Fix NCCL test hang

src/collective/communicator.cu

@@ -29,10 +29,18 @@ DeviceCommunicator* Communicator::GetDevice(int device_ordinal) {
     old_device_ordinal = device_ordinal;
     old_world_size = communicator_->GetWorldSize();
 #ifdef XGBOOST_USE_NCCL
-    if (type_ != CommunicatorType::kFederated) {
-      device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal));
-    } else {
-      device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal));
+    switch (type_) {
+      case CommunicatorType::kRabit:
+        device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal, false));
+        break;
+      case CommunicatorType::kFederated:
+        device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal));
+        break;
+      case CommunicatorType::kInMemory:
+        device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal, true));
+        break;
+      default:
+        device_communicator_.reset(new NcclDeviceCommunicator(device_ordinal, false));
     }
 #else
     device_communicator_.reset(new DeviceCommunicatorAdapter(device_ordinal));

src/collective/nccl_device_communicator.cu

@@ -7,8 +7,11 @@
 namespace xgboost {
 namespace collective {
 
-NcclDeviceCommunicator::NcclDeviceCommunicator(int device_ordinal)
-    : device_ordinal_{device_ordinal}, world_size_{GetWorldSize()}, rank_{GetRank()} {
+NcclDeviceCommunicator::NcclDeviceCommunicator(int device_ordinal, bool needs_sync)
+    : device_ordinal_{device_ordinal},
+      needs_sync_{needs_sync},
+      world_size_{GetWorldSize()},
+      rank_{GetRank()} {
   if (device_ordinal_ < 0) {
     LOG(FATAL) << "Invalid device ordinal: " << device_ordinal_;
   }
@@ -140,6 +143,9 @@ void NcclDeviceCommunicator::BitwiseAllReduce(void *send_receive_buffer, std::si
   // First gather data from all the workers.
   dh::safe_nccl(ncclAllGather(send_receive_buffer, device_buffer, count, GetNcclDataType(data_type),
                               nccl_comm_, cuda_stream_));
+  if (needs_sync_) {
+    dh::safe_cuda(cudaStreamSynchronize(cuda_stream_));
+  }
 
   // Then reduce locally.
   auto *out_buffer = static_cast<char *>(send_receive_buffer);

src/collective/nccl_device_communicator.cuh

@@ -12,7 +12,20 @@ namespace collective {
 
 class NcclDeviceCommunicator : public DeviceCommunicator {
  public:
-  explicit NcclDeviceCommunicator(int device_ordinal);
+  /**
+   * @brief Construct a new NCCL communicator.
+   * @param device_ordinal The GPU device id.
+   * @param needs_sync Whether extra CUDA stream synchronization is needed.
+   *
+   * In multi-GPU tests when multiple NCCL communicators are created in the same process, sometimes
+   * a deadlock happens because NCCL kernels are blocking. The extra CUDA stream synchronization
+   * makes sure that the NCCL kernels are caught up, thus avoiding the deadlock.
+   *
+   * The Rabit communicator runs with one process per GPU, so the additional synchronization is not
+   * needed. The in-memory communicator is used in tests with multiple threads, each thread
+   * representing a rank/worker, so the additional synchronization is needed to avoid deadlocks.
+   */
+  explicit NcclDeviceCommunicator(int device_ordinal, bool needs_sync);
   ~NcclDeviceCommunicator() override;
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
                  Operation op) override;
@@ -60,6 +73,7 @@ class NcclDeviceCommunicator : public DeviceCommunicator {
                         Operation op);
 
   int const device_ordinal_;
+  bool const needs_sync_;
   int const world_size_;
   int const rank_;
   ncclComm_t nccl_comm_{};

tests/cpp/collective/test_nccl_device_communicator.cu

@@ -16,7 +16,7 @@ namespace xgboost {
 namespace collective {
 
 TEST(NcclDeviceCommunicatorSimpleTest, ThrowOnInvalidDeviceOrdinal) {
-  auto construct = []() { NcclDeviceCommunicator comm{-1}; };
+  auto construct = []() { NcclDeviceCommunicator comm{-1, false}; };
   EXPECT_THROW(construct(), dmlc::Error);
 }