Index: clangd/Threading.h
===================================================================
--- clangd/Threading.h
+++ clangd/Threading.h
@@ -117,16 +117,6 @@
   std::size_t InFlightTasks = 0;
 };
 
-enum class ThreadPriority {
-  Low = 0,
-  Normal = 1,
-};
-void setCurrentThreadPriority(ThreadPriority Priority);
-// Avoid the use of scheduler policies that may starve low-priority threads.
-// This prevents tests from timing out on loaded systems.
-// Affects subsequent setThreadPriority() calls.
-void preventThreadStarvationInTests();
-
 } // namespace clangd
 } // namespace clang
 #endif
Index: clangd/Threading.cpp
===================================================================
--- clangd/Threading.cpp
+++ clangd/Threading.cpp
@@ -113,33 +113,5 @@
   CV.wait_until(Lock, D.time());
 }
 
-static std::atomic<bool> AvoidThreadStarvation = {false};
-
-void setCurrentThreadPriority(ThreadPriority Priority) {
-  // Some *really* old glibcs are missing SCHED_IDLE.
-#if defined(__linux__) && defined(SCHED_IDLE)
-  sched_param priority;
-  priority.sched_priority = 0;
-  pthread_setschedparam(
-      pthread_self(),
-      Priority == ThreadPriority::Low && !AvoidThreadStarvation ? SCHED_IDLE
-                                                                : SCHED_OTHER,
-      &priority);
-#elif defined(__APPLE__)
-  // https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/getpriority.2.html
-  setpriority(PRIO_DARWIN_THREAD, 0,
-              Priority == ThreadPriority::Low && !AvoidThreadStarvation
-                  ? PRIO_DARWIN_BG
-                  : 0);
-#elif defined(_WIN32)
-  SetThreadPriority(GetCurrentThread(),
-                    Priority == ThreadPriority::Low && !AvoidThreadStarvation
-                        ? THREAD_MODE_BACKGROUND_BEGIN
-                        : THREAD_MODE_BACKGROUND_END);
-#endif
-}
-
-void preventThreadStarvationInTests() { AvoidThreadStarvation = true; }
-
 } // namespace clangd
 } // namespace clang
Index: clangd/index/Background.h
===================================================================
--- clangd/index/Background.h
+++ clangd/index/Background.h
@@ -88,6 +88,10 @@
   LLVM_NODISCARD bool
   blockUntilIdleForTest(llvm::Optional<double> TimeoutSeconds = 10);
 
+  // Disables thread priority lowering in background index to make sure it can
+  // progress on loaded systems. Only affects tasks that run after the call.
+  static void preventThreadStarvationInTests();
+
 private:
   /// Given index results from a TU, only update symbols coming from files with
   /// different digests than \p DigestsSnapshot. Also stores new index
@@ -134,14 +138,14 @@
   // queue management
   using Task = std::function<void()>;
   void run(); // Main loop executed by Thread. Runs tasks from Queue.
-  void enqueueTask(Task T, ThreadPriority Prioirty);
+  void enqueueTask(Task T, llvm::ThreadPriority Prioirty);
   void enqueueLocked(tooling::CompileCommand Cmd,
                      BackgroundIndexStorage *IndexStorage);
   std::mutex QueueMu;
   unsigned NumActiveTasks = 0; // Only idle when queue is empty *and* no tasks.
   std::condition_variable QueueCV;
   bool ShouldStop = false;
-  std::deque<std::pair<Task, ThreadPriority>> Queue;
+  std::deque<std::pair<Task, llvm::ThreadPriority>> Queue;
   std::vector<std::thread> ThreadPool; // FIXME: Abstract this away.
   GlobalCompilationDatabase::CommandChanged::Subscription CommandsChanged;
 };
Index: clangd/index/Background.cpp
===================================================================
--- clangd/index/Background.cpp
+++ clangd/index/Background.cpp
@@ -26,7 +26,9 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/SHA1.h"
+#include "llvm/Support/Threading.h"
 
