[BugFix] Avoid race conditions in zero-copy tensor transmission (#17203)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/v1/test_serial_utils.py

@@ -32,6 +32,7 @@ class MyType:
     large_f_contig_tensor: torch.Tensor
     small_non_contig_tensor: torch.Tensor
     large_non_contig_tensor: torch.Tensor
+    empty_tensor: torch.Tensor
 
 
 def test_encode_decode():
@@ -58,6 +59,7 @@ def test_encode_decode():
         large_f_contig_tensor=torch.rand(1024, 4).t(),
         small_non_contig_tensor=torch.rand(2, 4)[:, 1:3],
         large_non_contig_tensor=torch.rand(1024, 512)[:, 10:20],
+        empty_tensor=torch.empty(0),
     )
 
     encoder = MsgpackEncoder(size_threshold=256)
@@ -193,3 +195,4 @@ def assert_equal(obj1: MyType, obj2: MyType):
                        obj2.small_non_contig_tensor)
     assert torch.equal(obj1.large_non_contig_tensor,
                        obj2.large_non_contig_tensor)
+    assert torch.equal(obj1.empty_tensor, obj2.empty_tensor)

vllm/v1/engine/core.py

@@ -5,6 +5,7 @@ import signal
 import sys
 import threading
 import time
+from collections import deque
 from concurrent.futures import Future
 from inspect import isclass, signature
 from logging import DEBUG
@@ -527,8 +528,12 @@ class EngineCoreProc(EngineCore):
 
         # Msgpack serialization encoding.
         encoder = MsgpackEncoder()
-        # Reuse send buffer.
+        # Send buffers to reuse.
-        buffer = bytearray()
+        reuse_buffers: list[bytearray] = []
+        # Keep references to outputs and buffers until zmq is finished
+        # with them (outputs may contain tensors/np arrays whose
+        # backing buffers were extracted for zero-copy send).
+        pending = deque[tuple[zmq.MessageTracker, Any, bytearray]]()
 
         # We must set linger to ensure the ENGINE_CORE_DEAD
         # message is sent prior to closing the socket.
@@ -541,8 +546,22 @@ class EngineCoreProc(EngineCore):
                     break
                 assert not isinstance(outputs, bytes)
                 outputs.engine_index = engine_index
+
+                # Reclaim buffers that zmq is finished with.
+                while pending and pending[-1][0].done:
+                    reuse_buffers.append(pending.pop()[2])
+
+                buffer = reuse_buffers.pop() if reuse_buffers else bytearray()
                 buffers = encoder.encode_into(outputs, buffer)
-                socket.send_multipart(buffers, copy=False)
+                tracker = socket.send_multipart(buffers,
+                                                copy=False,
+                                                track=True)
+                if not tracker.done:
+                    ref = outputs if len(buffers) > 1 else None
+                    pending.appendleft((tracker, ref, buffer))
+                elif len(reuse_buffers) < 2:
+                    # Keep at most 2 buffers to reuse.
+                    reuse_buffers.append(buffer)
 
 
 class DPEngineCoreProc(EngineCoreProc):

vllm/v1/engine/core_client.py

@@ -1,9 +1,11 @@
 # SPDX-License-Identifier: Apache-2.0
 import asyncio
+import contextlib
 import queue
 import uuid
 import weakref
 from abc import ABC, abstractmethod
+from collections import deque
 from collections.abc import Awaitable, Sequence
 from concurrent.futures import Future
 from dataclasses import dataclass, field
@@ -396,6 +398,12 @@ class MPClient(EngineCoreClient):
             self._wait_for_engine_startup()
 
             self.utility_results: dict[int, AnyFuture] = {}
+
+            # Request objects which may contain pytorch-allocated tensors
+            # that we need to keep references to until zmq is done with the
+            # underlying data.
+            self.pending_messages = deque[tuple[zmq.MessageTracker, Any]]()
+
             success = True
         finally:
             if not success:
@@ -459,6 +467,14 @@ class MPClient(EngineCoreClient):
         if self.resources.engine_dead:
             raise EngineDeadError()
 
+    def add_pending_message(self, tracker: zmq.MessageTracker, msg: Any):
+        if not tracker.done:
+            self.pending_messages.appendleft((tracker, msg))
+
+    def free_pending_messages(self):
+        while self.pending_messages and self.pending_messages[-1][0].done:
+            self.pending_messages.pop()
+
 
 def _process_utility_output(output: UtilityOutput,
                             utility_results: dict[int, AnyFuture]):
@@ -544,10 +560,18 @@ class SyncMPClient(MPClient):
 
     def _send_input(self, request_type: EngineCoreRequestType, request: Any):
         self.ensure_alive()
+        self.free_pending_messages()
         # (Identity, RequestType, SerializedRequest)
         msg = (self.core_engine.identity, request_type.value,
                *self.encoder.encode(request))
-        self.input_socket.send_multipart(msg, copy=False)
+
+        if len(msg) <= 3:
+            # No auxiliary buffers => no tensor backing buffers in request.
+            self.input_socket.send_multipart(msg, copy=False)
+            return
+
+        tracker = self.input_socket.send_multipart(msg, copy=False, track=True)
+        self.add_pending_message(tracker, request)
 
     def call_utility(self, method: str, *args) -> Any:
         call_id = uuid.uuid1().int >> 64
@@ -698,19 +722,38 @@ class AsyncMPClient(MPClient):
     def _send_input(self,
                     request_type: EngineCoreRequestType,
                     request: Any,
-                    engine: Optional[CoreEngine] = None) -> Awaitable[None]:
+                    engine: Optional[CoreEngine] = None) -> Awaitable[Any]:
         self.ensure_alive()
         if engine is None:
             engine = self.core_engine
 
         message = (request_type.value, *self.encoder.encode(request))
-        return self._send_input_message(message, engine)
+        return self._send_input_message(message, engine, request)
 
-    def _send_input_message(self, message: tuple[bytestr, ...],
+    def _send_input_message(self, message: tuple[bytestr,
-                            engine: CoreEngine) -> Awaitable[None]:
+                                                 ...], engine: CoreEngine,
+                            objects: Any) -> Awaitable[Any]:
+        """
+        objects is a reference to retain until zmq is finished with the
+        buffers, in case they were extracted from tensors in the request.
+        """
         self.ensure_alive()
-        message = (engine.identity, ) + message
+        self.free_pending_messages()
-        return self.input_socket.send_multipart(message, copy=False)
+
+        msg = (engine.identity, ) + message
+        if not objects or len(msg) <= 3:
+            # No auxiliary buffers => no tensor backing buffers in request.
+            return self.input_socket.send_multipart(msg, copy=False)
+
+        future: asyncio.Future[zmq.MessageTracker]
+        future = self.input_socket.send_multipart(msg, copy=False, track=True)
+
+        def add_pending(f: asyncio.Future[zmq.MessageTracker]):
+            with contextlib.suppress(BaseException):
+                self.add_pending_message(f.result(), objects)
+
+        future.add_done_callback(add_pending)
+        return future
 
     async def call_utility_async(self, method: str, *args) -> Any:
         return await self._call_utility_async(method,
@@ -724,7 +767,7 @@ class AsyncMPClient(MPClient):
         self.utility_results[call_id] = future
         message = (EngineCoreRequestType.UTILITY.value, *self.encoder.encode(
             (call_id, method, args)))
-        await self._send_input_message(message, engine)
+        await self._send_input_message(message, engine, args)
         self._ensure_output_queue_task()
         return await future