queue_status)
+      return false;
+
+    // If there were sent messages in the queue then ::PeekMessage internally
+    // dispatches the message and returns false. We return true in this case to
+    // ensure that the message loop peeks again instead of calling
+    // MsgWaitForMultipleObjectsEx.
     if (HIWORD(queue_status) & QS_SENDMESSAGE)
       more_work_is_plausible = true;
   }
@@ -552,9 +563,9 @@ bool MessagePumpForUI::ProcessPumpReplacementMessage() {
   // that peeked replacement. Note that the re-post of kMsgHaveWork may be
   // asynchronous to this thread!!
 
-  // Bump the work id since ::PeekMessage may process internal events.
-  state_->delegate->BeforeDoInternalWork();
-
+  MSG msg;
+  bool have_message = false;
+  {
     // The system headers don't define this; it's equivalent to PM_QS_INPUT |
     // PM_QS_PAINT | PM_QS_POSTMESSAGE. i.e., anything but QS_SENDMESSAGE. Since
     // we're looking to replace our kMsgHaveWork posted message, we can ignore
@@ -565,20 +576,33 @@ bool MessagePumpForUI::ProcessPumpReplacementMessage() {
         PM_QS_ALLEVENTS == (PM_QS_INPUT | PM_QS_PAINT | PM_QS_POSTMESSAGE), "");
     static_assert((PM_QS_ALLEVENTS & PM_QS_SENDMESSAGE) == 0, "");
 
-  MSG msg;
-  bool have_message = false;
+    static const auto peek_replacement_message_modifier =
+        FeatureList::IsEnabled(kPreventMessagePumpHangs) ? PM_QS_ALLEVENTS : 0;
+
+    // Always check messages under the non-experimental arm but skip in the
+    // experimental arm if GetQueueStatus() doesn't think there are any.
+    bool check_messages = true;
     {
-    TRACE_EVENT0("base",
-                 "MessagePumpForUI::ProcessPumpReplacementMessage PeekMessage");
+      if (peek_replacement_message_modifier == PM_QS_ALLEVENTS) {
+        TRACE_EVENT0(
+            "base",
+            "MessagePumpForUI::ProcessPumpReplacementMessage GetQueueStatus");
+        check_messages = ::GetQueueStatus(QS_ALLEVENTS);
+      }
+    }
 
-    static const auto peek_replacement_message_modifier =
-        base::FeatureList::IsEnabled(kPreventMessagePumpHangs) ? PM_QS_ALLEVENTS
-                                                               : 0;
+    if (check_messages) {
+      TRACE_EVENT0(
+          "base",
+          "MessagePumpForUI::ProcessPumpReplacementMessage PeekMessage");
 
+      // Bump the work id since ::PeekMessage may process internal events.
+      state_->delegate->BeforeDoInternalWork();
       have_message =
           ::PeekMessage(&msg, nullptr, 0, 0,
                         PM_REMOVE | peek_replacement_message_modifier) != FALSE;
     }
+  }
 
   // Expect no message or a message different than kMsgHaveWork.
   DCHECK(!have_message || kMsgHaveWork != msg.message ||