[gc] Use CreateJob().Join() in v8 gc to avoid extra context switches

CreateJob() doesn't schedule anything until Join() or Notify*() is called. CreateJob().Join() will thus schedule the right number of
workers for the job right away (taking into account the main thread
contributes), whereas PostJob().Join() schedules 1 worker that won't
be necessary once doing Join() and the main thread kicks in.
This has the effect of reducing 1 unnecessary context switch each time
the jobs are schedule.

Bug: chromium:1287665
Change-Id: Ie262f8904cc8ac78d9e5cbd23ef28dc5b013a625
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3746080Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82047}

src/heap/mark-compact.cc

@@ -4207,7 +4207,7 @@ size_t CreateAndExecuteEvacuationTasks(
     evacuators.push_back(std::move(evacuator));
   }
   V8::GetCurrentPlatform()
-      ->PostJob(
+      ->CreateJob(
           v8::TaskPriority::kUserBlocking,
           std::make_unique<PageEvacuationJob>(collector->isolate(), &evacuators,
                                               std::move(evacuation_items)))
@@ -4965,11 +4965,12 @@ void MarkCompactCollector::UpdatePointersAfterEvacuation() {
         std::make_unique<EphemeronTableUpdatingItem>(heap()));
 
     V8::GetCurrentPlatform()
-        ->PostJob(v8::TaskPriority::kUserBlocking,
-                  std::make_unique<PointersUpdatingJob>(
-                      isolate(), std::move(updating_items),
-                      GCTracer::Scope::MC_EVACUATE_UPDATE_POINTERS_PARALLEL,
-                      GCTracer::Scope::MC_BACKGROUND_EVACUATE_UPDATE_POINTERS))
+        ->CreateJob(
+            v8::TaskPriority::kUserBlocking,
+            std::make_unique<PointersUpdatingJob>(
+                isolate(), std::move(updating_items),
+                GCTracer::Scope::MC_EVACUATE_UPDATE_POINTERS_PARALLEL,
+                GCTracer::Scope::MC_BACKGROUND_EVACUATE_UPDATE_POINTERS))
         ->Join();
   }
 
@@ -5555,7 +5556,7 @@ void MinorMarkCompactCollector::UpdatePointersAfterEvacuation() {
     TRACE_GC(heap()->tracer(),
              GCTracer::Scope::MINOR_MC_EVACUATE_UPDATE_POINTERS_SLOTS);
     V8::GetCurrentPlatform()
-        ->PostJob(
+        ->CreateJob(
             v8::TaskPriority::kUserBlocking,
             std::make_unique<PointersUpdatingJob>(
                 isolate(), std::move(updating_items),
@@ -6054,10 +6055,10 @@ void MinorMarkCompactCollector::MarkRootSetInParallel(
       local_marking_worklists_->Publish();
       TRACE_GC(heap()->tracer(), GCTracer::Scope::MINOR_MC_MARK_ROOTS);
       V8::GetCurrentPlatform()
-          ->PostJob(v8::TaskPriority::kUserBlocking,
-                    std::make_unique<YoungGenerationMarkingJob>(
-                        isolate(), this, marking_worklists(),
-                        std::move(marking_items)))
+          ->CreateJob(v8::TaskPriority::kUserBlocking,
+                      std::make_unique<YoungGenerationMarkingJob>(
+                          isolate(), this, marking_worklists(),
+                          std::move(marking_items)))
           ->Join();
 
       DCHECK(local_marking_worklists_->IsEmpty());

src/heap/scavenger.cc

@@ -373,10 +373,10 @@ void ScavengerCollector::CollectGarbage() {
       // Parallel phase scavenging all copied and promoted objects.
       TRACE_GC(heap_->tracer(), GCTracer::Scope::SCAVENGER_SCAVENGE_PARALLEL);
       V8::GetCurrentPlatform()
-          ->PostJob(v8::TaskPriority::kUserBlocking,
-                    std::make_unique<JobTask>(this, &scavengers,
-                                              std::move(memory_chunks),
-                                              &copied_list, &promotion_list))
+          ->CreateJob(v8::TaskPriority::kUserBlocking,
+                      std::make_unique<JobTask>(this, &scavengers,
+                                                std::move(memory_chunks),
+                                                &copied_list, &promotion_list))
           ->Join();
       DCHECK(copied_list.IsEmpty());
       DCHECK(promotion_list.IsEmpty());