[Fix] Skip cross-node probe in MultimemAllGatherer on single-node runs (fixes mooncake EP segfault) (#30139)

python/sglang/srt/distributed/device_communicators/triton_symm_mem_ag.py

@@ -466,10 +466,19 @@ class MultimemAllGatherer:
             # Lazy import avoids a module-load dependency on the distributed facade.
             from sglang.srt.distributed import get_tp_group
             from sglang.srt.distributed.parallel_state import in_the_same_node_as
+            from sglang.srt.runtime_context import get_server_args
 
             tp_group = get_tp_group()
-            if tp_group.world_size > 1 and not all(
-                in_the_same_node_as(tp_group.cpu_group, source_rank=0)
+            # Only probe node topology when the deployment can actually span
+            # nodes. Check world_size first so a TP=1 gatherer short-circuits
+            # before reading server args (which may be unpublished on offline
+            # paths). On a single node every TP rank is co-located, so skip the
+            # in_the_same_node_as() all-reduce, which can segfault under some
+            # EP/mooncake setups, and keep multimem enabled.
+            if (
+                tp_group.world_size > 1
+                and get_server_args().nnodes > 1
+                and not all(in_the_same_node_as(tp_group.cpu_group, source_rank=0))
             ):
                 logger.warning(
                     "multimem all-gather disabled because the TP group spans "