7120038: G1: ParallelGCThreads==0 is broken

Summary: Running G1 with ParallelGCThreads==0 results in various crashes and asserts. Most of these are caused by unguarded references to the worker threads array or an incorrect number of active workers.
Reviewed-by: jmasa, tonyp

src/share/vm/gc_implementation/g1/concurrentMark.cpp

@@ -1117,12 +1117,9 @@ public:
 
 // Calculates the number of active workers for a concurrent
 // phase.
-int ConcurrentMark::calc_parallel_marking_threads() {
-
-  size_t n_conc_workers;
-  if (!G1CollectedHeap::use_parallel_gc_threads()) {
-    n_conc_workers = 1;
-  } else {
+size_t ConcurrentMark::calc_parallel_marking_threads() {
+  if (G1CollectedHeap::use_parallel_gc_threads()) {
+    size_t n_conc_workers = 0;
     if (!UseDynamicNumberOfGCThreads ||
         (!FLAG_IS_DEFAULT(ConcGCThreads) &&
          !ForceDynamicNumberOfGCThreads)) {
@@ -1137,9 +1134,13 @@ int ConcurrentMark::calc_parallel_marking_threads() {
       // Don't scale down "n_conc_workers" by scale_parallel_threads() because
       // that scaling has already gone into "_max_parallel_marking_threads".
     }
-  }
     assert(n_conc_workers > 0, "Always need at least 1");
-  return (int) MAX2(n_conc_workers, (size_t) 1);
+    return n_conc_workers;
+  }
+  // If we are not running with any parallel GC threads we will not
+  // have spawned any marking threads either. Hence the number of
+  // concurrent workers should be 0.
+  return 0;
 }
 
 void ConcurrentMark::markFromRoots() {
@@ -1151,24 +1152,24 @@ void ConcurrentMark::markFromRoots() {
   // stop-the-world GC happens even as we mark in this generation.
 
   _restart_for_overflow = false;
-
-  // Parallel task terminator is set in "set_phase()".
   force_overflow_conc()->init();
 
   // _g1h has _n_par_threads
-
   _parallel_marking_threads = calc_parallel_marking_threads();
   assert(parallel_marking_threads() <= max_parallel_marking_threads(),
     "Maximum number of marking threads exceeded");
-  _parallel_workers->set_active_workers((int)_parallel_marking_threads);
-  // Don't set _n_par_threads because it affects MT in proceess_strong_roots()
-  // and the decisions on that MT processing is made elsewhere.
 
-  assert( _parallel_workers->active_workers() > 0, "Should have been set");
-  set_phase(_parallel_workers->active_workers(), true /* concurrent */);
+  size_t active_workers = MAX2((size_t) 1, parallel_marking_threads());
+
+  // Parallel task terminator is set in "set_phase()"
+  set_phase(active_workers, true /* concurrent */);
 
   CMConcurrentMarkingTask markingTask(this, cmThread());
   if (parallel_marking_threads() > 0) {
+    _parallel_workers->set_active_workers((int)active_workers);
+    // Don't set _n_par_threads because it affects MT in proceess_strong_roots()
+    // and the decisions on that MT processing is made elsewhere.
+    assert(_parallel_workers->active_workers() > 0, "Should have been set");
     _parallel_workers->run_task(&markingTask);
   } else {
     markingTask.work(0);
@@ -1765,8 +1766,7 @@ void ConcurrentMark::cleanup() {
 
   HeapRegionRemSet::reset_for_cleanup_tasks();
 
-  g1h->set_par_threads();
-  size_t n_workers = g1h->n_par_threads();
+  size_t n_workers;
 
   // Do counting once more with the world stopped for good measure.
   G1ParFinalCountTask g1_par_count_task(g1h, nextMarkBitMap(),
@@ -1776,6 +1776,8 @@ void ConcurrentMark::cleanup() {
                                                HeapRegion::InitialClaimValue),
            "sanity check");
 
+    g1h->set_par_threads();
+    n_workers = g1h->n_par_threads();
     assert(g1h->n_par_threads() == (int) n_workers,
            "Should not have been reset");
     g1h->workers()->run_task(&g1_par_count_task);
@@ -1786,6 +1788,7 @@ void ConcurrentMark::cleanup() {
                                              HeapRegion::FinalCountClaimValue),
            "sanity check");
   } else {
+    n_workers = 1;
     g1_par_count_task.work(0);
   }
 
@@ -1851,7 +1854,6 @@ void ConcurrentMark::cleanup() {
                            (note_end_end - note_end_start)*1000.0);
   }
 
-
   // call below, since it affects the metric by which we sort the heap
   // regions.
   if (G1ScrubRemSets) {
@@ -2329,9 +2331,9 @@ public:
     }
   }
 
-  CMRemarkTask(ConcurrentMark* cm) :
+  CMRemarkTask(ConcurrentMark* cm, int active_workers) :
     AbstractGangTask("Par Remark"), _cm(cm) {
-    _cm->terminator()->reset_for_reuse(cm->_g1h->workers()->active_workers());
+    _cm->terminator()->reset_for_reuse(active_workers);
   }
 };
 
@@ -2357,7 +2359,7 @@ void ConcurrentMark::checkpointRootsFinalWork() {
     // constructor and pass values of the active workers
     // through the gang in the task.
 