+#include <atomic>
 #include <chrono>
 #include <memory>
 #include <numeric>
@@ -38,6 +40,9 @@
 namespace clang {
 namespace clangd {
 namespace {
+
+static std::atomic<bool> PreventStarvation = {false};
+
 // Resolves URI to file paths with cache.
 class URIToFileCache {
 public:
@@ -172,7 +177,7 @@
   WithContext Background(BackgroundContext.clone());
   while (true) {
     llvm::Optional<Task> Task;
-    ThreadPriority Priority;
+    llvm::ThreadPriority Priority;
     {
       std::unique_lock<std::mutex> Lock(QueueMu);
       QueueCV.wait(Lock, [&] { return ShouldStop || !Queue.empty(); });
@@ -186,11 +191,11 @@
       Queue.pop_front();
     }
 
-    if (Priority != ThreadPriority::Normal)
-      setCurrentThreadPriority(Priority);
+    if (Priority != llvm::ThreadPriority::Default && !PreventStarvation.load())
+      llvm::set_thread_priority(Priority);
     (*Task)();
-    if (Priority != ThreadPriority::Normal)
-      setCurrentThreadPriority(ThreadPriority::Normal);
+    if (Priority != llvm::ThreadPriority::Default)
+      llvm::set_thread_priority(llvm::ThreadPriority::Default);
 
     {
       std::unique_lock<std::mutex> Lock(QueueMu);
@@ -223,7 +228,7 @@
         for (auto &Elem : NeedsReIndexing)
           enqueue(std::move(Elem.first), Elem.second);
       },
-      ThreadPriority::Normal);
+      llvm::ThreadPriority::Default);
 }
 
 void BackgroundIndex::enqueue(tooling::CompileCommand Cmd,
@@ -238,10 +243,10 @@
                            std::move(Error));
                   },
                   std::move(Cmd)),
-              ThreadPriority::Low);
+              llvm::ThreadPriority::Background);
 }
 
-void BackgroundIndex::enqueueTask(Task T, ThreadPriority Priority) {
+void BackgroundIndex::enqueueTask(Task T, llvm::ThreadPriority Priority) {
   {
     std::lock_guard<std::mutex> Lock(QueueMu);
     auto I = Queue.end();
@@ -249,10 +254,11 @@
     // Then we store low priority tasks. Normal priority tasks are pretty rare,
     // they should not grow beyond single-digit numbers, so it is OK to do
     // linear search and insert after that.
-    if (Priority == ThreadPriority::Normal) {
-      I = llvm::find_if(Queue, [](const std::pair<Task, ThreadPriority> &Elem) {
-        return Elem.second == ThreadPriority::Low;
-      });
+    if (Priority == llvm::ThreadPriority::Default) {
+      I = llvm::find_if(
+          Queue, [](const std::pair<Task, llvm::ThreadPriority> &Elem) {
+            return Elem.second == llvm::ThreadPriority::Background;
+          });
     }
     Queue.insert(I, {std::move(T), Priority});
   }
@@ -611,5 +617,9 @@
   return NeedsReIndexing;
 }
 
+void BackgroundIndex::preventThreadStarvationInTests() {
+  PreventStarvation.store(true);
+}
+
 } // namespace clangd
 } // namespace clang
Index: clangd/tool/ClangdMain.cpp
===================================================================
--- clangd/tool/ClangdMain.cpp
+++ clangd/tool/ClangdMain.cpp
@@ -13,6 +13,7 @@
 #include "Protocol.h"
 #include "Trace.h"
 #include "Transport.h"
+#include "index/Background.h"
 #include "index/Serialization.h"
 #include "clang/Basic/Version.h"
 #include "llvm/ADT/Optional.h"
@@ -332,7 +333,8 @@
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;
     PrettyPrint = true;
-    preventThreadStarvationInTests(); // Ensure background index makes progress.
+    // Ensure background index makes progress.
+    BackgroundIndex::preventThreadStarvationInTests();
   }
   if (Test || EnableTestScheme) {
     static URISchemeRegistry::Add<TestScheme> X(
Index: unittests/clangd/BackgroundIndexTests.cpp
===================================================================
--- unittests/clangd/BackgroundIndexTests.cpp
+++ unittests/clangd/BackgroundIndexTests.cpp
@@ -70,7 +70,7 @@
 
 class BackgroundIndexTest : public ::testing::Test {
 protected:
-  BackgroundIndexTest() { preventThreadStarvationInTests(); }
+  BackgroundIndexTest() { BackgroundIndex::preventThreadStarvationInTests(); }
 };
 
 TEST_F(BackgroundIndexTest, NoCrashOnErrorFile) {