[Base] Use background mode for ThreadPriority::BACKGROUND threads (behind feature) (reland).

The previous attempt to land this CL was reverted because an early call
to PlatformThread::SetCurrentThreadPriority() in headless caused the
FeatureList to be accessed before it was initialized. This CL avoids
accessing an uninitialized FeatureList if the target priority is not
BACKGROUND.

Diff between ps 1 and 5 shows changes made since revert.

Previous attempts to land:
  https://chromium-review.googlesource.com/1171482
  https://chromium-review.googlesource.com/1185529
  https://chromium-review.googlesource.com/1190842

NOTE: If this causes new test failures, we'll add the value returned
by GetThreadPriority() to the switch/case of allowed values.

This CL adds a feature to use THREAD_MODE_BACKGROUND_BEGIN instead of
THREAD_PRIORITY_LOWEST for ThreadPriority::BACKGROUND threads. This
lowers the disk and network I/O priority of the thread in addition to
the CPU scheduling priority. MSDN recommends using this setting for
threads that perform background work.
https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-processthreadsapi-setthreadpriority

Bug: 872820
Change-Id: I2c9f17390eaaba2f8f454faa29836bbc37e3d47c
Reviewed-on: https://chromium-review.googlesource.com/1195677
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587349}

base/BUILD.gn

@@ -939,6 +939,7 @@ jumbo_component("base") {
     "threading/platform_thread_linux.cc",
     "threading/platform_thread_mac.mm",
     "threading/platform_thread_win.cc",
+    "threading/platform_thread_win.h",
     "threading/post_task_and_reply_impl.cc",
     "threading/post_task_and_reply_impl.h",
     "threading/scoped_blocking_call.cc",

base/threading/platform_thread_unittest.cc

@@ -7,6 +7,7 @@
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/test/scoped_feature_list.h"
 #include "base/threading/platform_thread.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -15,6 +16,7 @@
 #include "base/threading/platform_thread_internal_posix.h"
 #elif defined(OS_WIN)
 #include <windows.h>
+#include "base/threading/platform_thread_win.h"
 #endif
 
 namespace base {
@@ -288,16 +290,7 @@ class ThreadPriorityTestThread : public FunctionTestThread {
   DISALLOW_COPY_AND_ASSIGN(ThreadPriorityTestThread);
 };
 
-}  // namespace
-// Test changing a created thread's priority (which has different semantics on
-// some platforms).
-#if defined(OS_FUCHSIA)
-// TODO(crbug.com/851759): Thread priorities are not implemented in Fuchsia.
-#define MAYBE_SetCurrentThreadPriority DISABLED_SetCurrentThreadPriority
-#else
-#define MAYBE_SetCurrentThreadPriority SetCurrentThreadPriority
-#endif
-TEST(PlatformThreadTest, MAYBE_SetCurrentThreadPriority) {
+void TestSetCurrentThreadPriority() {
   ThreadPriorityTestThread thread;
   PlatformThreadHandle handle;
 
@@ -311,6 +304,30 @@ TEST(PlatformThreadTest, MAYBE_SetCurrentThreadPriority) {
   ASSERT_FALSE(thread.IsRunning());
 }
 
+}  // namespace
+
+// Test changing a created thread's priority.
+#if defined(OS_FUCHSIA)
+// TODO(crbug.com/851759): Thread priorities are not implemented in Fuchsia.
+#define MAYBE_SetCurrentThreadPriority DISABLED_SetCurrentThreadPriority
+#else
+#define MAYBE_SetCurrentThreadPriority SetCurrentThreadPriority
+#endif
+TEST(PlatformThreadTest, MAYBE_SetCurrentThreadPriority) {
+  TestSetCurrentThreadPriority();
+}
+
+#if defined(OS_WIN)
+// Test changing a created thread's priority, with the
+// kWindowsThreadModeBackground feature enabled.
+TEST(PlatformThreadTest, SetCurrentThreadPriorityWithThreadModeBackground) {
+  test::ScopedFeatureList scoped_feature_list;
+  scoped_feature_list.InitAndEnableFeature(
+      features::kWindowsThreadModeBackground);
+  TestSetCurrentThreadPriority();
+}
+#endif  // defined(OS_WIN)
+
 // This tests internal PlatformThread APIs used under some POSIX platforms,
 // with the exception of Mac OS X, iOS and Fuchsia.
 #if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_IOS) && \

base/threading/platform_thread_win.cc

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/threading/platform_thread.h"
+#include "base/threading/platform_thread_win.h"
 
 #include <stddef.h>
 
@@ -15,6 +15,7 @@
 #include "base/threading/thread_id_name_manager.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/win/scoped_handle.h"
+#include "base/win/windows_version.h"
 
 #include <windows.h>
 
@@ -22,6 +23,14 @@ namespace base {
 
 namespace {
 
+// The value returned by ::GetThreadPriority() after background thread mode is
+// enabled on Windows 7.
+constexpr int kWin7BackgroundThreadModePriority = 4;
+
+// The value returned by ::GetThreadPriority() after background thread mode is
+// enabled on Windows 8+.
+constexpr int kWin8AboveBackgroundThreadModePriority = -4;
+
 // The information on how to set the thread name comes from
 // a MSDN article: http://msdn2.microsoft.com/en-us/library/xcb2z8hs.aspx
 const DWORD kVCThreadNameException = 0x406D1388;
@@ -144,6 +153,11 @@ bool CreateThreadInternal(size_t stack_size,
 
