[distributed] fix dp group (#15355)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/distributed/parallel_state.py

@@ -897,29 +897,22 @@ def initialize_model_parallel(
         get_world_group().device_group)
 
     data_parallel_size = 1
-    has_external_dp = False
     from vllm.config import get_current_vllm_config
     config = get_current_vllm_config()
     if config is not None:
-        if config.parallel_config.world_size != world_size:
-            # detect external data parallelism.
-            # dp in vllm means all dp instances need to run together.
-            # if the world size does not match, it means this dp is external,
-            # and the dp instances can run independently, e.g. in rlhf workflow
-            # from https://github.com/volcengine/verl .
-            # in that case, we treat the rest dimensions as if they are
-            # data parallel, and create a dummy dp group that is not used.
-            data_parallel_size = world_size // (pipeline_model_parallel_size *
-                                                tensor_model_parallel_size)
-            has_external_dp = True
-        else:
-            data_parallel_size = config.parallel_config.data_parallel_size
-
-    # the layout order is: DP x PP x TP
+        data_parallel_size = config.parallel_config.data_parallel_size
+
+    # the layout order is: ExternalDP x DP x PP x TP
+    # ExternalDP is the data parallel group that is not part of the model,
+    # every dp rank can generate independently (in verl integration).
+    # DP is the data parallel group that is part of the model,
+    # all the ranks in the same DP group should generate simultaneously,
+    # i.e. the `generate` call in the same DP group should be called together,
+    # otherwise it will cause deadlock.
     # to get group_ranks for each dimension, transpose that dimension to the
     # last dimension, then reshape to 2D, then unbind the last dimension
     all_ranks = torch.arange(world_size).reshape(
-        data_parallel_size, pipeline_model_parallel_size,
+        -1, data_parallel_size, pipeline_model_parallel_size,
         tensor_model_parallel_size)  # noqa
 
     # Build the tensor model-parallel groups.
@@ -939,7 +932,7 @@ def initialize_model_parallel(
     global _PP
     assert _PP is None, (
         "pipeline model parallel group is already initialized")
-    group_ranks = all_ranks.transpose(1, 2).reshape(
+    group_ranks = all_ranks.transpose(2, 3).reshape(
         -1, pipeline_model_parallel_size).unbind(0)
     group_ranks = [x.tolist() for x in group_ranks]
     _PP = init_model_parallel_group(group_ranks,
@@ -949,16 +942,10 @@ def initialize_model_parallel(
 
     global _DP
     assert _DP is None, ("data parallel group is already initialized")
-    group_ranks = all_ranks.transpose(0,
-                                      2).reshape(-1,
+    group_ranks = all_ranks.transpose(1,
+                                      3).reshape(-1,
                                                  data_parallel_size).unbind(0)
     group_ranks = [x.tolist() for x in group_ranks]
-    if has_external_dp:
-        # create a dummy dp group that is not used actually,
-        # since this dp is external.
-        # a dummy dp group means every rank is a group itself.
-        # this way, no communication is needed, no memory is wasted.
-        group_ranks = [[x] for x in range(world_size)]
     _DP = init_model_parallel_group(group_ranks,
                                     get_world_group().local_rank,
                                     backend,