Add weighted telemetry probes for parallel restyles

components/style/driver.rs

@@ -7,7 +7,7 @@
 
 #![deny(missing_docs)]
 
-use context::{StyleContext, ThreadLocalStyleContext};
+use context::{StyleContext, ThreadLocalStyleContext, TraversalStatistics};
 use dom::{SendNode, TElement, TNode};
 use parallel;
 use parallel::{DispatchMode, WORK_UNIT_MAX};
@@ -26,12 +26,14 @@ use traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
 /// parallel traversal would parallelize it. If a thread pool is provided, we
 /// then transfer control over to the parallel traversal.
 ///
-/// Returns true if the traversal was parallel
+/// Returns true if the traversal was parallel, and also returns the statistics
+/// object containing information on nodes traversed (on nightly only). Not
+/// all of its fields will be initialized since we don't call finish().
 pub fn traverse_dom<E, D>(
     traversal: &D,
     token: PreTraverseToken<E>,
     pool: Option<&rayon::ThreadPool>
-) -> bool
+) -> (bool, Option<TraversalStatistics>)
 where
     E: TElement,
     D: DomTraversal<E>,
@@ -40,6 +42,7 @@ where
         token.traversal_root().expect("Should've ensured we needed to traverse");
 
     let dump_stats = traversal.shared_context().options.dump_style_statistics;
+    let is_nightly  = traversal.shared_context().options.is_nightly();
     let mut used_parallel = false;
     let start_time = if dump_stats { Some(time::precise_time_s()) } else { None };
 
@@ -112,9 +115,9 @@ where
             nodes_remaining_at_current_depth = discovered.len();
         }
     }
-
-    // Dump statistics to stdout if requested.
-    if dump_stats {
+    let mut maybe_stats = None;
+    // Accumulate statistics
+    if dump_stats || is_nightly {
         let mut aggregate =
             mem::replace(&mut context.thread_local.statistics, Default::default());
         let parallel = maybe_tls.is_some();
@@ -127,11 +130,14 @@ where
                 }
             });
         }
-        aggregate.finish(traversal, parallel, start_time.unwrap());
-        if aggregate.is_large_traversal() {
-            println!("{}", aggregate);
+
+        // dump to stdout if requested
+        if dump_stats && aggregate.is_large_traversal() {
+            aggregate.finish(traversal, parallel, start_time.unwrap());
+             println!("{}", aggregate);
         }
+        maybe_stats = Some(aggregate);
     }
 
-    used_parallel
+    (used_parallel, maybe_stats)
 }