[ThreadPool]: Add kFixedMaxBestEffortTasks

Feature to disable max_best_effort_tasks increases with ScopedBlockingCall.
This feature goes along with MayBlockWithoutDelay.
It was made a separate feature to enable multi-arm and detect possible
regressions from deadlock if all BEST_EFFORT tasks are blocked, although this
should not happen in practice.

Bug: 1026785, 1059767
Change-Id: I3a0cb71f0fec3b0dbe812b11084f4d391cba1c69
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2116649Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753629}

base/task/task_features.cc

@@ -17,6 +17,9 @@ const Feature kNoDetachBelowInitialCapacity = {
 const Feature kMayBlockWithoutDelay = {"MayBlockWithoutDelay",
                                        base::FEATURE_DISABLED_BY_DEFAULT};
 
+const Feature kFixedMaxBestEffortTasks = {"FixedMaxBestEffortTasks",
+                                          base::FEATURE_DISABLED_BY_DEFAULT};
+
 #if defined(OS_WIN) || defined(OS_MACOSX)
 const Feature kUseNativeThreadPool = {"UseNativeThreadPool",
                                       base::FEATURE_DISABLED_BY_DEFAULT};

base/task/task_features.h

@@ -23,6 +23,12 @@ extern const BASE_EXPORT Feature kNoDetachBelowInitialCapacity;
 // instead of waiting for a threshold in the foreground thread group.
 extern const BASE_EXPORT Feature kMayBlockWithoutDelay;
 
+// Under this feature, best effort capacity is never increased.
+// While it's unlikely we'd ship this as-is, this experiment allows us to
+// determine whether blocked worker replacement logic on best-effort tasks has
+// any impact on guardian metrics.
+extern const BASE_EXPORT Feature kFixedMaxBestEffortTasks;
+
 #if defined(OS_WIN) || defined(OS_MACOSX)
 #define HAS_NATIVE_THREAD_POOL() 1
 #else

base/task/thread_pool/thread_group_impl.cc

@@ -417,8 +417,9 @@ void ThreadGroupImpl::Start(
   DCHECK(!replacement_thread_group_);
 
   in_start().may_block_without_delay =
-      FeatureList::IsEnabled(kMayBlockWithoutDelay) &&
-      priority_hint_ == ThreadPriority::NORMAL;
+      FeatureList::IsEnabled(kMayBlockWithoutDelay);
+  in_start().fixed_max_best_effort_tasks =
+      FeatureList::IsEnabled(kFixedMaxBestEffortTasks);
   in_start().may_block_threshold =
       may_block_threshold ? may_block_threshold.value()
                           : (priority_hint_ == ThreadPriority::NORMAL
@@ -1217,16 +1218,20 @@ void ThreadGroupImpl::DecrementMaxTasksLockRequired(TaskPriority priority) {
   DCHECK_GT(num_running_tasks_, 0U);
   DCHECK_GT(max_tasks_, 0U);
   --max_tasks_;
-  if (priority == TaskPriority::BEST_EFFORT)
+  if (priority == TaskPriority::BEST_EFFORT &&
+      !after_start().fixed_max_best_effort_tasks) {
     --max_best_effort_tasks_;
+  }
   UpdateMinAllowedPriorityLockRequired();
 }
 
 void ThreadGroupImpl::IncrementMaxTasksLockRequired(TaskPriority priority) {
   DCHECK_GT(num_running_tasks_, 0U);
   ++max_tasks_;
-  if (priority == TaskPriority::BEST_EFFORT)
+  if (priority == TaskPriority::BEST_EFFORT &&
+      !after_start().fixed_max_best_effort_tasks) {
     ++max_best_effort_tasks_;
+  }
   UpdateMinAllowedPriorityLockRequired();
 }
 

base/task/thread_pool/thread_group_impl.h

@@ -242,6 +242,7 @@ class BASE_EXPORT ThreadGroupImpl : public ThreadGroup {
     WorkerThreadObserver* worker_thread_observer = nullptr;
 
     bool may_block_without_delay;
+    bool fixed_max_best_effort_tasks;
 
     // Threshold after which the max tasks is increased to compensate for a
     // worker that is within a MAY_BLOCK ScopedBlockingCall.