fix(dp-attn): consistent overlap disable decision across DP ranks (#20853)

python/sglang/srt/managers/scheduler.py

@@ -1326,12 +1326,21 @@ class Scheduler(
     def is_disable_overlap_for_batch(self, batch: ScheduleBatch) -> bool:
         # For two consecutive prefill batches, we disable overlap to improve the TTFT of the first batch.
         # This might slightly hurt the throughput, so we use an environment variable to control it.
+        # In DP attention mode, use the globally synchronized is_extend_in_batch
+        # so all DP ranks make the same overlap decision (avoiding deadlock).
+        # In non-DP mode, use the local forward_mode directly.
+        if self.require_mlp_sync:
+            is_extend = lambda b: b and b.is_extend_in_batch
+        else:
+            is_extend = lambda b: b and b.forward_mode.is_extend()
+
+        batch_is_extend = is_extend(batch)
+        last_batch_is_extend = is_extend(self.last_batch)
+
         disable_overlap_for_batch = (
             envs.SGLANG_DISABLE_CONSECUTIVE_PREFILL_OVERLAP.get()
-            and batch
-            and batch.forward_mode.is_extend()
-            and self.last_batch
-            and self.last_batch.forward_mode.is_extend()
+            and batch_is_extend
+            and last_batch_is_extend
         )
 
         # We do not support overlap + spec + grammar yet,

test/registered/distributed/test_dp_attention.py

@@ -38,6 +38,10 @@ class TestDPAttentionDP2TP2(
     def setUpClass(cls):
         cls.model = DEFAULT_MLA_MODEL_NAME_FOR_TEST
         cls.base_url = DEFAULT_URL_FOR_TEST
+        cls._env_override = envs.SGLANG_DISABLE_CONSECUTIVE_PREFILL_OVERLAP.override(
+            True
+        )
+        cls._env_override.__enter__()
         cls.process = popen_launch_server(
             cls.model,
             cls.base_url,
@@ -58,6 +62,7 @@ class TestDPAttentionDP2TP2(
     @classmethod
     def tearDownClass(cls):
         kill_process_tree(cls.process.pid)
+        cls._env_override.__exit__(None, None, None)
 
     def test_mgsm_en(self):
         args = SimpleNamespace(