More minor RL fixes (#3551)

* fix: handle get_open_port import across TRL versions

TRL 0.29+ removed get_open_port from exports; fall back to importing
directly from vllm.utils or vllm.utils.network_utils.

* support DP with vllm and make generation_batch_size confifurable

src/axolotl/core/trainers/grpo/__init__.py

@@ -89,6 +89,9 @@ class GRPOStrategy:
         if trl.num_generations:
             grpo_args_kwargs["num_generations"] = trl.num_generations
 
+        if trl.generation_batch_size is not None:
+            grpo_args_kwargs["generation_batch_size"] = trl.generation_batch_size
+
         if trl.sync_ref_model:
             grpo_args_kwargs["sync_ref_model"] = trl.sync_ref_model
 

src/axolotl/scripts/vllm_serve_lora.py

@@ -26,8 +26,15 @@ from trl.scripts.vllm_serve import (
     ScriptArguments,
     chunk_list,
     extract_logprobs,
-    get_open_port,
 )
+
+try:
+    from trl.scripts.vllm_serve import get_open_port
+except ImportError:
+    try:
+        from vllm.utils import get_open_port
+    except ImportError:
+        from vllm.utils.network_utils import get_open_port
 from vllm import LLM, SamplingParams
 from vllm.lora.request import LoRARequest
 
@@ -63,10 +70,21 @@ def llm_worker(
     connection: Connection,
 ) -> None:
     """Worker process that creates a vLLM LLM with LoRA enabled."""
-    os.environ["VLLM_DP_RANK"] = str(data_parallel_rank)
-    os.environ["VLLM_DP_RANK_LOCAL"] = str(data_parallel_rank)
-    os.environ["VLLM_DP_SIZE"] = str(script_args.data_parallel_size)
-    os.environ["VLLM_DP_MASTER_PORT"] = str(master_port)
+    # For DP with TP=1: pin each worker to its own GPU via CUDA_VISIBLE_DEVICES.
+    # vLLM's LLM() offline mode doesn't support DP env vars natively, so we
+    # isolate each worker to a single GPU and let vLLM think it's the only one.
+    if script_args.data_parallel_size > 1 and script_args.tensor_parallel_size == 1:
+        visible = os.environ.get("CUDA_VISIBLE_DEVICES", "")
+        if visible:
+            gpu_ids = visible.split(",")
+            os.environ["CUDA_VISIBLE_DEVICES"] = gpu_ids[data_parallel_rank]
+        else:
+            os.environ["CUDA_VISIBLE_DEVICES"] = str(data_parallel_rank)
+    else:
+        os.environ["VLLM_DP_RANK"] = str(data_parallel_rank)
+        os.environ["VLLM_DP_RANK_LOCAL"] = str(data_parallel_rank)
+        os.environ["VLLM_DP_SIZE"] = str(script_args.data_parallel_size)
+        os.environ["VLLM_DP_MASTER_PORT"] = str(master_port)
 
     llm = LLM(
         model=script_args.model,

src/axolotl/utils/schemas/trl.py

@@ -66,6 +66,14 @@ class TRLConfig(BaseModel):
             "description": "List of reward weights for the reward functions."
         },
     )
+    generation_batch_size: int | None = Field(
+        default=None,
+        json_schema_extra={
+            "description": "Batch size for generation. Controls how many unique "
+            "prompts are generated per step. For full DP utilization, set to "
+            "num_generations * data_parallel_size (or a multiple thereof)."
+        },
+    )
     num_generations: int | None = Field(
         default=None,
         json_schema_extra={"description": "Number of generations to sample."},