[Bugfix] Do not crash V0 engine on input errors (#13101)

Signed-off-by: Joe Runde <Joseph.Runde@ibm.com>

tests/mq_llm_engine/test_error_handling.py

@@ -18,6 +18,7 @@ from vllm.engine.multiprocessing.engine import MQLLMEngine
 from vllm.entrypoints.openai.api_server import build_async_engine_client
 from vllm.entrypoints.openai.cli_args import make_arg_parser
 from vllm.lora.request import LoRARequest
+from vllm.sequence import SequenceGroupMetadata
 from vllm.usage.usage_lib import UsageContext
 from vllm.utils import FlexibleArgumentParser
 
@@ -292,3 +293,80 @@ async def test_engine_process_death(tmp_socket):
             await client.check_health()
 
         client.close()
+
+
+def run_with_evil_input_processing(engine_args: AsyncEngineArgs,
+                                   ipc_path: str):
+    """Simulate an exception while preparing inputs for the model.
+    In the wild, this could be something like a multimodal input processor
+    failing on invalid image data."""
+
+    # Make engine.
+    engine = MQLLMEngine.from_engine_args(
+        engine_args=engine_args,
+        usage_context=UsageContext.UNKNOWN_CONTEXT,
+        ipc_path=ipc_path)
+
+    runner = engine.engine.model_executor.driver_worker.worker.model_runner
+
+    # Raise error in the model runner when adding a sequence group.
+    # See class ModelInputForGPUBuilder
+    def raiser(_, seq_group_metadata: SequenceGroupMetadata):
+        if seq_group_metadata.request_id.startswith("evil"):
+            raise RAISED_ERROR(RAISED_VALUE)
+
+    runner.builder.per_seq_group_compute_fns.append(raiser)
+
+    # Run engine.
+    engine.start()
+
+
+@pytest.mark.asyncio
+async def test_failed_inputs(tmp_socket):
+    with RemoteMQLLMEngine(engine_args=ENGINE_ARGS,
+                           ipc_path=tmp_socket,
+                           run_fn=run_with_evil_input_processing) as engine:
+
+        client = await engine.make_client()
+        assert client.is_running
+
+        # Engine should be healthy
+        await client.check_health()
+
+        async def run_failing_request():
+            async for _ in client.generate(
+                    prompt="Hello my name is",
+                    sampling_params=SamplingParams(max_tokens=10),
+                    request_id="evil" + str(uuid.uuid4())):
+                pass
+
+        async def run_passing_request():
+            async for _ in client.generate(
+                    prompt="Hello my name is",
+                    sampling_params=SamplingParams(max_tokens=10),
+                    request_id=str(uuid.uuid4())):
+                pass
+
+        passing_tasks = [
+            asyncio.create_task(run_passing_request()) for _ in range(10)
+        ]
+        failing_tasks = [
+            asyncio.create_task(run_failing_request()) for _ in range(10)
+        ]
+        await asyncio.gather(*failing_tasks, return_exceptions=True)
+        await asyncio.gather(*passing_tasks)
+
+        # All the bad inputs should have raised
+        for task in failing_tasks:
+            with pytest.raises(RAISED_ERROR):
+                task.result()
+
+        # But all good inputs should have still succeeded
+        for task in passing_tasks:
+            task.result()
+
+        # And the engine should remain healthy
+        assert not client.errored
+        await client.check_health()
+
+        client.close()

vllm/engine/llm_engine.py

@@ -60,6 +60,7 @@ from vllm.usage.usage_lib import (UsageContext, is_usage_stats_enabled,
 from vllm.utils import (Counter, Device, deprecate_kwargs,
                         resolve_obj_by_qualname, weak_bind)
 from vllm.version import __version__ as VLLM_VERSION
+from vllm.worker.model_runner_base import InputProcessingError
 
 logger = init_logger(__name__)
 _LOCAL_LOGGING_INTERVAL_SEC = 5
@@ -410,6 +411,10 @@ class LLMEngine:
 
         self.seq_id_to_seq_group: Dict[str, SequenceGroupBase] = {}
 
+        # Flag to set when an input fails to process and the engine should run
+        # the next step without re-scheduling.
+        self._skip_scheduling_next_step = False
+
     def _initialize_kv_caches(self) -> None:
         """Initialize the KV cache in the worker(s).
 
@@ -1334,7 +1339,11 @@ class LLMEngine:
         # Skip the scheduler if there are any remaining steps in the seq groups.
         # This ensures that the scheduler is only called again when the current
         # batch has completed.
-        if not self._has_remaining_steps(seq_group_metadata_list):
+        # The scheduler is also skipped if a single request caused the last
+        # engine step to fail, and the previous schedule needs to be rerun.
+        if not self._has_remaining_steps(
+                seq_group_metadata_list
+        ) and not self._skip_scheduling_next_step:
             # Schedule iteration
             (seq_group_metadata_list, scheduler_outputs,
              allow_async_output_proc
@@ -1388,8 +1397,23 @@ class LLMEngine:
                 execute_model_req.async_callback = self.async_callbacks[
                     virtual_engine]
 
