[Bugfix][NIXL] Fix Async Scheduler timeout issue (#25808)

Signed-off-by: NickLucche <nlucches@redhat.com>

Author: NickLucche

vllm/distributed/kv_transfer/kv_connector/v1/nixl_connector.py

@@ -105,6 +105,7 @@ def __init__(self):
         self.reqs_to_recv: dict[ReqId, ReqMeta] = {}
         self.reqs_to_save: dict[ReqId, ReqMeta] = {}
         self.reqs_to_send: dict[ReqId, float] = {}
+        self.reqs_in_batch: set[ReqId] = set()
 
     def add_new_req(
         self,
@@ -278,6 +279,7 @@ def __init__(self, vllm_config: VllmConfig, engine_id: str):
         self._reqs_need_save: dict[ReqId, tuple[Request, list[int]]] = {}
         # Reqs to send and their expiration time
         self._reqs_need_send: dict[ReqId, float] = {}
+        self._reqs_in_batch: set[ReqId] = set()
 
     def get_num_new_matched_tokens(
             self, request: "Request",
@@ -324,6 +326,9 @@ def update_state_after_alloc(self, request: "Request",
 
         if not params:
             return
+
+        if params.get("do_remote_decode"):
+            self._reqs_in_batch.add(request.request_id)
         if self.use_host_buffer and params.get("do_remote_decode"):
             # NOTE: when accelerator is not directly supported by Nixl,
             # prefilled blocks need to be saved to host memory before transfer.
@@ -373,6 +378,8 @@ def build_connector_meta(
                 request_id=req_id,
                 local_block_ids=block_ids,
                 kv_transfer_params=req.kv_transfer_params,
+                load_remote_cache=True,
+                save_to_host=False,
             )
 
         for req_id, (req, block_ids) in self._reqs_need_save.items():
@@ -386,10 +393,12 @@ def build_connector_meta(
             )
 
         meta.reqs_to_send = self._reqs_need_send
+        meta.reqs_in_batch = self._reqs_in_batch
 
         # Clear the list once workers start the transfers
         self._reqs_need_recv.clear()
         self._reqs_need_save.clear()
+        self._reqs_in_batch = set()
         self._reqs_need_send = {}
 
         return meta
@@ -546,6 +555,8 @@ def __init__(self, vllm_config: VllmConfig, engine_id: str):
         self._recving_transfers = defaultdict[ReqId, list[Transfer]](list)
         # Track the expiration time of requests that are waiting to be sent.
         self._reqs_to_send: dict[ReqId, float] = {}
+        # Set of requests that have been part of a batch, regardless of status.
+        self._reqs_to_process: set[ReqId] = set()
 
         # Background thread for handling new handshake requests.
         self._nixl_handshake_listener_t: Optional[threading.Thread] = None
@@ -1082,6 +1093,7 @@ def get_finished(self) -> tuple[set[str], set[str]]:
                 "Releasing expired KV blocks for request %s which were "
                 "retrieved by %d decode worker(s) within %d seconds.", req_id,
                 count, envs.VLLM_NIXL_ABORT_REQUEST_TIMEOUT)
+            self._reqs_to_process.remove(req_id)
             del self._reqs_to_send[req_id]
             done_sending.add(req_id)
 
@@ -1097,7 +1109,8 @@ def _get_new_notifs(self) -> set[str]:
         for notifs in self.nixl_wrapper.get_new_notifs().values():
             for notif in notifs:
                 req_id, tp_ratio = notif.decode("utf-8").rsplit(":", 1)
-                if req_id not in self._reqs_to_send:
+                if (req_id not in self._reqs_to_send
+                        and req_id not in self._reqs_to_process):
                     logger.error(
                         "Potentially invalid KV blocks for "
                         "unrecognized request %s were retrieved by "
@@ -1110,7 +1123,8 @@ def _get_new_notifs(self) -> set[str]:
                         tp_ratio):
                     notified_req_ids.add(req_id)
                     del self.consumer_notification_counts_by_req[req_id]
-                    del self._reqs_to_send[req_id]
+                    self._reqs_to_process.remove(req_id)
+                    self._reqs_to_send.pop(req_id, None)
         return notified_req_ids
 
     def _pop_done_transfers(
@@ -1171,8 +1185,19 @@ def start_load_kv(self, metadata: NixlConnectorMetadata):
         while not self._ready_requests.empty():
             self._read_blocks_for_req(*self._ready_requests.get_nowait())
 
+        # Keep around the requests that have been part of a batch. This is
+        # needed because async scheduling pushes the misalignment between the
+        # moment in which requests expiration is set (P side) and the moment in
+        # which blocks are read from D. As P can now more easily lag behind D
+        # while processing the next batch, we make sure to only set an
+        # expiration for requests that have not been read from D yet.
+        for req_id in metadata.reqs_in_batch:
+            self._reqs_to_process.add(req_id)
+
         # Add to requests that are waiting to be read and track expiration.
-        self._reqs_to_send.update(metadata.reqs_to_send)
+        for req_id, expiration_time in metadata.reqs_to_send.items():
+            if req_id in self._reqs_to_process:
+                self._reqs_to_send[req_id] = expiration_time
 
     def _read_blocks_for_req(self, req_id: str, meta: ReqMeta):
         logger.debug(