[Task Scheduler]: Merge blocking and non-blocking pools.

This CL creates a feature experiment that removes blocking worker pools
from the task scheduler. Tasks with MayBlock trait are instead posted to
the usual foreground/background pool, giving us better control over scheduling.

Note: ScopedBlockingCall is used to add extra workers to the pool when a task is
blocking. In this CL, we take advantage of this behavior to make sure enough
workers are running on the CPU.

Bug: 874080
Change-Id: I7889a941f82203388c259b0d96073eb5ddf3de69
Reviewed-on: https://chromium-review.googlesource.com/c/1249836
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600401}

base/task/task_scheduler/task_scheduler_impl.cc

@@ -11,6 +11,7 @@
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/compiler_specific.h"
+#include "base/feature_list.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/stl_util.h"
@@ -28,6 +29,27 @@
 namespace base {
 namespace internal {
 
+namespace {
+
+// Returns worker pool EnvironmentType for given arguments |is_background| and
+// |is_blocking|.
+EnvironmentType GetEnvironmentIndex(bool is_background, bool is_blocking) {
+  if (is_background) {
+    if (is_blocking)
+      return BACKGROUND_BLOCKING;
+    return BACKGROUND;
+  }
+
+  if (is_blocking)
+    return FOREGROUND_BLOCKING;
+  return FOREGROUND;
+}
+
+}  // namespace
+
+const base::Feature kMergeBlockingNonBlockingPools = {
+    "MergeBlockingNonBlockingPools", base::FEATURE_DISABLED_BY_DEFAULT};
+
 TaskSchedulerImpl::TaskSchedulerImpl(StringPiece histogram_label)
     : TaskSchedulerImpl(histogram_label,
                         std::make_unique<TaskTrackerImpl>(histogram_label)) {}
@@ -65,22 +87,20 @@ TaskSchedulerImpl::TaskSchedulerImpl(
         task_tracker_->GetTrackedRef(), &delayed_task_manager_));
   }
 
-  // Map environment indexes to pools.
-  environment_to_worker_pool_[FOREGROUND] = worker_pools_[FOREGROUND].get();
+  // Map environment indexes to pools. |kMergeBlockingNonBlockingPools| is
+  // assumed to be disabled.
+  environment_to_worker_pool_[FOREGROUND] =
+      worker_pools_[GetEnvironmentIndex(false, false)].get();
   environment_to_worker_pool_[FOREGROUND_BLOCKING] =
-      worker_pools_[FOREGROUND_BLOCKING].get();
-
-  if (CanUseBackgroundPriorityForSchedulerWorker()) {
-    environment_to_worker_pool_[BACKGROUND] = worker_pools_[BACKGROUND].get();
-    environment_to_worker_pool_[BACKGROUND_BLOCKING] =
-        worker_pools_[BACKGROUND_BLOCKING].get();
-  } else {
-    // On platforms without background thread priority, tasks posted to the
-    // background environment are run by foreground pools.
-    environment_to_worker_pool_[BACKGROUND] = worker_pools_[FOREGROUND].get();
-    environment_to_worker_pool_[BACKGROUND_BLOCKING] =
-        worker_pools_[FOREGROUND_BLOCKING].get();
-  }
+      worker_pools_[GetEnvironmentIndex(false, true)].get();
+  environment_to_worker_pool_[BACKGROUND] =
+      worker_pools_[GetEnvironmentIndex(
+                        CanUseBackgroundPriorityForSchedulerWorker(), false)]
+          .get();
+  environment_to_worker_pool_[BACKGROUND_BLOCKING] =
+      worker_pools_[GetEnvironmentIndex(
+                        CanUseBackgroundPriorityForSchedulerWorker(), true)]
+          .get();
 }
 
 TaskSchedulerImpl::~TaskSchedulerImpl() {
@@ -99,6 +119,26 @@ void TaskSchedulerImpl::Start(
     all_tasks_user_blocking_.Set();
   }
 
+  const bool use_blocking_pools =
+      !base::FeatureList::IsEnabled(kMergeBlockingNonBlockingPools);
+
+  // Remap environment indexes to pools with |use_blocking_pools|.
+  // TODO(etiennep): This is only necessary because of the kMergeBlockingNonBlockingPools
+  // experiment. Remove this after the experiment.
+  environment_to_worker_pool_[FOREGROUND] =
+      worker_pools_[GetEnvironmentIndex(false, false)].get();
+  environment_to_worker_pool_[FOREGROUND_BLOCKING] =
+      worker_pools_[GetEnvironmentIndex(false, use_blocking_pools)].get();
+  environment_to_worker_pool_[BACKGROUND] =
+      worker_pools_[GetEnvironmentIndex(
+                        CanUseBackgroundPriorityForSchedulerWorker(), false)]
+          .get();
+  environment_to_worker_pool_[BACKGROUND_BLOCKING] =
+      worker_pools_[GetEnvironmentIndex(
+                        CanUseBackgroundPriorityForSchedulerWorker(),
+                        use_blocking_pools)]
+          .get();
+
   // Start the service thread. On platforms that support it (POSIX except NaCL
   // SFI), the service thread runs a MessageLoopForIO which is used to support
   // FileDescriptorWatcher in the scope in which tasks run.
@@ -153,10 +193,10 @@ void TaskSchedulerImpl::Start(
       max_best_effort_tasks_in_foreground_pool, service_thread_task_runner,
       scheduler_worker_observer, worker_environment);
   const int max_best_effort_tasks_in_foreground_blocking_pool = std::max(
-      1,
-      std::min(
-          init_params.background_blocking_worker_pool_params.max_tasks(),
-          init_params.foreground_blocking_worker_pool_params.max_tasks() / 2));
+      1, std::min(
+             init_params.background_blocking_worker_pool_params.max_tasks(),
+             init_params.foreground_blocking_worker_pool_params.max_tasks() /
+                 2));
   worker_pools_[FOREGROUND_BLOCKING]->Start(
       init_params.foreground_blocking_worker_pool_params,
       max_best_effort_tasks_in_foreground_blocking_pool,

base/task/task_scheduler/task_scheduler_impl.h

@@ -38,9 +38,12 @@ namespace base {
 
 class HistogramBase;
 class Thread;
+struct Feature;
 
 namespace internal {
 
+extern const BASE_EXPORT base::Feature kMergeBlockingNonBlockingPools;
+
 // Default TaskScheduler implementation. This class is thread-safe.
 class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
  public:

testing/variations/fieldtrial_testing_config.json

@@ -2549,6 +2549,25 @@
             ]
         }
     ],
+    "MergeBlockingNonBlockingPools": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "MergeBlockingNonBlockingPools"
+                    ]
+                }
+            ]
+        }
+    ],
     "ModernMediaControls": [
         {
             "platforms": [