TaskScheduler: Remove TaskScheduler.BlockShutdownTasksPostedDuringShutdown.

The histogram shows that 99.9% of the time, less than
1000 BLOCK_SHUTDOWN tasks are posted during shutdown. This CL replaces
the histogram with a crash when the number is to high, which will be
more actionnable.

Note: Current histogram data shows that this crash will happen
less than 0.1% of the time.

Bug: 697731
Change-Id: I553caf93b0109a1bd44c5c841d0f557e6cad16cd
Reviewed-on: https://chromium-review.googlesource.com/c/1280670Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600892}

base/task/task_scheduler/task_tracker.cc

@@ -133,17 +133,11 @@ HistogramBase* GetHistogramForTaskTraits(
                         : 0];
 }
 
-// Upper bound for the
-// TaskScheduler.BlockShutdownTasksPostedDuringShutdown histogram.
-constexpr HistogramBase::Sample kMaxBlockShutdownTasksPostedDuringShutdown =
-    1000;
-
-void RecordNumBlockShutdownTasksPostedDuringShutdown(
-    HistogramBase::Sample value) {
-  UMA_HISTOGRAM_CUSTOM_COUNTS(
-      "TaskScheduler.BlockShutdownTasksPostedDuringShutdown", value, 1,
-      kMaxBlockShutdownTasksPostedDuringShutdown, 50);
-}
+// Maximum number of BLOCK_SHUTDOWN tasks that can be posted during shutdown. If
+// that many BLOCK_SHUTDOWN tasks are posted during shutdown, it is possible
+// that buggy code is posting an infinite number of tasks and that shutdown will
+// never complete. The mitigation is to induce a crash.
+constexpr int kMaxBlockShutdownTasksPostedDuringShutdown = 1000;
 
 // Returns the maximum number of TaskPriority::BEST_EFFORT sequences that can be
 // scheduled concurrently based on command line flags.
@@ -707,20 +701,6 @@ void TaskTracker::PerformShutdown() {
     base::ThreadRestrictions::ScopedAllowWait allow_wait;
     shutdown_event_->Wait();
   }
-
-  {
-    AutoSchedulerLock auto_lock(shutdown_lock_);
-
-    // Record TaskScheduler.BlockShutdownTasksPostedDuringShutdown if less than
-    // |kMaxBlockShutdownTasksPostedDuringShutdown| BLOCK_SHUTDOWN tasks were
-    // posted during shutdown. Otherwise, the histogram has already been
-    // recorded in BeforePostTask().
-    if (num_block_shutdown_tasks_posted_during_shutdown_ <
-        kMaxBlockShutdownTasksPostedDuringShutdown) {
-      RecordNumBlockShutdownTasksPostedDuringShutdown(
-          num_block_shutdown_tasks_posted_during_shutdown_);
-    }
-  }
 }
 
 void TaskTracker::SetMaxNumScheduledSequences(int max_scheduled_sequences,
@@ -819,16 +799,8 @@ bool TaskTracker::BeforePostTask(
       }
 
       ++num_block_shutdown_tasks_posted_during_shutdown_;
-
-      if (num_block_shutdown_tasks_posted_during_shutdown_ ==
-          kMaxBlockShutdownTasksPostedDuringShutdown) {
-        // Record the TaskScheduler.BlockShutdownTasksPostedDuringShutdown
-        // histogram as soon as its upper bound is hit. That way, a value will
-        // be recorded even if an infinite number of BLOCK_SHUTDOWN tasks are
-        // posted, preventing shutdown to complete.
-        RecordNumBlockShutdownTasksPostedDuringShutdown(
-            num_block_shutdown_tasks_posted_during_shutdown_);
-      }
+      CHECK_LT(num_block_shutdown_tasks_posted_during_shutdown_,
+               kMaxBlockShutdownTasksPostedDuringShutdown);
     }
 
     return true;

tools/metrics/histograms/histograms.xml

@@ -109987,6 +109987,12 @@ uploading your change for review.
 
 <histogram name="TaskScheduler.BlockShutdownTasksPostedDuringShutdown"
     units="tasks">
+  <obsolete>
+    Deprecated 10/2018. Instead of recording a histogram with the number of
+    BLOCK_SHUTDOWN tasks posted during shutdown, we generate a crash when too
+    many BLOCK_SHUTDOWN tasks are posted during shutdown. A crash dump is more
+    actionnable than a histogram.
+  </obsolete>
   <owner>fdoray@chromium.org</owner>
   <summary>
     Number of BLOCK_SHUTDOWN tasks that were posted to a base::TaskScheduler