[SPARK-25502]Empty Page when page number exceeds the reatinedTask size.

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -117,7 +117,9 @@ private[ui] class StagePage(parent: StagesTab, store: AppStatusStore) extends We
 
     val localitySummary = store.localitySummary(stageData.stageId, stageData.attemptId)
 
-    val totalTasks = taskCount(stageData)
+    val totalTasks = stageData.numActiveTasks + stageData.numCompleteTasks +
+      stageData.numFailedTasks + stageData.numKilledTasks
+
     if (totalTasks == 0) {
       val content =
         <div>
@@ -685,7 +687,7 @@ private[ui] class TaskDataSource(
 
   private var _tasksToShow: Seq[TaskData] = null
 
-  override def dataSize: Int = taskCount(stage)
+  override def dataSize: Int = store.taskCount(stage.stageId, stage.attemptId).toInt
 
   override def sliceData(from: Int, to: Int): Seq[TaskData] = {
     if (_tasksToShow == null) {
@@ -1051,9 +1053,4 @@ private[ui] object ApiHelper {
     (stage.map(_.name).getOrElse(""), stage.flatMap(_.description).getOrElse(job.name))
   }
 
-  def taskCount(stageData: StageData): Int = {
-    stageData.numActiveTasks + stageData.numCompleteTasks + stageData.numFailedTasks +
-      stageData.numKilledTasks
-  }
-
 }