diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index 2bb064cc0a..69cc6180ea 100644
--- a/libs/ardour/audioengine.cc
+++ b/libs/ardour/audioengine.cc
@@ -180,6 +180,9 @@ AudioEngine::buffer_size_change (pframes_t bufsiz)
 /** Method called by our ::process_thread when there is work to be done.
  *  @param nframes Number of frames to process.
  */
+#ifdef __clang__
+__attribute__((annotate("realtime")))
+#endif
 int
 AudioEngine::process_callback (pframes_t nframes)
 {