6919638: CMS: ExplicitGCInvokesConcurrent misinteracts with gc locker

Summary: GC-locker induced concurrent full gc should be asynchronous; policy now controlled by a separate flag, which defaults to false.
Reviewed-by: jmasa

src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp

@@ -1424,10 +1424,9 @@ bool ConcurrentMarkSweepGeneration::should_collect(bool   full,
 
 bool CMSCollector::shouldConcurrentCollect() {
   if (_full_gc_requested) {
-    assert(ExplicitGCInvokesConcurrent, "Unexpected state");
     if (Verbose && PrintGCDetails) {
       gclog_or_tty->print_cr("CMSCollector: collect because of explicit "
-                             " gc request");
+                             " gc request (or gc_locker)");
     }
     return true;
   }

src/share/vm/gc_implementation/concurrentMarkSweep/vmCMSOperations.cpp

@@ -163,6 +163,7 @@ void VM_CMS_Final_Remark::doit() {
 // GenCollectedHeap heap.
 void VM_GenCollectFullConcurrent::doit() {
   assert(Thread::current()->is_VM_thread(), "Should be VM thread");
+  assert(GCLockerInvokesConcurrent || ExplicitGCInvokesConcurrent, "Unexpected");
 
   GenCollectedHeap* gch = GenCollectedHeap::heap();
   if (_gc_count_before == gch->total_collections()) {
@@ -190,7 +191,7 @@ void VM_GenCollectFullConcurrent::doit() {
     CMSCollector::disable_icms();
     // In case CMS thread was in icms_wait(), wake it up.
     CMSCollector::start_icms();
-    // Nudge the CMS thread to start a concurrent collection
+    // Nudge the CMS thread to start a concurrent collection.
     CMSCollector::request_full_gc(_full_gc_count_before);
   } else {
     FullGCCount_lock->notify_all();  // Inform the Java thread its work is done
@@ -231,7 +232,9 @@ void VM_GenCollectFullConcurrent::doit_epilogue() {
   // e.g. at the rate of 1 full gc per ms, this could
   // overflow in about 1000 years.
   GenCollectedHeap* gch = GenCollectedHeap::heap();
-  if (gch->total_full_collections_completed() <= _full_gc_count_before) {
+  if (_gc_cause != GCCause::_gc_locker &&
+      gch->total_full_collections_completed() <= _full_gc_count_before) {
+    assert(ExplicitGCInvokesConcurrent, "Error");
     // Now, wait for witnessing concurrent gc cycle to complete,
     // but do so in native mode, because we want to lock the
     // FullGCEvent_lock, which may be needed by the VM thread

src/share/vm/gc_implementation/concurrentMarkSweep/vmCMSOperations.hpp

@@ -126,8 +126,7 @@ class VM_GenCollectFullConcurrent: public VM_GC_Operation {
                               GCCause::Cause gc_cause)
     : VM_GC_Operation(gc_count_before, full_gc_count_before, true /* full */) {
     _gc_cause = gc_cause;
-    assert(FullGCCount_lock != NULL && UseConcMarkSweepGC &&
-           ExplicitGCInvokesConcurrent, "Otherwise shouldn't be here");
+    assert(FullGCCount_lock != NULL, "Error");
     assert(UseAsyncConcMarkSweepGC, "Else will hang caller");
   }
   ~VM_GenCollectFullConcurrent() {}

src/share/vm/memory/genCollectedHeap.cpp

@@ -410,9 +410,9 @@ bool GenCollectedHeap::must_clear_all_soft_refs() {
 }
 
 bool GenCollectedHeap::should_do_concurrent_full_gc(GCCause::Cause cause) {
-  return (cause == GCCause::_java_lang_system_gc ||
-          cause == GCCause::_gc_locker) &&
-         UseConcMarkSweepGC && ExplicitGCInvokesConcurrent;
+  return UseConcMarkSweepGC &&
+         ((cause == GCCause::_gc_locker && GCLockerInvokesConcurrent) ||
+          (cause == GCCause::_java_lang_system_gc && ExplicitGCInvokesConcurrent));
 }
 
 void GenCollectedHeap::do_collection(bool  full,

src/share/vm/runtime/globals.hpp

@@ -1303,6 +1303,10 @@ class CommandLineFlags {
           "also unloads classes during such a concurrent gc cycle "         \
           "(effective only when UseConcMarkSweepGC)")                       \
                                                                             \
+  product(bool, GCLockerInvokesConcurrent, false,                           \
+          "The exit of a JNI CS necessitating a scavenge also"              \
+          " kicks off a bkgrd concurrent collection")                       \
+                                                                            \
   develop(bool, UseCMSAdaptiveFreeLists, true,                              \
           "Use Adaptive Free Lists in the CMS generation")                  \
                                                                             \