Add stat force_incremental_marking_finish_count

This commit adds key force_incremental_marking_finish_count to
GC.stat_heap. This statistic returns the number of times the size pool
has forced incremental marking to finish due to running out of slots.

gc.c

@@ -707,6 +707,7 @@ typedef struct rb_size_pool_struct {
     size_t total_allocated_pages;
     size_t total_freed_pages;
     size_t force_major_gc_count;
+    size_t force_incremental_marking_finish_count;
 
     /* Sweeping statistics */
     size_t freed_slots;
@@ -8504,6 +8505,7 @@ gc_marks_continue(rb_objspace_t *objspace, rb_size_pool_t *size_pool, rb_heap_t
     else {
         gc_report(2, objspace, "gc_marks_continue: no more pooled pages (stack depth: %"PRIdSIZE").\n",
                   mark_stack_size(&objspace->mark_stack));
+        size_pool->force_incremental_marking_finish_count++;
         gc_marks_rest(objspace);
     }
 
@@ -11182,6 +11184,7 @@ enum gc_stat_heap_sym {
     gc_stat_heap_sym_total_allocated_pages,
     gc_stat_heap_sym_total_freed_pages,
     gc_stat_heap_sym_force_major_gc_count,
+    gc_stat_heap_sym_force_incremental_marking_finish_count,
     gc_stat_heap_sym_last
 };
 
@@ -11201,6 +11204,7 @@ setup_gc_stat_heap_symbols(void)
         S(total_allocated_pages);
         S(total_freed_pages);
         S(force_major_gc_count);
+        S(force_incremental_marking_finish_count);
 #undef S
     }
 }
@@ -11244,6 +11248,7 @@ gc_stat_heap_internal(int size_pool_idx, VALUE hash_or_sym)
     SET(total_allocated_pages, size_pool->total_allocated_pages);
     SET(total_freed_pages, size_pool->total_freed_pages);
     SET(force_major_gc_count, size_pool->force_major_gc_count);
+    SET(force_incremental_marking_finish_count, size_pool->force_incremental_marking_finish_count);
 #undef SET
 
     if (!NIL_P(key)) { /* matched key should return above */

gc.rb

@@ -244,6 +244,9 @@ module GC
   # [force_major_gc_count]
   #   The number of times major garbage collection cycles this heap has forced
   #   to start due to running out of free slots.
+  # [force_incremental_marking_finish_count]
+  #   The number of times this heap has forced incremental marking to complete
+  #   due to running out of pooled slots.
   #
   def self.stat_heap heap_name = nil, hash_or_key = nil
     Primitive.gc_stat_heap heap_name, hash_or_key

test/ruby/test_gc.rb

@@ -163,6 +163,7 @@ class TestGc < Test::Unit::TestCase
       assert_operator stat_heap[:total_allocated_pages], :>=, 0
       assert_operator stat_heap[:total_freed_pages], :>=, 0
       assert_operator stat_heap[:force_major_gc_count], :>=, 0
+      assert_operator stat_heap[:force_incremental_marking_finish_count], :>=, 0
     end
 
     GC.stat_heap(0, stat_heap)