6816154: G1: introduce flags to enable/disable RSet updating and scanning

Summary: Introduces two flags, -XX:-/+G1EnableParallelRSetUpdating and -XX:-/+G1EnableParallelRSetScanning, to turn on/off the "band aid" fix that serializes RSet updating / scanning during GCs.
Reviewed-by: iveresov

src/share/vm/gc_implementation/g1/g1RemSet.cpp

@@ -502,14 +502,17 @@ HRInto_G1RemSet::oops_into_collection_set_do(OopsInHeapRegionClosure* oc,
   }
 
   if (ParallelGCThreads > 0) {
-    // This is a temporary change to serialize the update and scanning
-    // of remembered sets. There are some race conditions when this is
-    // done in parallel and they are causing failures. When we resolve
-    // said race conditions, we'll revert back to parallel remembered
-    // set updating and scanning. See CRs 6677707 and 6677708.
-    if (worker_i == 0) {
+    // The two flags below were introduced temporarily to serialize
+    // the updating and scanning of remembered sets. There are some
+    // race conditions when these two operations are done in parallel
+    // and they are causing failures. When we resolve said race
+    // conditions, we'll revert back to parallel remembered set
+    // updating and scanning. See CRs 6677707 and 6677708.
+    if (G1EnableParallelRSetUpdating || (worker_i == 0)) {
       updateRS(worker_i);
       scanNewRefsRS(oc, worker_i);
+    }
+    if (G1EnableParallelRSetScanning || (worker_i == 0)) {
       scanRS(oc, worker_i);
     }
   } else {

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -295,6 +295,14 @@
                                                                             \
   product(uintx, G1FixedSurvivorSpaceSize, 0,                               \
           "If non-0 is the size of the G1 survivor space, "                 \
-          "otherwise SurvivorRatio is used to determine the size")
+          "otherwise SurvivorRatio is used to determine the size")          \
+                                                                            \
+  experimental(bool, G1EnableParallelRSetUpdating, false,                   \
+          "Enables the parallelization of remembered set updating "         \
+          "during evacuation pauses")                                       \
+                                                                            \
+  experimental(bool, G1EnableParallelRSetScanning, false,                   \
+          "Enables the parallelization of remembered set scanning "         \
+          "during evacuation pauses")
 
 G1_FLAGS(DECLARE_DEVELOPER_FLAG, DECLARE_PD_DEVELOPER_FLAG, DECLARE_PRODUCT_FLAG, DECLARE_PD_PRODUCT_FLAG, DECLARE_DIAGNOSTIC_FLAG, DECLARE_EXPERIMENTAL_FLAG, DECLARE_NOTPRODUCT_FLAG, DECLARE_MANAGEABLE_FLAG, DECLARE_PRODUCT_RW_FLAG)