[BugFix] Fix Memory Leak (#17567)

Signed-off-by: rshaw@neuralmagic.com <robertgshaw2@gmail.com>

tests/v1/core/test_scheduler.py

@@ -1165,3 +1165,80 @@ def test_kv_connector_handles_preemption():
     # All memory should be freed since nothing is running.
     assert scheduler.kv_cache_manager.block_pool.get_num_free_blocks() \
         == NUM_BLOCKS - 1
+
+
+def make_output(scheduler: Scheduler):
+    return ModelRunnerOutput(
+        req_ids=[req.request_id for req in scheduler.running],
+        req_id_to_index={
+            req.request_id: i
+            for i, req in enumerate(scheduler.running)
+        },
+        sampled_token_ids=[[1000]] * len(scheduler.running),
+        spec_token_ids=None,
+        logprobs=None,
+        prompt_logprobs_dict={},
+    )
+
+
+def assert_scheduler_empty(scheduler: Scheduler):
+    """Confirm the scheduler is "empty" - i.e. no leaks."""
+    # Scheduler Metadata.
+    assert len(scheduler.requests) == 0
+    assert len(scheduler.waiting) == 0
+    assert len(scheduler.running) == 0
+    assert len(scheduler.finished_req_ids) == 0
+    assert len(scheduler._cached_reqs_data) == 0
+
+    # EncoderCacheManager.
+    assert len(scheduler.encoder_cache_manager.freed) == 0
+    assert len(scheduler.encoder_cache_manager.cached) == 0
+
+    # KVCache Manager.
+    assert len(scheduler.kv_cache_manager.req_to_blocks) == 0
+    assert len(scheduler.kv_cache_manager.req_to_block_hashes) == 0
+    assert len(scheduler.kv_cache_manager.num_cached_block) == 0
+    num_free_blocks = (
+        scheduler.kv_cache_manager.block_pool.free_block_queue.num_free_blocks)
+    assert num_free_blocks == (
+        scheduler.kv_cache_manager.block_pool.num_gpu_blocks - 1)
+
+    # NOTE(rob): just the ref count on blocks will be 0. The hash
+    # value, etc will remain since we lazily evict for prefix cache.
+    for block in scheduler.kv_cache_manager.block_pool.blocks:
+        assert block.ref_cnt == 0
+        # assert block._block_hash is None
+    # assert (
+    #     len(scheduler.kv_cache_manager.block_pool.cached_block_hash_to_block
+    #           ) == 0)
+
+
+def test_memory_leak():
+    """Test that we do not have a memory leak."""
+
+    scheduler = create_scheduler(enable_prefix_caching=True)
+
+    NUM_REQUESTS = 5
+    NUM_TOKENS = 10
+    MAX_TOKENS = 10
+    requests = create_requests(num_requests=NUM_REQUESTS,
+                               num_tokens=NUM_TOKENS,
+                               max_tokens=MAX_TOKENS)
+
+    # Add each request.
+    for request in requests:
+        scheduler.add_request(request)
+        scheduler_output = scheduler.schedule()
+        model_runner_output = make_output(scheduler)
+        scheduler.update_from_output(scheduler_output, model_runner_output)
+
+    # Iterate until done.
+    while True:
+        scheduler_output = scheduler.schedule()
+        if len(scheduler.running) == 0:
+            break
+        model_runner_output = make_output(scheduler)
+        scheduler.update_from_output(scheduler_output, model_runner_output)
+
+    # Confirm no memory leak.
+    assert_scheduler_empty(scheduler)

vllm/v1/core/sched/scheduler.py

@@ -761,7 +761,10 @@ class Scheduler(SchedulerInterface):
 
         # Return the cached request data to the queue so they can be reused.
         for req_data in scheduler_output.scheduled_cached_reqs:
-            self._cached_reqs_data[req_data.req_id].append(req_data)
+            # NOTE(rob): since we free stopped reqs above, adding stopped reqs
+            # to _cached_reqs_data will cause a memory leak.
+            if req_data.req_id not in self.finished_req_ids:
+                self._cached_reqs_data[req_data.req_id].append(req_data)
 
         self.running = new_running
         engine_core_outputs = EngineCoreOutputs(