-    CMRemarkTask remarkTask(this);
+    CMRemarkTask remarkTask(this, active_workers);
     g1h->set_par_threads(active_workers);
     g1h->workers()->run_task(&remarkTask);
     g1h->set_par_threads(0);
@@ -2367,7 +2369,7 @@ void ConcurrentMark::checkpointRootsFinalWork() {
     int active_workers = 1;
     set_phase(active_workers, false /* concurrent */);
 
-    CMRemarkTask remarkTask(this);
+    CMRemarkTask remarkTask(this, active_workers);
     // We will start all available threads, even if we decide that the
     // active_workers will be fewer. The extra ones will just bail out
     // immediately.
@@ -3123,13 +3125,12 @@ void ConcurrentMark::complete_marking_in_collection_set() {
   }
 
   double start = os::elapsedTime();
-  int n_workers = g1h->workers()->total_workers();
-
   G1ParCompleteMarkInCSetTask complete_mark_task(g1h, this);
 
   assert(g1h->check_cset_heap_region_claim_values(HeapRegion::InitialClaimValue), "sanity");
 
   if (G1CollectedHeap::use_parallel_gc_threads()) {
+    int n_workers = g1h->workers()->active_workers();
     g1h->set_par_threads(n_workers);
     g1h->workers()->run_task(&complete_mark_task);
     g1h->set_par_threads(0);

src/share/vm/gc_implementation/g1/concurrentMark.hpp

@@ -718,7 +718,7 @@ public:
   size_t scale_parallel_threads(size_t n_par_threads);
 
   // Calculates the number of GC threads to be used in a concurrent phase.
-  int calc_parallel_marking_threads();
+  size_t calc_parallel_marking_threads();
 
   // The following three are interaction between CM and
   // G1CollectedHeap

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -3787,8 +3787,9 @@ G1CollectedHeap::do_collection_pause_at_safepoint(double target_pause_time_ms) {
         double end_time_sec = os::elapsedTime();
         double pause_time_ms = (end_time_sec - start_time_sec) * MILLIUNITS;
         g1_policy()->record_pause_time_ms(pause_time_ms);
-        int active_gc_threads = workers()->active_workers();
-        g1_policy()->record_collection_pause_end(active_gc_threads);
+        int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
+                                workers()->active_workers() : 1);
+        g1_policy()->record_collection_pause_end(active_workers);
 
         MemoryService::track_memory_usage();
 
@@ -5312,8 +5313,10 @@ void G1CollectedHeap::process_discovered_references() {
   int active_workers = (G1CollectedHeap::use_parallel_gc_threads() ?
                         workers()->active_workers() : 1);
 
-  assert(active_workers == workers()->active_workers(),
+  assert(!G1CollectedHeap::use_parallel_gc_threads() ||
+           active_workers == workers()->active_workers(),
            "Need to reset active_workers");
+
   set_par_threads(active_workers);
   G1ParPreserveCMReferentsTask keep_cm_referents(this, active_workers, _task_queues);
 
@@ -5451,13 +5454,13 @@ void G1CollectedHeap::evacuate_collection_set() {
     assert(UseDynamicNumberOfGCThreads ||
            n_workers == workers()->total_workers(),
            "If not dynamic should be using all the  workers");
+    workers()->set_active_workers(n_workers);
     set_par_threads(n_workers);
   } else {
     assert(n_par_threads() == 0,
            "Should be the original non-parallel value");
     n_workers = 1;
   }
-  workers()->set_active_workers(n_workers);
 
   G1ParTask g1_par_task(this, _task_queues);
 
@@ -5479,6 +5482,7 @@ void G1CollectedHeap::evacuate_collection_set() {
     workers()->run_task(&g1_par_task);
   } else {
     StrongRootsScope srs(this);
+    g1_par_task.set_for_termination(n_workers);
     g1_par_task.work(0);
   }
 
@@ -5727,8 +5731,8 @@ void G1CollectedHeap::cleanUpCardTable() {
     // Iterate over the dirty cards region list.
     G1ParCleanupCTTask cleanup_task(ct_bs, this);
 
-    if (ParallelGCThreads > 0) {
-      set_par_threads(workers()->total_workers());
+    if (G1CollectedHeap::use_parallel_gc_threads()) {
+      set_par_threads();
       workers()->run_task(&cleanup_task);
       set_par_threads(0);
     } else {
@@ -6136,6 +6140,7 @@ HeapRegion* MutatorAllocRegion::allocate_new_region(size_t word_size,
 void G1CollectedHeap::set_par_threads() {
   // Don't change the number of workers.  Use the value previously set
   // in the workgroup.
+  assert(G1CollectedHeap::use_parallel_gc_threads(), "shouldn't be here otherwise");
   int n_workers = workers()->active_workers();
   assert(UseDynamicNumberOfGCThreads ||
            n_workers == workers()->total_workers(),