[Bugfix][Ray] Set the cuda context eagerly in the ray worker

.buildkite/test-pipeline.yaml

@@ -271,6 +271,15 @@ steps:
   commands:
     - pytest -v -s prefix_caching
 
+
+- label: Platform Tests (CUDA)
+  mirror_hardwares: [amdexperimental]
+  source_file_dependencies:
+  - vllm/
+  - tests/cuda
+  commands:
+    - pytest -v -s cuda/test_cuda_context.py
+
 - label: Samplers Test # 36min
   mirror_hardwares: [amdexperimental]
   source_file_dependencies:

tests/cuda/test_cuda_context.py

@@ -0,0 +1,80 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import ctypes
+from concurrent.futures import ThreadPoolExecutor
+
+import pytest
+import torch
+
+from vllm.platforms import current_platform
+
+
+def check_cuda_context():
+    """Check CUDA driver context status"""
+    try:
+        cuda = ctypes.CDLL('libcuda.so')
+        device = ctypes.c_int()
+        result = cuda.cuCtxGetDevice(ctypes.byref(device))
+        return (True, device.value) if result == 0 else (False, None)
+    except Exception:
+        return False, None
+
+
+def run_cuda_test_in_thread(device_input, expected_device_id):
+    """Run CUDA context test in separate thread for isolation"""
+    try:
+        # New thread should have no CUDA context initially
+        valid_before, device_before = check_cuda_context()
+        if valid_before:
+            return False, \
+                "CUDA context should not exist in new thread, " \
+                f"got device {device_before}"
+
+        # Test setting CUDA context
+        current_platform.set_device(device_input)
+
+        # Verify context is created correctly
+        valid_after, device_id = check_cuda_context()
+        if not valid_after:
+            return False, "CUDA context should be valid after set_cuda_context"
+        if device_id != expected_device_id:
+            return False, \
+                f"Expected device {expected_device_id}, got {device_id}"
+
+        return True, "Success"
+    except Exception as e:
+        return False, f"Exception in thread: {str(e)}"
+
+
+class TestSetCudaContext:
+    """Test suite for the set_cuda_context function."""
+
+    @pytest.mark.skipif(not current_platform.is_cuda(),
+                        reason="CUDA not available")
+    @pytest.mark.parametrize(argnames="device_input,expected_device_id",
+                             argvalues=[
+                                 (0, 0),
+                                 (torch.device('cuda:0'), 0),
+                                 ('cuda:0', 0),
+                             ],
+                             ids=["int", "torch_device", "string"])
+    def test_set_cuda_context_parametrized(self, device_input,
+                                           expected_device_id):
+        """Test setting CUDA context in isolated threads."""
+        with ThreadPoolExecutor(max_workers=1) as executor:
+            future = executor.submit(run_cuda_test_in_thread, device_input,
+                                     expected_device_id)
+            success, message = future.result(timeout=30)
+        assert success, message
+
+    @pytest.mark.skipif(not current_platform.is_cuda(),
+                        reason="CUDA not available")
+    def test_set_cuda_context_invalid_device_type(self):
+        """Test error handling for invalid device type."""
+        with pytest.raises(ValueError, match="Expected a cuda device"):
+            current_platform.set_device(torch.device('cpu'))
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])

vllm/platforms/cuda.py

@@ -71,6 +71,17 @@ def supported_dtypes(self) -> list[torch.dtype]:
         # though vLLM doesn't support these GPUs.
         return [torch.float32]
 
+    @classmethod
+    def set_device(cls, device: torch.device) -> None:
+        """
+        Set the device for the current platform.
+        """
+        super().set_device(device)
+        # With this trick we can force the device to be set eagerly
+        # see https://github.com/pytorch/pytorch/issues/155668
+        # for why and when it is needed
+        _ = torch.zeros(1, device=device)
+
     @classmethod
     def get_device_capability(cls,
                               device_id: int = 0

vllm/platforms/interface.py

@@ -298,6 +298,13 @@ def seed_everything(cls, seed: Optional[int] = None) -> None:
             np.random.seed(seed)
             torch.manual_seed(seed)
 
+    @classmethod
+    def set_device(cls, device: torch.device) -> None:
+        """
+        Set the device for the current platform.
+        """
+        torch.cuda.set_device(device)
+
     @classmethod
     def pre_register_and_update(cls,
                                 parser: Optional[FlexibleArgumentParser] = None