[Bug] Fix DBO IMA issue for DeepEPHT (vllm-project#27666)

Signed-off-by: yewentao256 <zhyanwentao@126.com>

Author: yewentao256

vllm/model_executor/layers/fused_moe/deepep_ht_prepare_finalize.py

@@ -16,6 +16,7 @@
 from vllm.v1.worker.ubatching import (
     dbo_current_ubatch_id,
     dbo_enabled,
+    dbo_get_previous_event,
     dbo_switch_to_comm,
     dbo_switch_to_compute,
     dbo_switch_to_compute_sync,
@@ -110,6 +111,10 @@ def _do_dispatch(
         # for the other ubatch before the dispatch kernel starts.
         dbo_yield_and_switch_from_compute_to_comm()
 
+        # capture a DeepEP event and pass it as previous_event so
+        # DeepEP honors the dependency internally.
+        previous_event = dbo_get_previous_event(self.buffer.capture)
+
         (
             num_tokens_per_rank,
             num_tokens_per_rdma_rank,
@@ -119,7 +124,7 @@ def _do_dispatch(
         ) = self.buffer.get_dispatch_layout(
             topk_idx=rank_topk_ids,
             num_experts=num_experts,
-            previous_event=None,
+            previous_event=previous_event,
             async_finish=False,
             allocate_on_comm_stream=False,
         )
@@ -148,7 +153,7 @@ def _do_dispatch(
             # to this value.
             expert_alignment=1,
             config=self._get_dispatch_config(),
-            previous_event=None,
+            previous_event=previous_event,
             async_finish=self.async_prepare and not dbo_enabled(),
             allocate_on_comm_stream=False,
         )
@@ -339,13 +344,14 @@ def _finalize(
         assert fused_expert_output.dtype == torch.bfloat16, (
             f"Expected fused_expert_output bfloat16, got {fused_expert_output.dtype}"
         )
+        previous_event = dbo_get_previous_event(self.buffer.capture)
         combined_x, _, event = self.buffer.combine(
             # HT combine only supports BF16
             x=fused_expert_output,
             handle=handle,
             topk_weights=None,
             config=self._get_combine_config(),
-            previous_event=None,
+            previous_event=previous_event,
             async_finish=do_async and not dbo_enabled(),
             allocate_on_comm_stream=False,
         )

vllm/v1/worker/ubatching.py

@@ -185,6 +185,15 @@ def dbo_register_recv_hook(recv_hook):
         next_ctx.recv_hook = recv_hook
 
 
+def dbo_get_previous_event(func, *args, **kwargs):
+    if len(_THREAD_ID_TO_CONTEXT) > 0:
+        ctx_idx = _THREAD_ID_TO_CONTEXT[threading.get_ident()]
+        ctx = _CURRENT_CONTEXTS[ctx_idx]
+        # execute callable on the ubatch compute stream to record/wait events there
+        with torch.cuda.stream(ctx.compute_stream):
+            return func(*args, **kwargs)
+
+
 def make_ubatch_contexts(
     num_micro_batches: int,
     compute_stream: torch.cuda.Stream,