fix：将 VLLM_ENABLE_OUTPUT_PLACEHOLDERS 替换为 VLLM_SCHED_ENABLE_MINIMAL_INJECTION，以改进调度逻辑

- 更新环境变量以控制流水线并行调度的最小注入。
- 从 Request 类中移除 num_output_placeholders，并调整 Scheduler 逻辑以使用新的最小注入功能。
- 增强 Scheduler，根据批次队列状态管理最小进度注入。

vllm/envs.py

@@ -175,7 +175,7 @@ if TYPE_CHECKING:
     USE_FUSED_SILU_MUL_QUANT: bool = False
     VLLM_P2P_ASYNC: bool = False
     VLLM_P2P_BUF_TOKENS: int = 30000
-    VLLM_ENABLE_OUTPUT_PLACEHOLDERS: bool = False
+    VLLM_SCHED_ENABLE_MINIMAL_INJECTION: bool = False
 
 def get_default_cache_root():
     return os.getenv(
@@ -1143,9 +1143,10 @@ environment_variables: dict[str, Callable[[], Any]] = {
     # pd separation p2p async buf tokens
     "VLLM_P2P_BUF_TOKENS":
     lambda: int(os.getenv("VLLM_P2P_BUF_TOKENS", "30000")),
-    # vllm will enable output placeholders
-    "VLLM_ENABLE_OUTPUT_PLACEHOLDERS":
-    lambda: bool(int(os.getenv("VLLM_ENABLE_OUTPUT_PLACEHOLDERS", "0"))),
+    # vllm will enable minimal injection for pipeline parallel scheduling
+    "VLLM_SCHED_ENABLE_MINIMAL_INJECTION":
+        lambda: (os.getenv("VLLM_SCHED_ENABLE_MINIMAL_INJECTION", "0").lower() in
+                 ("true", "1")),
 }
 
 # --8<-- [end:env-vars-definition]

vllm/v1/core/sched/interface.py

@@ -148,3 +148,6 @@ class SchedulerInterface(ABC):
 
     def get_kv_connector(self) -> Optional["KVConnectorBase_V1"]:
         return None
+
+    def set_allow_minimal_injection(self, allow: bool) -> None:  
+        return
\ No newline at end of file

vllm/v1/core/sched/scheduler.py

@@ -75,8 +75,7 @@ class Scheduler(SchedulerInterface):
         self.enable_kv_cache_events = (
             self.kv_events_config is not None
             and self.kv_events_config.enable_kv_cache_events)
-        # Cache the output placeholders feature flag to avoid repeated envs access
-        self.enable_output_placeholders = envs.VLLM_ENABLE_OUTPUT_PLACEHOLDERS
+        self.enable_minimal_injection = envs.VLLM_SCHED_ENABLE_MINIMAL_INJECTION
 
         # Create KVConnector for the Scheduler. Note that each Worker
         # will have a corresponding KVConnector with Role=WORKER.
@@ -165,6 +164,11 @@ class Scheduler(SchedulerInterface):
             enable_kv_cache_events=self.enable_kv_cache_events,
         )
         self.use_pp = self.parallel_config.pipeline_parallel_size > 1
+        self.allow_minimal_injection: bool = False
+
+    # Engine hint to gate minimal-progress injection (0-token breaker)
+    def set_allow_minimal_injection(self, allow: bool) -> None:
+        self.allow_minimal_injection = allow
 
     def schedule_default(self) -> SchedulerOutput:
         # NOTE(woosuk) on the scheduling algorithm:
@@ -208,11 +212,8 @@ class Scheduler(SchedulerInterface):
         while req_index < len(self.running) and token_budget > 0:
             request = self.running[req_index]
 