 }  // namespace
 
+namespace features {
+const Feature kWindowsThreadModeBackground{"WindowsThreadModeBackground",
+                                           FEATURE_DISABLED_BY_DEFAULT};
+}  // namespace features
+
 // static
 PlatformThreadId PlatformThread::CurrentId() {
   return ::GetCurrentThreadId();
@@ -263,10 +277,32 @@ bool PlatformThread::CanIncreaseCurrentThreadPriority() {
 
 // static
 void PlatformThread::SetCurrentThreadPriority(ThreadPriority priority) {
+  // A DCHECK is triggered on FeatureList initialization if the state of a
+  // feature has been checked before. We only want to trigger that DCHECK if the
+  // priority has been set to BACKGROUND before, so we are careful not to access
+  // the state of the feature needlessly. We don't DCHECK here because it is ok
+  // if the FeatureList is never initialized in the process (e.g. in tests).
+  //
+  // TODO(fdoray): Remove experiment code. https://crbug.com/872820
+  const bool use_thread_mode_background =
+      (priority == ThreadPriority::BACKGROUND
+           ? FeatureList::IsEnabled(features::kWindowsThreadModeBackground)
+           : (FeatureList::GetInstance() &&
+              FeatureList::IsEnabled(features::kWindowsThreadModeBackground)));
+
+  if (use_thread_mode_background && priority != ThreadPriority::BACKGROUND) {
+    // Exit background mode if the new priority is not BACKGROUND. This is a
+    // no-op if not in background mode.
+    ::SetThreadPriority(PlatformThread::CurrentHandle().platform_handle(),
+                        THREAD_MODE_BACKGROUND_END);
+  }
+
   int desired_priority = THREAD_PRIORITY_ERROR_RETURN;
   switch (priority) {
     case ThreadPriority::BACKGROUND:
-      desired_priority = THREAD_PRIORITY_LOWEST;
+      desired_priority = use_thread_mode_background
+                             ? THREAD_MODE_BACKGROUND_BEGIN
+                             : THREAD_PRIORITY_LOWEST;
       break;
     case ThreadPriority::NORMAL:
       desired_priority = THREAD_PRIORITY_NORMAL;
@@ -290,13 +326,23 @@ void PlatformThread::SetCurrentThreadPriority(ThreadPriority priority) {
                           desired_priority);
   DPLOG_IF(ERROR, !success) << "Failed to set thread priority to "
                             << desired_priority;
+
+  // Sanity check that GetCurrentThreadPriority() is consistent with
+  // SetCurrentThreadPriority().
+  DCHECK_EQ(GetCurrentThreadPriority(), priority);
 }
 
 // static
 ThreadPriority PlatformThread::GetCurrentThreadPriority() {
-  int priority =
+  const int priority =
       ::GetThreadPriority(PlatformThread::CurrentHandle().platform_handle());
+
   switch (priority) {
+    case THREAD_PRIORITY_IDLE:
+    case kWin7BackgroundThreadModePriority:
+      DCHECK_EQ(win::GetVersion(), win::VERSION_WIN7);
+      FALLTHROUGH;
+    case kWin8AboveBackgroundThreadModePriority:
     case THREAD_PRIORITY_LOWEST:
       return ThreadPriority::BACKGROUND;
     case THREAD_PRIORITY_NORMAL:
@@ -307,11 +353,10 @@ ThreadPriority PlatformThread::GetCurrentThreadPriority() {
       return ThreadPriority::REALTIME_AUDIO;
     case THREAD_PRIORITY_ERROR_RETURN:
       DPCHECK(false) << "GetThreadPriority error";
-      FALLTHROUGH;
-    default:
-      NOTREACHED() << "Unexpected priority: " << priority;
-      return ThreadPriority::NORMAL;
   }
+
+  NOTREACHED() << "GetCurrentThreadPriority returned " << priority << ".";
+  return ThreadPriority::NORMAL;
 }
 
 }  // namespace base

base/threading/platform_thread_win.h

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_THREADING_PLATFORM_THREAD_WIN_H_
+#define BASE_THREADING_PLATFORM_THREAD_WIN_H_
+
+#include "base/threading/platform_thread.h"
+
+#include "base/base_export.h"
+#include "base/feature_list.h"
+
+namespace base {
+namespace features {
+
+// Use THREAD_MODE_BACKGROUND_BEGIN instead of THREAD_PRIORITY_LOWEST for
+// ThreadPriority::BACKGROUND threads. This lowers the disk and network I/O
+// priority of the thread in addition to the CPU scheduling priority. MSDN
+// recommends using this setting for threads that perform background work.
+// https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-processthreadsapi-setthreadpriority
+BASE_EXPORT extern const Feature kWindowsThreadModeBackground;
+
+}  // namespace features
+}  // namespace base
+
+#endif  // BASE_THREADING_PLATFORM_THREAD_WIN_H_