Collect samples for background threads.

runtime/vm/gc_sweeper.cc

Index: runtime/vm/gc_sweeper.cc
diff --git a/runtime/vm/gc_sweeper.cc b/runtime/vm/gc_sweeper.cc
index 0dcf62fb276c7b944f512602fd4847b59aaf6d27..4d714bafd266da0040889afed0e76f7ae20660da 100644
--- a/runtime/vm/gc_sweeper.cc
+++ b/runtime/vm/gc_sweeper.cc
@@ -11,6 +11,7 @@
 #include "vm/pages.h"
 #include "vm/safepoint.h"
 #include "vm/thread_pool.h"
+#include "vm/timeline.h"
 
 namespace dart {
 
@@ -116,30 +117,33 @@ class SweeperTask : public ThreadPool::Task {
   virtual void Run() {
     bool result = Thread::EnterIsolateAsHelper(task_isolate_);
     ASSERT(result);
-    Thread* thread = Thread::Current();
-    GCSweeper sweeper;
-
-    HeapPage* page = first_;
-    HeapPage* prev_page = NULL;
-
-    while (page != NULL) {
-      thread->CheckForSafepoint();
-      HeapPage* next_page = page->next();
-      ASSERT(page->type() == HeapPage::kData);
-      bool page_in_use = sweeper.SweepPage(page, freelist_, false);
-      if (page_in_use) {
-        prev_page = page;
-      } else {
-        old_space_->FreePage(page, prev_page);
-      }
-      {
-        // Notify the mutator thread that we have added elements to the free
-        // list or that more capacity is available.
-        MonitorLocker ml(old_space_->tasks_lock());
-        ml.Notify();
+    {
+      Thread* thread = Thread::Current();
+      TimelineDurationScope tds(thread, Timeline::GetGCStream(), "SweeperTask");
+      GCSweeper sweeper;
+
+      HeapPage* page = first_;
+      HeapPage* prev_page = NULL;
+
+      while (page != NULL) {
+        thread->CheckForSafepoint();
+        HeapPage* next_page = page->next();
+        ASSERT(page->type() == HeapPage::kData);
+        bool page_in_use = sweeper.SweepPage(page, freelist_, false);
+        if (page_in_use) {
+          prev_page = page;
+        } else {
+          old_space_->FreePage(page, prev_page);
+        }
+        {
+          // Notify the mutator thread that we have added elements to the free
+          // list or that more capacity is available.
+          MonitorLocker ml(old_space_->tasks_lock());
+          ml.Notify();
+        }
+        if (page == last_) break;
+        page = next_page;
       }
-      if (page == last_) break;
-      page = next_page;
     }
     // Exit isolate cleanly *before* notifying it, to avoid shutdown race.
     Thread::ExitIsolateAsHelper();