-            # Include output placeholders so that decode keeps a minimal
-            # backlog, avoiding 0-token plateaus when resuming from WAITING
-            # or immediately after prefill finishes.
-            num_new_tokens = (request.num_tokens_with_spec +
-                              request.num_output_placeholders -
+
+            num_new_tokens = (request.num_tokens_with_spec  -
                               request.num_computed_tokens)
             if (0 < self.scheduler_config.long_prefill_token_threshold <
                     num_new_tokens):
@@ -243,9 +244,21 @@ class Scheduler(SchedulerInterface):
                 #    not finished yet.
                 # 2. The encoder budget is exhausted.
                 # 3. The encoder cache is exhausted.
-                # Allow lower-priority requests to be scheduled in this step.
-                req_index += 1
-                continue
+                if (self.use_pp and self.enable_minimal_injection and self.allow_minimal_injection
+                        and len(self.waiting) == 0 and token_budget > 0):
+                    # Ensure we are truly at a boundary (no target tokens)
+                    # and have room to advance without exceeding model len.
+                    if (request.num_tokens_with_spec
+                            == request.num_computed_tokens):
+                        max_additional = (self.max_model_len - 1
+                                          - request.num_computed_tokens)
+                        if max_additional > 0:
+                            # Advance with a single decode token.
+                            num_new_tokens = 1
+                if num_new_tokens == 0:
+                    # Still cannot schedule; allow lower-priority requests.
+                    req_index += 1
+                    continue
 
             num_draft_tokens = max(
                 num_new_tokens + request.num_computed_tokens -
@@ -811,11 +824,8 @@ class Scheduler(SchedulerInterface):
             while req_index < len(self.running) and token_budget > 0:
                 request = self.running[req_index]
 
-                # Include output placeholders to maintain a minimal backlog
-                # for decode, avoiding 0-token plateaus after prefill or when
-                # resuming from WAITING under PP/speculative paths.
-                num_new_tokens = (request.num_tokens_with_spec +
-                                  request.num_output_placeholders -
+               
+                num_new_tokens = (request.num_tokens_with_spec -
                                   request.num_computed_tokens)
                 if (0 < self.scheduler_config.long_prefill_token_threshold <
                         num_new_tokens):
@@ -1035,15 +1045,6 @@ class Scheduler(SchedulerInterface):
             request = self.requests[req_id]
             request.num_computed_tokens += num_scheduled_token
             
-        # 在推进已计算的 token 数量之后，对于本步骤将生成新 token 的请求，
-        # 增加一个输出占位符。这将创建一个小的赤字，确保即使在赤字本来为0的边界处，
-        # 随后的调度步骤也能继续安排解码任务。
-        if self.enable_output_placeholders:
-            for req_id in scheduler_output.num_scheduled_tokens:
-                request = self.requests[req_id]
-                if (request.num_computed_tokens
-                        == request.num_tokens + request.num_output_placeholders):
-                    request.num_output_placeholders += 1
 
         # Clear the finished request IDs.
         # NOTE: We shouldn't do self.finished_req_ids.clear() here because
@@ -1263,12 +1264,6 @@ class Scheduler(SchedulerInterface):
                 # NOTE: once we support N tokens per step (spec decode),
                 # the outer lists can be of length > 1.
                 new_logprobs = logprobs.slice(req_index, req_index + 1)
-                
-            # 按照本步实际生成的token数量（如果有），减少输出占位符的数量。
-            if self.enable_output_placeholders:
-                if new_token_ids:
-                    request.num_output_placeholders = max(
-                        0, request.num_output_placeholders - len(new_token_ids))
 
             if new_token_ids and self.structured_output_manager.should_advance(
                     request):

vllm/v1/engine/core.py

@@ -265,6 +265,14 @@ class EngineCore:
         # the scheduler may return an empty batch if all requests are scheduled.
         # Note that this is not blocking.
         if not self.batch_queue.full():
+            # Tell the scheduler to try minimal progress injection only when there is no running batch (queue is empty),
+            # to avoid unnecessary 0-token batches while a batch is already running.
+            try:
+                self.scheduler.set_allow_minimal_injection(
+                    self.batch_queue.empty())
+            except Exception:
+                # Schedulers without this hook simply ignore it.
+                pass
             scheduler_output = self.scheduler.schedule()
             if scheduler_output.total_num_scheduled_tokens > 0:
                 future = self.model_executor.execute_model(scheduler_output)

vllm/v1/request.py

@@ -82,11 +82,6 @@ class Request:
         self.num_generated_token_ids = 0
         self.cache_salt: Optional[str] = cache_salt
         
-        # 用于异步调度，确保解码过程能够持续推进：
-        # “输出占位符”的数量，表示我们预计即将生成的token数量。
-        # 其作用类似一个小的积压队列，用来避免在从WAITING状态恢复或解码初始时，
-        # 因待生成token数量为0而导致调度停滞的情况。
-        self.num_output_placeholders = 0
 
         # Multi-modal related
         self.mm_positions = multi_modal_placeholders or []