fix(sglang): populate total_kv_blocks via canonical Engine scheduler path (#8439)

components/src/dynamo/sglang/_compat.py

@@ -133,6 +133,38 @@ async def mm_encode(encoder: Any, mm_items: Any, modality: Any) -> tuple:
     return result
 
 
+def get_scheduler_info(engine: Any) -> dict:
+    """Return the scheduler-info dict for rank-0 of an ``sgl.Engine``.
+
+    SGLang exposes per-rank scheduler stats (``max_total_num_tokens``,
+    ``max_req_input_len``, ...) on the ``Engine`` via ``_scheduler_init_result``.
+    We return the rank-0 dict, or ``{}`` if it is not reachable on this build.
+
+    Covers:
+      - sglang 0.5.10+: ``engine._scheduler_init_result.scheduler_infos[0]``
+        (canonical; also what ``Engine.get_server_info`` reads internally).
+      - Older probed attributes (``engine.scheduler_info``,
+        ``engine.tokenizer_manager.scheduler_info``) as a best-effort fallback
+        for forks/experimental branches that surfaced the dict directly.
+    """
+    result = getattr(engine, "_scheduler_init_result", None)
+    if result is not None:
+        infos = getattr(result, "scheduler_infos", None)
+        if infos:
+            return infos[0]
+
+    direct = getattr(engine, "scheduler_info", None)
+    if direct:
+        return direct
+
+    tm = getattr(engine, "tokenizer_manager", None)
+    tm_info = getattr(tm, "scheduler_info", None) if tm is not None else None
+    if tm_info:
+        return tm_info
+
+    return {}
+
+
 def enable_disjoint_streaming_output(server_args: Any) -> None:
     """
     Enable SGLang's disjoint streaming output across ServerArgs field renames.
@@ -170,6 +202,7 @@ __all__ = [
     "NetworkAddress",
     "enable_disjoint_streaming_output",
     "get_local_ip_auto",
+    "get_scheduler_info",
     "get_zmq_socket",
     "mm_encode",
 ]

components/src/dynamo/sglang/llm_engine.py

@@ -25,6 +25,7 @@ from dynamo.common.backend.engine import (
 from dynamo.common.backend.worker import WorkerConfig
 from dynamo.common.utils.input_params import InputParamManager
 from dynamo.llm import ModelInput
+from dynamo.sglang._compat import get_scheduler_info
 from dynamo.sglang.args import parse_args
 
 logger = logging.getLogger(__name__)
@@ -73,7 +74,7 @@ class SglangLLMEngine(LLMEngine):
         # Capacity fields -- sourced the same way as register.py in the
         # non-unified path so the Rust runtime gets consistent values.
         total_kv_blocks = None
-        scheduler_info = getattr(self.engine, "scheduler_info", None) or {}
+        scheduler_info = get_scheduler_info(self.engine)
         max_total_tokens = scheduler_info.get("max_total_num_tokens")
         page_size = self.server_args.page_size
         if max_total_tokens and page_size:

components/src/dynamo/sglang/register.py

@@ -11,7 +11,7 @@ from sglang.srt.server_args import ServerArgs
 from dynamo._core import Endpoint
 from dynamo.common.utils.output_modalities import get_output_modalities
 from dynamo.llm import ModelInput, ModelRuntimeConfig, ModelType, register_model
-from dynamo.sglang._compat import NetworkAddress, get_local_ip_auto
+from dynamo.sglang._compat import NetworkAddress, get_local_ip_auto, get_scheduler_info
 from dynamo.sglang.args import DynamoConfig
 
 
@@ -170,12 +170,11 @@ async def _get_runtime_config(
         runtime_config.enable_eagle = True
 
     try:
-        # Try to check if the engine has a scheduler attribute with the computed values
-        if hasattr(engine, "scheduler_info") and engine.scheduler_info is not None:
-            # Get max_total_num_tokens from scheduler_info
-            max_total_tokens = engine.scheduler_info.get("max_total_num_tokens")
-            if max_total_tokens and hasattr(engine.tokenizer_manager, "server_args"):
-                page_size = engine.tokenizer_manager.server_args.page_size
+        scheduler_info = get_scheduler_info(engine)
+        max_total_tokens = scheduler_info.get("max_total_num_tokens")
+
+        if max_total_tokens:
+            page_size = server_args.page_size
             if page_size:
                 runtime_config.total_kv_blocks = (
                     max_total_tokens + page_size - 1
@@ -188,21 +187,21 @@ async def _get_runtime_config(
             # When max_prefill_tokens is not explicitly set by the user, fall back
             # to max_total_num_tokens from the scheduler. This ensures the planner
             # always has a prefill load signal for aggregated scaling decisions.
-            if not max_prefill_tokens and max_total_tokens:
+            if not max_prefill_tokens:
                 runtime_config.max_num_batched_tokens = max_total_tokens
                 logging.info(
                     f"max_prefill_tokens not set, using max_total_num_tokens "
                     f"from scheduler as max_num_batched_tokens: {max_total_tokens}"
                 )
-
-            return runtime_config
-
-        # If scheduler approach doesn't work, log and return None to indicate we'll skip runtime config
+        else:
+            unpublished = "total_kv_blocks"
+            if not max_prefill_tokens:
+                unpublished += " and max_num_batched_tokens"
             logging.warning(
-            "Could not access runtime config from SGLang engine. "
-            "The engine may compute these values internally after initialization. "
-            "Proceeding without runtime config - SGLang will use its internal defaults."
+                f"Could not access scheduler info from SGLang engine. "
+                f"{unpublished} will not be published; SGLang will use its internal defaults."
             )
+
         return runtime_config
 
     except Exception as e: