[PD+DP] Allow PrefillDelayer in disaggregated-prefill mode (#23588)

python/sglang/srt/managers/prefill_delayer.py

@@ -70,9 +70,6 @@ class PrefillDelayer:
         self._curr_state: Optional[_State] = None
         self.skip_first_delayer = True
 
-        assert (
-            server_args.disaggregation_mode == "null"
-        ), "To use PrefillDelayer, disaggregation_mode must be null."
         assert (
             not server_args.disable_overlap_schedule
         ), "To use PrefillDelayer, disable_overlap_schedule must be False."

python/sglang/srt/managers/scheduler.py

@@ -982,6 +982,12 @@ class Scheduler(
         self.prefill_delayer: Optional[PrefillDelayer] = None
         self.max_prefill_bs: int = 0
         if self.server_args.enable_prefill_delayer:
+            if self.server_args.disaggregation_mode == "decode":
+                logger.info(
+                    "Ignoring --enable-prefill-delayer on decode engine "
+                    "(no prefill scheduling path; delayer would be a no-op)."
+                )
+            else:
                 self.prefill_delayer = PrefillDelayer(
                     dp_size=self.dp_size,
                     attn_tp_size=self.attn_tp_size,