Revert "[Base] Use background mode for ThreadPriority::BACKGROUND threads...

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

This reverts commit d88c3ec1.

Reason for revert: PlatformThreadTest.SetCurrentThreadPriorityWithThreadModeBackground is Flaky:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=base_unittests&tests=SetCurrentThreadPriorityWithThreadModeBackground

Original change's description:
> [Base] Use background mode for ThreadPriority::BACKGROUND threads (behind feature) (reland).
>
> Relanding a CL that was reverted because of a variable that was
> unused in non-DCHECK builds and caused a compile failure.
> Original CL: Reviewed-on: https://chromium-review.googlesource.com/1171482
>
> 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
>
> TBR=gab@chromium.org
>
> Bug: 872820
> Change-Id: I212fb77bb035088595b4944ce145a74d333c38eb
> Reviewed-on: https://chromium-review.googlesource.com/1185529
> Commit-Queue: François Doray <fdoray@chromium.org>
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#585272}

TBR=gab@chromium.org,fdoray@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 872820, 877628
Change-Id: I6cf6625fc1f01106e8fff1e206a6a256f3a6160a
Reviewed-on: https://chromium-review.googlesource.com/1188644
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585999}

base/BUILD.gn

@@ -939,7 +939,6 @@ 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

@@ -7,7 +7,6 @@
 #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"
@@ -16,7 +15,6 @@
 #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 {
@@ -290,7 +288,16 @@ class ThreadPriorityTestThread : public FunctionTestThread {
   DISALLOW_COPY_AND_ASSIGN(ThreadPriorityTestThread);
 };
 
-void TestSetCurrentThreadPriority() {
+}  // 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) {
   ThreadPriorityTestThread thread;
   PlatformThreadHandle handle;
 
@@ -304,30 +311,6 @@ void TestSetCurrentThreadPriority() {
   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_win.h"
+#include "base/threading/platform_thread.h"
 
 #include <stddef.h>
 
@@ -15,7 +15,6 @@
 #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>
 
@@ -23,14 +22,6 @@ 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;
@@ -153,11 +144,6 @@ 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();
@@ -277,22 +263,10 @@ bool PlatformThread::CanIncreaseCurrentThreadPriority() {
 
 // static
 void PlatformThread::SetCurrentThreadPriority(ThreadPriority priority) {
-  const bool use_thread_mode_background =
-      base::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 = use_thread_mode_background
-                             ? THREAD_MODE_BACKGROUND_BEGIN
-                             : THREAD_PRIORITY_LOWEST;
+      desired_priority = THREAD_PRIORITY_LOWEST;
       break;
     case ThreadPriority::NORMAL:
       desired_priority = THREAD_PRIORITY_NORMAL;
@@ -316,22 +290,13 @@ 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() {
-  const int priority =
+  int priority =
       ::GetThreadPriority(PlatformThread::CurrentHandle().platform_handle());
-
   switch (priority) {
-    case kWin7BackgroundThreadModePriority:
-      DCHECK_EQ(win::GetVersion(), win::VERSION_WIN7);
-      FALLTHROUGH;
-    case kWin8AboveBackgroundThreadModePriority:
     case THREAD_PRIORITY_LOWEST:
       return ThreadPriority::BACKGROUND;
     case THREAD_PRIORITY_NORMAL:
@@ -341,11 +306,12 @@ ThreadPriority PlatformThread::GetCurrentThreadPriority() {
     case THREAD_PRIORITY_TIME_CRITICAL:
       return ThreadPriority::REALTIME_AUDIO;
     case THREAD_PRIORITY_ERROR_RETURN:
-      PCHECK(false) << "GetThreadPriority error";
+      DPCHECK(false) << "GetThreadPriority error";
+      FALLTHROUGH;
+    default:
+      NOTREACHED() << "Unexpected priority: " << priority;
+      return ThreadPriority::NORMAL;
   }
-
-  NOTREACHED();
-  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_