[CPU] Fix macOS/Apple Silicon hang by enabling OpenMP in the build (#46769)

Signed-off-by: mgoin <mgoin64@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/actionlint.yaml

@@ -3,3 +3,5 @@
 self-hosted-runner:
   labels:
     - vllm-runners
+    # Not yet in actionlint's known-label set.
+    - macos-26

.github/workflows/macos-smoke-test.yml

@@ -11,7 +11,19 @@ permissions:
 
 jobs:
   macos-m1-smoke-test:
-    runs-on: macos-latest
+    # macos-26 (the supported target) is still a preview runner, so gate on GA
+    # macos-15 and keep macos-26 non-blocking.
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: macos-15
+            required: true
+          - os: macos-26
+            required: false
+    name: macos-m1-smoke-test (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ !matrix.required }}
     timeout-minutes: 30
 
     steps:
@@ -72,14 +84,11 @@ jobs:
           # Test health endpoint
           curl -f http://localhost:8000/health
 
-          # Test completion
-          curl -f http://localhost:8000/v1/completions \
+          # Long prompt: hits the split-KV path that short prompts skip (#46769).
+          PAYLOAD=$(python -c "import json; print(json.dumps({'model': 'Qwen/Qwen3-0.6B', 'prompt': 'The quick brown fox jumps over the lazy dog. ' * 24, 'max_tokens': 16}))")
+          curl -f --max-time 120 http://localhost:8000/v1/completions \
             -H "Content-Type: application/json" \
-            -d '{
-              "model": "Qwen/Qwen3-0.6B",
-              "prompt": "Hello",
-              "max_tokens": 5
-            }'
+            -d "$PAYLOAD"
 
           # Cleanup
           kill "$SERVER_PID"

cmake/cpu_extension.cmake

@@ -24,7 +24,10 @@ set (ENABLE_NUMA TRUE)
 # Check the compile flags
 #
 if(MACOSX_FOUND)
+    # Apple clang needs -Xpreprocessor to enable OpenMP. No runtime link is
+    # needed: _C is a dynamic_lookup bundle and resolves libomp from torch.
     list(APPEND CXX_COMPILE_FLAGS
+        "-Xpreprocessor" "-fopenmp"
         "-DVLLM_CPU_EXTENSION")
 else()
     list(APPEND CXX_COMPILE_FLAGS

csrc/cpu/cpu_attn_impl.hpp

@@ -124,7 +124,7 @@ struct AttentionMetadata {
         workitem_group_num(workitem_group_num),
         reduction_item_num(reduction_item_num),
         reduction_split_num(reduction_split_num),
-        thread_num(omp_get_max_threads()),
+        thread_num(cpu_utils::get_max_threads()),
         effective_thread_num(thread_num),
         split_kv_q_token_num_threshold(split_kv_q_token_num_threshold),
         attention_scratchpad_size_per_thread(0),
@@ -405,7 +405,7 @@ class AttentionScheduler {
   torch::Tensor schedule(const ScheduleInput& input) const {
     const bool causal = input.causal;
     const bool is_dynamic_causal = input.dynamic_causal != nullptr;
-    const int32_t thread_num = omp_get_max_threads();
+    const int32_t thread_num = cpu_utils::get_max_threads();
     const int64_t cache_size = cpu_utils::get_available_l2_size();
     const int32_t max_num_q_per_iter = input.max_num_q_per_iter;
     const int32_t kv_len_alignment = input.kv_block_alignment;
@@ -1423,7 +1423,7 @@ class AttentionMainLoop {
 
  public:
   void operator()(const AttentionInput* input) {
-    const int thread_num = omp_get_max_threads();
+    const int thread_num = cpu_utils::get_max_threads();
     TORCH_CHECK_EQ(input->metadata->thread_num, thread_num);
     std::atomic<int32_t> guard_counter(0);
     std::atomic<int32_t>* guard_counter_ptr = &guard_counter;

csrc/cpu/cpu_fused_moe.cpp

@@ -267,7 +267,7 @@ void fused_moe_impl(scalar_t* __restrict__ output, scalar_t* __restrict__ input,
   TORCH_CHECK_EQ(output_size_2 % gemm_n_tile_size, 0);
   TORCH_CHECK_EQ(output_size_13 / 2, input_size_2);
 
-  const int32_t thread_num = omp_get_max_threads();
+  const int32_t thread_num = cpu_utils::get_max_threads();
 
   const int32_t w13_input_buffer_size = cpu_utils::round_up<64>(
       gemm_m_tile_size * input_size_13 * sizeof(scalar_t));

csrc/cpu/cpu_types.hpp

@@ -25,4 +25,20 @@
   #include <omp.h>
 #endif
 
+#include <c10/util/Exception.h>
+
+namespace cpu_utils {
+// Without OpenMP the omp pragmas compile to serial loops, so report 1: kernels
+// that barrier on the thread count would otherwise deadlock.
+inline int get_max_threads() {
+#ifdef _OPENMP
+  return omp_get_max_threads();
+#else
+  TORCH_WARN_ONCE(
+      "vLLM CPU was built without OpenMP; running single-threaded.");
+  return 1;
+#endif
+}
+}  // namespace cpu_utils
+
 #endif

csrc/cpu/cpu_wna16.cpp

@@ -155,7 +155,7 @@ void cpu_gemm_wna16_impl(
   constexpr int32_t gemm_m_tile_size = gemm_t::MaxMSize;
   constexpr int32_t n_block_size = 16;
   static_assert(gemm_n_tile_size % n_block_size == 0);
-  const int32_t thread_num = omp_get_max_threads();
+  const int32_t thread_num = cpu_utils::get_max_threads();
 
   // a simple schedule policy, just to hold more B tiles in L2 and make sure
   // each thread has tasks

csrc/cpu/dnnl_kernels.cpp

@@ -202,7 +202,7 @@ void dynamic_quant_epilogue(const float* input, scalar_t* output,
   using cvt_vec_t = typename KernelVecType<scalar_t>::cvt_vec_type;
   constexpr int vec_elem_num = load_vec_t::VEC_ELEM_NUM;
 
-  const int64_t thread_num = omp_get_max_threads();
+  const int64_t thread_num = cpu_utils::get_max_threads();
   if (num_tokens > thread_num) {
 #pragma omp parallel for
     for (int64_t i = 0; i < num_tokens; ++i) {

csrc/cpu/mla_decode.cpp

@@ -251,7 +251,7 @@ void mla_decode_kvcache_cpu_impl(
   constexpr int QK_NUM_ELEM = qk_vec_type::VEC_ELEM_NUM;
 
   // shared across threads
-  const int max_threads = omp_get_max_threads();
+  const int max_threads = cpu_utils::get_max_threads();
   const int acc_out_nbytes =
       max_threads * num_heads * V_HEAD_DIM * sizeof(float);
   float* acc_out = static_cast<float*>(std::aligned_alloc(64, acc_out_nbytes));

docs/getting_started/installation/cpu.apple.inc.md

@@ -15,6 +15,10 @@ Currently the CPU implementation for macOS supports FP32 and FP16 datatypes.
 - SDK: `XCode 15.4` or later with Command Line Tools
 - Compiler: `Apple Clang >= 15.0.0`
 
+!!! note
+    The macOS CPU build is smoke-tested in CI on the latest GA Apple Silicon
+    runner; other macOS or Apple Clang versions are best-effort.
+
 --8<-- [end:requirements]
 --8<-- [start:set-up-using-python]