fix(kv-router): correct decode-worker scoring for unregistered workers in selector (#7919)

Signed-off-by: Matej Kosec <mkosec@nvidia.com>

components/src/dynamo/common/configuration/groups/kv_router_args.py

@@ -256,7 +256,8 @@ class KvRouterArgGroup(ArgGroup):
             help=(
                 "KV Router: Queue threshold fraction for prefill token capacity. "
                 "Requests are queued if all workers exceed this fraction of "
-                "max_num_batched_tokens. Must be > 0."
+                "max_num_batched_tokens. Must be >= 0. Use 0.0 for maximum "
+                "queueing sensitivity (queue as soon as any tokens are active)."
             ),
             arg_type=float,
         )

lib/kv-router/src/scheduling/config.rs

@@ -187,7 +187,7 @@ pub struct KvRouterConfig {
     /// Queue threshold fraction for prefill token capacity.
     /// When set, requests are queued if all workers exceed this fraction of max_num_batched_tokens.
     /// If None, queueing is disabled and all requests go directly to ready.
-    /// Default: 2.0. Must be > 0.
+    /// Default: 4.0. Must be >= 0. Use 0.0 for maximum queueing sensitivity.
     #[validate(range(min = 0.0))]
     pub router_queue_threshold: Option<f64>,
 

lib/kv-router/src/scheduling/selector.rs

@@ -145,7 +145,12 @@ impl<C: WorkerConfigLike> WorkerSelector<C> for DefaultWorkerSelector {
 
         let get_score = |worker: WorkerWithDpRank| -> f64 {
             let overlap = *overlaps.get(&worker).unwrap_or(&0);
-            let prefill_token = *prefill_tokens.get(&worker).unwrap_or(&isl);
+            // Use 0 for unregistered decode workers (track_prefill_tokens=false)
+            // to match registered idle workers; use isl otherwise.
+            let default_prefill_token = if request.track_prefill_tokens { isl } else { 0 };
+            let prefill_token = *prefill_tokens
+                .get(&worker)
+                .unwrap_or(&default_prefill_token);
             let potential_prefill_block = (prefill_token as f64) / (block_size as f64);
             let decode_block = *decode_blocks
                 .get(&worker)