[Fix] Spawn, don't fork, the benchmark server process (#34712)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

python/sglang/benchmark/endpoint.py

@@ -50,14 +50,13 @@ def _launch_server_target(launch_server_func: Callable, server_args: ServerArgs)
 
 
 def launch_or_reuse_server(launch_server_func: Callable, server_args: ServerArgs):
-    # Resolve in the parent, before the fork. The pipeline probes the device
-    # (the default attention backend reads the CUDA capability), and a forked
-    # child cannot re-initialize CUDA once this process has.
+    # Resolving probes the device: the default attention backend reads the CUDA
+    # capability, and XPU reads mem_get_info. This process owns a live context after.
     server_args.resolve_once()
 
     base_url = resolve_base_url("", server_args.host, server_args.port)
 
-    # Reuse an already-running server instead of forking a second one onto the
+    # Reuse an already-running server instead of launching a second one onto the
     # occupied port, where it would orphan, compete for the GPU, and OOM.
     if server_is_up(base_url, timeout=5):
         print(
@@ -66,7 +65,9 @@ def launch_or_reuse_server(launch_server_func: Callable, server_args: ServerArgs
         )
         return None, base_url
 
-    proc = multiprocessing.Process(
+    # Spawn, not the platform default: a fork inherits the context resolve_once()
+    # initialized above, and CUDA/XPU cannot be re-initialized in a forked child.
+    proc = multiprocessing.get_context("spawn").Process(
         target=_launch_server_target,
         args=(
             launch_server_func,
@@ -99,7 +100,8 @@ class BenchEndpoint:
     """
 
     base_url: str
-    _proc: Optional[multiprocessing.Process] = None
+    # SpawnProcess is a sibling of multiprocessing.Process, not a subclass.
+    _proc: Optional[multiprocessing.process.BaseProcess] = None
 
     def close(self) -> None:
         if self._proc is not None: