[BugFix] scheduler: Fix resuming of preempted requests after async load (#31583)

Signed-off-by: Or Ozeri <oro@il.ibm.com>

tests/v1/core/test_scheduler.py

@@ -1261,10 +1261,11 @@ def test_kv_connector_unable_to_allocate(use_ec_connector, ec_role):
     assert len(scheduler.waiting) == 0
 
 
+@pytest.mark.parametrize("is_async", [False, True])
 @pytest.mark.parametrize(
     "use_ec_connector, ec_role", [(False, None), (True, "ec_consumer")]
 )
-def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
+def test_kv_connector_handles_preemption(is_async, use_ec_connector, ec_role):
     """
     Test whether scheduler with KVConnector is able to handle
     unable to allocate (run out of blocks in allocate_slots().
@@ -1277,7 +1278,9 @@ def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
     NUM_MATCHED_NEW_TOKENS = BLOCK_SIZE
     scheduler = create_scheduler(
         enable_prefix_caching=True,
-        use_kv_connector=mock_kv(matched_tokens=NUM_MATCHED_NEW_TOKENS, is_async=False),
+        use_kv_connector=mock_kv(
+            matched_tokens=NUM_MATCHED_NEW_TOKENS, is_async=is_async
+        ),
         block_size=BLOCK_SIZE,
         num_blocks=NUM_BLOCKS,
         # encoder connector should not affect test results
@@ -1315,6 +1318,12 @@ def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
 
     # All can be scheduled - 1st token.
     output = scheduler.schedule()
+    if is_async:
+        assert len(scheduler.waiting) == 2
+        assert scheduler.running == []
+        _step_until_kv_transfer_finished(scheduler, req_ids)
+        output = scheduler.schedule()
+
     _assert_right_scheduler_output(
         output,
         # 2 remote kv cache hits.
@@ -1367,6 +1376,12 @@ def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
     # Restarts the preempted request - generate 3rd token.
     # This will have a local and remote cache hit.
     output = scheduler.schedule()
+    if is_async:
+        waiting_req_ids = [req.request_id for req in scheduler.waiting]
+        assert len(waiting_req_ids) == 1
+        _step_until_kv_transfer_finished(scheduler, waiting_req_ids)
+        output = scheduler.schedule()
+
     _assert_right_scheduler_output(
         output,
         # 1 remote kv_cache hit!
@@ -1377,6 +1392,8 @@ def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
     )
     assert len(scheduler.running) == 1
     assert len(scheduler.waiting) == 0
+    assert output.scheduled_cached_reqs.num_reqs == 1
+    assert output.scheduled_new_reqs == []
     _ = scheduler.update_from_output(output, MODEL_RUNNER_OUTPUT)
     assert len(scheduler.running) == 1
     assert len(scheduler.waiting) == 0
@@ -1389,6 +1406,8 @@ def test_kv_connector_handles_preemption(use_ec_connector, ec_role):
         num_requests=0,
         expected_num_scheduled_tokens=1,
     )
+    assert output.scheduled_cached_reqs.num_reqs == 1
+    assert output.scheduled_new_reqs == []
     assert len(scheduler.running) == 1
     _ = scheduler.update_from_output(output, MODEL_RUNNER_OUTPUT)
     assert len(scheduler.running) == 0

vllm/v1/core/sched/scheduler.py

@@ -445,7 +445,12 @@ class Scheduler(SchedulerInterface):
                 if request.status == RequestStatus.WAITING_FOR_REMOTE_KVS:
                     is_ready = self._update_waiting_for_remote_kv(request)
                     if is_ready:
-                        request.status = RequestStatus.WAITING
+                        if request.num_preemptions:
+                            # We must be loading for a resumed preemption
+                            # rather than a new request.
+                            request.status = RequestStatus.PREEMPTED
+                        else:
+                            request.status = RequestStatus.WAITING
                     else:
                         logger.debug(
                             "%s is still in WAITING_FOR_REMOTE_KVS state.",

vllm/v1/request.py

@@ -123,7 +123,7 @@ class Request:
         # indicates that the output is corrupted
         self.num_nans_in_logits = 0
 
-        # The number of requests being preempted by the scheduler
+        # The number of times this request has been preempted by the scheduler.
         self.num_preemptions = 0
 
         # The number of tokens that have been computed remotely.