-            outputs = self.model_executor.execute_model(
-                execute_model_req=execute_model_req)
+            try:
+                outputs = self.model_executor.execute_model(
+                    execute_model_req=execute_model_req)
+                self._skip_scheduling_next_step = False
+            except InputProcessingError as e:
+                # The input for this request cannot be processed, so we must
+                # abort it. If there are remaining requests in the batch that
+                # have been scheduled, they will be retried on the next step.
+                invalid_request_id = e.request_id
+                self._abort_and_cache_schedule(
+                    request_id=invalid_request_id,
+                    virtual_engine=virtual_engine,
+                    seq_group_metadata_list=seq_group_metadata_list,
+                    scheduler_outputs=scheduler_outputs,
+                    allow_async_output_proc=allow_async_output_proc)
+                # Raise so the caller is notified that this request failed
+                raise
 
             # We need to do this here so that last step's sampled_token_ids can
             # be passed to the next iteration for PP.
@@ -1464,6 +1488,38 @@ class LLMEngine:
 
         return ctx.request_outputs
 
+    def _abort_and_cache_schedule(
+            self, request_id: str, virtual_engine: int,
+            seq_group_metadata_list: List[SequenceGroupMetadata],
+            scheduler_outputs: SchedulerOutputs,
+            allow_async_output_proc: bool) -> None:
+        """Aborts a single request, and caches the scheduler outputs minus that
+        request. This allows the next step to continue processing the remaining
+        requests without having to re-run the scheduler."""
+
+        # Abort the request and remove its sequence group from the current
+        # schedule
+        self.abort_request(request_id)
+        for i, metadata in enumerate(seq_group_metadata_list):
+            if metadata.request_id == request_id:
+                del seq_group_metadata_list[i]
+                break
+        for i, group in enumerate(scheduler_outputs.scheduled_seq_groups):
+            if group.seq_group.request_id == request_id:
+                del scheduler_outputs.scheduled_seq_groups[i]
+                break
+
+        # If there are still other sequence groups left in the schedule, cache
+        # them and flag the engine to reuse the schedule.
+        if len(seq_group_metadata_list) > 0:
+            self._skip_scheduling_next_step = True
+            # Reuse multi-step caching logic
+            self._cache_scheduler_outputs_for_multi_step(
+                virtual_engine=virtual_engine,
+                scheduler_outputs=scheduler_outputs,
+                seq_group_metadata_list=seq_group_metadata_list,
+                allow_async_output_proc=allow_async_output_proc)
+
     def _has_remaining_steps(
         self, seq_group_metadata_list: Optional[List[SequenceGroupMetadata]]
     ) -> bool:

vllm/engine/multiprocessing/engine.py

@@ -27,6 +27,7 @@ from vllm.engine.multiprocessing import (ENGINE_DEAD_ERROR, IPC_DATA_EXT,
 from vllm.logger import init_logger
 from vllm.outputs import RequestOutput
 from vllm.usage.usage_lib import UsageContext
+from vllm.worker.model_runner_base import InputProcessingError
 
 logger = init_logger(__name__)
 
@@ -210,6 +211,14 @@ class MQLLMEngine:
             return self.engine.step()
         except SystemExit:
             raise
+        except InputProcessingError as e:
+            # Special case where we handle an error preparing the inputs for
+            # a single request in the batch
+            rpc_err = RPCError(request_id=e.request_id,
+                               is_engine_errored=False,
+                               exception=e.__cause__)
+            self._send_outputs(rpc_err)
+            return []
         except BaseException as e:
             self._set_errored(e)
             rpc_err = RPCError(request_id=None,

vllm/worker/model_runner.py

@@ -53,8 +53,8 @@ from vllm.utils import (DeviceMemoryProfiler, GiB_bytes, PyObjectCache,
                         is_pin_memory_available, supports_dynamo,
                         weak_ref_tensor)
 from vllm.worker.model_runner_base import (
-    ModelRunnerBase, ModelRunnerInputBase, ModelRunnerInputBuilderBase,
-    _add_attn_metadata_broadcastable_dict,
+    InputProcessingError, ModelRunnerBase, ModelRunnerInputBase,
+    ModelRunnerInputBuilderBase, _add_attn_metadata_broadcastable_dict,
     _add_sampling_metadata_broadcastable_dict,
     _init_attn_metadata_from_tensor_dict,
     _init_sampling_metadata_from_tensor_dict)
@@ -1216,7 +1216,12 @@ class GPUModelRunnerBase(ModelRunnerBase[TModelInputForGPU]):
         """
         self.builder.prepare(finished_requests_ids)
         for seq_group_metadata in seq_group_metadata_list:
-            self.builder.add_seq_group(seq_group_metadata)
+            try:
+                self.builder.add_seq_group(seq_group_metadata)
+            except Exception as e:
+                # Raise an exception that tracks the ID of the bad request
+                raise InputProcessingError(seq_group_metadata.request_id,
+                                           str(e)) from e
 
         self.builder.reset_cached_inter_data()
 

vllm/worker/model_runner_base.py

@@ -261,3 +261,21 @@ class ModelRunnerWrapperBase:
 
     def __getattr__(self, attr):
         return getattr(self.model_runner, attr)
+
+
+class InputProcessingError(Exception):
+    """This exception is raised when an error occurs preparing the inputs for
+    a single sequence group.
+    This allows the engine to gracefully handle errors with a single sequence
+    group without having to fail the entire batch.
+    """
+
+    def __init__(self, request_id, message):
+        """request_id is the id of the offending sequence group"""
+        self.request_id = request_id
+        self.message = message
+        super().__init__(self.message)
+
+    def __str__(self):
+        return "Failed to prepare inputs for sequence group with request id: " \
+                f"{self.request_id}, Error: {self.message}"