Print safepoint message only after some attempts.

runtime/vm/safepoint.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/safepoint.h"
 
 #include "vm/thread.h"
 #include "vm/thread_registry.h"
 
 namespace dart {
@@ skipping 92 matching lines @@
     }
   }
   // Now wait for all threads that are not already at a safepoint to check-in.
   {
     MonitorLocker sl(safepoint_lock_);
     intptr_t num_attempts = 0;
     while (number_threads_not_at_safepoint_ > 0) {
       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_);
+        }
       }
     }
   }
 }
 
 
 void SafepointHandler::ResumeThreads(Thread* T) {
   // First resume all the threads which are blocked for the safepoint
   // operation.
   MonitorLocker sl(threads_lock());
@@ skipping 63 matching lines @@
     while (T->IsSafepointRequested()) {
       T->SetBlockedForSafepoint(true);
       tl.Wait();
       T->SetBlockedForSafepoint(false);
     }
     T->SetAtSafepoint(false);
   }
 }
 
 }  // namespace dart