[Bugfix] In LongRoPE, decide short vs long based on max_model_len (#27431)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

vllm/config/model.py

@@ -2142,8 +2142,18 @@ def _get_and_verify_max_len(
     # If the user didn't specify `max_model_len`, then use that derived from
     # the model config as a default value.
     if max_model_len is None:
-        max_model_len = int(derived_max_model_len)
+        # For LongRoPE, default to original_max_position_embeddings to avoid
+        # performance degradation for shorter sequences
+        if rope_scaling is not None and rope_scaling["rope_type"] == "longrope":
+            max_model_len = int(
+                getattr(
+                    hf_config, "original_max_position_embeddings", derived_max_model_len
+                )
+            )
+        else:
+            max_model_len = int(derived_max_model_len)
         max_model_len = current_platform.check_max_model_len(max_model_len)
+
     # If the user specified a max length, make sure it is smaller than the
     # derived length from the HF model config.
     elif max_model_len > derived_max_model_len: