[PD] Add a queues.prealloc_ready counter to the load snapshot (#33133)

python/sglang/srt/managers/load_snapshot.py

@@ -171,6 +171,7 @@ class QueueMetrics(msgspec.Struct, array_like=True):
     grammar: int
     paused: int
     retracted: int
+    prealloc_ready: int
 
 
 # LoadSnapshot's nested sub-struct fields; every other struct field is a flat

python/sglang/srt/managers/scheduler_components/load_inquirer.py

@@ -166,6 +166,7 @@ class SchedulerLoadInquirer:
         mode_str = "null"
         prefill_bootstrap = prefill_inflight = 0
         decode_prealloc = decode_transfer = decode_retracted = 0
+        decode_prealloc_ready = 0
         if self.disaggregation_mode == DisaggregationMode.PREFILL:
             mode_str = "prefill"
             prefill_bootstrap = len(self.get_disagg_prefill_bootstrap_queue().queue)
@@ -177,6 +178,11 @@ class SchedulerLoadInquirer:
             decode_retracted = len(
                 self.get_disagg_decode_prealloc_queue().retracted_queue
             )
+            decode_prealloc_ready = sum(
+                1
+                for decode_req in self.get_disagg_decode_prealloc_queue().queue
+                if decode_req.waiting_for_input
+            )
         disaggregation = DisaggregationMetrics(
             mode=mode_str,
             prefill_bootstrap_queue_reqs=prefill_bootstrap,
@@ -193,6 +199,7 @@ class SchedulerLoadInquirer:
             grammar=stats.num_grammar_queue_reqs,
             paused=stats.num_paused_reqs,
             retracted=stats.num_retracted_reqs,
+            prealloc_ready=decode_prealloc_ready,
         )
 
         totals = self.get_decode_moment_totals()

test/registered/unit/entrypoints/test_v1_loads_aggregate.py

@@ -171,7 +171,9 @@ class TestGetLoads(CustomTestCase):
                     disaggregation=DisaggregationMetrics(
                         mode="decode", decode_transfer_queue_reqs=4
                     ),
-                    queues=QueueMetrics(waiting=2, grammar=1, paused=0, retracted=3),
+                    queues=QueueMetrics(
+                        waiting=2, grammar=1, paused=0, retracted=3, prealloc_ready=1
+                    ),
                 )
             )