[clangd] Use llvm::set_thread_priority in background-index

clangd/Threading.h

public:
   void runAsync(const llvm::Twine &Name, llvm::unique_function<void()> Action);
 
 private:
   mutable std::mutex Mutex;
   mutable std::condition_variable TasksReachedZero;
   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

clangd/Threading.cpp

void wait(std::unique_lock<std::mutex> &Lock, std::condition_variable &CV,
           Deadline D) {
   if (D == Deadline::zero())
     return;
   if (D == Deadline::infinity())
     return CV.wait(Lock);
   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

clangd/index/Background.h

public:
   // Cause background threads to stop after ther current task, any remaining
   // tasks will be discarded.
   void stop();
 
   // Wait until the queue is empty, to allow deterministic testing.
   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
   /// information on IndexStorage.
   void update(llvm::StringRef MainFile, IndexFileIn Index,
               const llvm::StringMap<FileDigest> &DigestsSnapshot,
               BackgroundIndexStorage *IndexStorage);
 
private:
   // Tries to load shards for the ChangedFiles.
   std::vector<std::pair<tooling::CompileCommand, BackgroundIndexStorage *>>
   loadShards(std::vector<std::string> ChangedFiles);
   void enqueue(tooling::CompileCommand Cmd, BackgroundIndexStorage *Storage);
 
   // 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;
 };
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/index/Background.cpp

 #include "index/SymbolCollector.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/ScopeExit.h"
 #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>
 #include <queue>
 #include <random>
 #include <string>
 #include <thread>
 
 namespace clang {
 namespace clangd {
 namespace {
+
+static std::atomic<bool> PreventStarvation = {false};
+
 // Resolves URI to file paths with cache.
 class URIToFileCache {
 public:
   URIToFileCache(llvm::StringRef HintPath) : HintPath(HintPath) {}
 
   llvm::StringRef resolve(llvm::StringRef FileURI) {
     auto I = URIToPathCache.try_emplace(FileURI);
     if (I.second) {
void BackgroundIndex::stop() {
   QueueCV.notify_all();
   IndexCV.notify_all();
 }
 
 void BackgroundIndex::run() {
   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(); });
       if (ShouldStop) {
         Queue.clear();
         QueueCV.notify_all();
         return;
       }
       ++NumActiveTasks;
       std::tie(Task, Priority) = std::move(Queue.front());
       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);
       assert(NumActiveTasks > 0 && "before decrementing");
       --NumActiveTasks;
     }
     QueueCV.notify_all();
   }
enqueueTask(
 
         auto NeedsReIndexing = loadShards(std::move(ChangedFiles));
         // Run indexing for files that need to be updated.
         std::shuffle(NeedsReIndexing.begin(), NeedsReIndexing.end(),
                      std::mt19937(std::random_device{}()));
         for (auto &Elem : NeedsReIndexing)
           enqueue(std::move(Elem.first), Elem.second);
       },
-      ThreadPriority::Normal);
+      llvm::ThreadPriority::Default);
 }
 
 void BackgroundIndex::enqueue(tooling::CompileCommand Cmd,
                               BackgroundIndexStorage *Storage) {
   enqueueTask(Bind(
                   [this, Storage](tooling::CompileCommand Cmd) {
                     // We can't use llvm::StringRef here since we are going to
                     // move from Cmd during the call below.
                     const std::string FileName = Cmd.Filename;
                     if (auto Error = index(std::move(Cmd), Storage))
                       elog("Indexing {0} failed: {1}", FileName,
                            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();
     // We first store the tasks with Normal priority in the front of the queue.
     // 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});
   }
   QueueCV.notify_all();
 }
 
 /// Given index results from a TU, only update symbols coming from files that
 /// are different or missing from than \p DigestsSnapshot. Also stores new index
BackgroundIndex::loadShards(std::vector<std::string> ChangedFiles) {
   vlog("Loaded all shards");
   reset(IndexedSymbols.buildIndex(IndexType::Heavy, DuplicateHandling::Merge));
   vlog("BackgroundIndex: built symbol index with estimated memory {0} "
        "bytes.",
        estimateMemoryUsage());
   return NeedsReIndexing;
 }
 
+void BackgroundIndex::preventThreadStarvationInTests() {
+  PreventStarvation.store(true);
+}
+
 } // namespace clangd
 } // namespace clang

clangd/tool/ClangdMain.cpp

 //===--- ClangdMain.cpp - clangd server loop ------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "ClangdLSPServer.h"
 #include "CodeComplete.h"
 #include "Features.inc"
 #include "Path.h"
 #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"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
llvm::cl::ParseCommandLineOptions(
       "For more information, see:"
       "\n\thttps://clang.llvm.org/extra/clangd.html"
       "\n\thttps://microsoft.github.io/language-server-protocol/");
   if (Test) {
     RunSynchronously = true;
     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(
         "test", "Test scheme for clangd lit tests.");
   }
 
   if (!RunSynchronously && WorkerThreadsCount == 0) {
     llvm::errs() << "A number of worker threads cannot be 0. Did you mean to "

unittests/clangd/BackgroundIndexTests.cpp

loadShard(llvm::StringRef ShardIdentifier) const override {
     return llvm::make_unique<IndexFileIn>(std::move(*IndexFile));
   }
 
   mutable llvm::StringSet<> AccessedPaths;
 };
 
 class BackgroundIndexTest : public ::testing::Test {
 protected:
-  BackgroundIndexTest() { preventThreadStarvationInTests(); }
+  BackgroundIndexTest() { BackgroundIndex::preventThreadStarvationInTests(); }
 };
 
 TEST_F(BackgroundIndexTest, NoCrashOnErrorFile) {
   MockFSProvider FS;
   FS.Files[testPath("root/A.cc")] = "error file";
   llvm::StringMap<std::string> Storage;
   size_t CacheHits = 0;
   MemoryShardStorage MSS(Storage, CacheHits);