[Scheduler] Defer ring-block free under overlapping steps

A circular-buffer ring block could be freed while an earlier, still-executing
step was reading it. The free is deferred until the overlapping step has
committed.

Co-Authored-By: Hinata <hinata@idihore.id>
Signed-off-by: MOVZX <movzx@yahoo.com>

vllm/v1/core/sched/scheduler.py

@@ -56,10 +56,12 @@ from vllm.v1.core.sched.request_queue import (
 from vllm.v1.core.sched.utils import check_stop, remove_all
 from vllm.v1.engine import EngineCoreEventType, EngineCoreOutput, EngineCoreOutputs
 from vllm.v1.kv_cache_interface import (
+    CircularBufferSpec,
     KVCacheConfig,
     MambaSpec,
     get_mamba_prefill_checkpoint_position,
     is_mamba_prefill_checkpoint_valid,
+    iter_layer_specs,
 )
 from vllm.v1.metrics.perf import ModelMetrics, PerfStats
 from vllm.v1.metrics.stats import (
@@ -192,6 +194,26 @@ class Scheduler(SchedulerInterface):
 
             self.requires_kv_delivery = self.connector.requires_kv_delivery
 
+        # Ring-skip telemetry: the QSA ring block (CircularBufferSpec) is rewritten
+        # by every step that touches the request, so with overlapping batches
+        # (async scheduling or PP) an in-flight step may still be committing a
+        # freed/preempted request's suffix rows when the scheduler hands that
+        # block to another request. A KV connector is not the only writer that
+        # can race the stale in-flight write; the next request's own first
+        # step is. Defer the return of freed blocks to the pool until the
+        # newest scheduled step's output has been processed whenever a ring
+        # group exists. Same mechanism and fence as the connector gate above.
+        if (
+            not self.defer_block_free
+            and self.vllm_config.max_concurrent_batches > 1
+            and any(
+                isinstance(spec, CircularBufferSpec)
+                for group in kv_cache_config.kv_cache_groups
+                for spec in iter_layer_specs(group.kv_cache_spec)
+            )
+        ):
+            self.defer_block_free = True
+
         self.kv_event_publisher = EventPublisherFactory.create(
             self.kv_events_config,
             self.parallel_config.data_parallel_index,
@@ -816,6 +838,7 @@ class Scheduler(SchedulerInterface):
                     else:
                         preempted_req = self.running.pop()
 
+                    num_deferred_before = len(self.deferred_frees)
                     self._preempt_request(
                         preempted_req,
                         scheduled_timestamp,
@@ -825,6 +848,16 @@ class Scheduler(SchedulerInterface):
                     if preempted_req == request:
                         # No more request to preempt. Cannot schedule this request.
                         break
+                    if len(self.deferred_frees) > num_deferred_before:
+                        # The ring-skip fix: the victim's blocks are fenced
+                        # behind its in-flight step and return to the pool only
+                        # when that step's output is processed, so retrying the
+                        # allocation now cannot succeed on their account and would
+                        # preempt the next request for nothing -- with every running
+                        # request in flight (PP / async) the whole batch. Stop at one
+                        # deferred victim; this request stays running, unscheduled,
+                        # and retries next step after the drain.
+                        break
 
             if new_blocks is None:
                 # Cannot schedule this request.