Print safepoint message only after some attempts.

runtime/vm/safepoint.cc

Index: runtime/vm/safepoint.cc
diff --git a/runtime/vm/safepoint.cc b/runtime/vm/safepoint.cc
index ae1405a81825630f58d6e025e092b4e1714c08ca..7f5d92d0d240bdc0590e04192b1158b33f63603a 100644
--- a/runtime/vm/safepoint.cc
+++ b/runtime/vm/safepoint.cc
@@ -110,9 +110,13 @@ void SafepointHandler::SafepointThreads(Thread* T) {
       Monitor::WaitResult retval = sl.Wait(1000);
       if (retval == Monitor::kTimedOut) {
         num_attempts += 1;
-        OS::Print("Attempt:%" Pd " waiting for %d threads to check in\n",
-                  num_attempts,
-                  number_threads_not_at_safepoint_);
+        if (num_attempts > 10) {
+          // We have been waiting too long, start logging this as we might
+          // have an issue where a thread is not checking in for a safepoint.
+          OS::Print("Attempt:%" Pd " waiting for %d threads to check in\n",
+                    num_attempts,
+                    number_threads_not_at_safepoint_);
+        }
       }
     }
   }