fix(llm): preserve unresolved dp rank routing (#8000)

Signed-off-by: PeaBrane <yanrpei@gmail.com>

components/src/dynamo/sglang/request_handlers/llm/prefill_handler.py

@@ -13,8 +13,9 @@ from dynamo.sglang.args import Config
 from dynamo.sglang.publisher import DynamoSglangPublisher
 from dynamo.sglang.request_handlers.handler_base import BaseWorkerHandler
 
-# Sentinel value matching u32::MAX from prefill_router.rs SimpleRouter path,
-# indicating no specific data-parallel rank was selected.
+# Sentinel value matching u32::MAX from the C/Go prefill-routing ABI.
+# This remains as a compatibility fallback for older callers that still encode
+# an unresolved data-parallel rank in-band instead of omitting the field.
 _DP_RANK_UNSET = 2**32 - 1
 
 

deploy/inference-gateway/epp/pkg/plugins/disagg/prefill_scorer.go

@@ -120,7 +120,6 @@ func (s *DynPrefillScorer) Score(ctx context.Context, cycleState *schedtypes.Cyc
 	}
 
 	prefillWorkerID := strconv.FormatUint(result.WorkerID, 10)
-	prefillDpRank := strconv.FormatUint(uint64(result.DpRank), 10)
 	logger.V(logutil.DEFAULT).Info("DynPrefillScorer: prefill worker selected",
 		"prefillWorkerID", prefillWorkerID,
 		"prefillDpRank", result.DpRank,
@@ -134,7 +133,11 @@ func (s *DynPrefillScorer) Score(ctx context.Context, cycleState *schedtypes.Cyc
 		req.Headers = map[string]string{}
 	}
 	req.Headers[PrefillWorkerIDHeader] = prefillWorkerID
-	req.Headers[PrefillDpRankHeader] = prefillDpRank
+	if result.DpRank != dynscorer.UnsetDpRank {
+		req.Headers[PrefillDpRankHeader] = strconv.FormatUint(uint64(result.DpRank), 10)
+	} else {
+		delete(req.Headers, PrefillDpRankHeader)
+	}
 
 	// Score: 1.0 for all pods. The label-filter has already restricted to prefill workers,
 	// and the FFI router's internal selection is authoritative.

deploy/inference-gateway/epp/pkg/plugins/dynamo_kv_scorer/plugin.go

@@ -124,6 +124,10 @@ var (
 	routerHandlesMutex sync.RWMutex
 )
 
+// UnsetDpRank is the ABI sentinel used by the Rust C bindings when a prefill
+// route selected a worker but left the DP rank unresolved.
+const UnsetDpRank = ^uint32(0)
+
 func loadDynamoConfig() {
 	ffiNamespace = getEnvOrDefault("DYN_NAMESPACE_PREFIX", getEnvOrDefault("DYN_NAMESPACE", "vllm-agg"))
 	ffiComponent = "backend" // This is not the same as DYN_COMPONENT=epp (in this case)

lib/llm/src/discovery/worker_monitor.rs

@@ -27,6 +27,7 @@ use dynamo_runtime::transports::event_plane::EventSubscriber;
 
 // Re-export worker type constants from timing.rs (single source of truth)
 pub use crate::protocols::common::timing::{WORKER_TYPE_DECODE, WORKER_TYPE_PREFILL};
+const UNSET_DP_RANK_LABEL: &str = "none";
 
 /// Clean up all Prometheus metrics for a worker across the specified dp_ranks.
 ///
@@ -44,6 +45,11 @@ fn cleanup_worker_metrics(worker_id: u64, dp_ranks: &[u32], worker_type: &str) {
         let _ = WORKER_LAST_INPUT_SEQUENCE_TOKENS_GAUGE.remove_label_values(labels);
         let _ = WORKER_LAST_INTER_TOKEN_LATENCY_GAUGE.remove_label_values(labels);
     }
+
+    let unset_labels = &[worker_id_str.as_str(), UNSET_DP_RANK_LABEL, worker_type];
+    let _ = WORKER_LAST_TIME_TO_FIRST_TOKEN_GAUGE.remove_label_values(unset_labels);
+    let _ = WORKER_LAST_INPUT_SEQUENCE_TOKENS_GAUGE.remove_label_values(unset_labels);
+    let _ = WORKER_LAST_INTER_TOKEN_LATENCY_GAUGE.remove_label_values(unset_labels);
 }
 
 /// Scale factor for storing f64 thresholds as u32 (10000 = 4 decimal places)

lib/llm/src/http/service/metrics.rs

@@ -42,6 +42,7 @@ use super::RouteDoc;
 
 /// Worker type label values for Prometheus timing metrics
 pub use crate::discovery::{WORKER_TYPE_DECODE, WORKER_TYPE_PREFILL};
+const UNSET_DP_RANK_LABEL: &str = "none";
 
 /// Global Prometheus gauge for last observed TTFT per worker (in seconds)
 /// Labels: worker_id, dp_rank, worker_type
@@ -1342,7 +1343,7 @@ impl ResponseMetricCollector {
                 let worker_id_str = worker_id.to_string();
                 let dp_rank_str = self
                     .prefill_dp_rank
-                    .map_or("0".to_string(), |r| r.to_string());
+                    .map_or(UNSET_DP_RANK_LABEL.to_string(), |r| r.to_string());
                 let worker_type = self
                     .prefill_worker_type
                     .as_deref()
@@ -1385,7 +1386,7 @@ impl ResponseMetricCollector {
                 let worker_id_str = worker_id.to_string();
                 let dp_rank_str = self
                     .decode_dp_rank
-                    .map_or("0".to_string(), |r| r.to_string());
+                    .map_or(UNSET_DP_RANK_LABEL.to_string(), |r| r.to_string());
                 let worker_type = self
                     .decode_worker_type
                     .as_deref()

lib/llm/src/kv_router.rs

@@ -131,6 +131,7 @@ where
 {
     indexer: Indexer,
     scheduler: KvScheduler<Sel>,
+    workers_with_configs: RuntimeConfigWatch,
     block_size: u32,
     kv_router_config: KvRouterConfig,
     prefill_load_estimator: Option<Arc<dyn PrefillLoadEstimator>>,
@@ -230,6 +231,7 @@ where
         Ok(Self {
             indexer,
             scheduler,
+            workers_with_configs,
             block_size,
             kv_router_config,
             prefill_load_estimator,
@@ -473,6 +475,13 @@ where
         self.scheduler.worker_type()
     }
 
+    /// Return the worker's unique global DP rank when it owns exactly one rank.
+    pub fn unique_dp_rank_for_worker(&self, worker_id: WorkerId) -> Option<u32> {
+        let configs = self.workers_with_configs.borrow();
+        let config = configs.get(&worker_id)?;
+        (config.data_parallel_size == 1).then_some(config.data_parallel_start_rank)
+    }
+
     pub fn add_output_block(
         &self,
         request_id: &str,

lib/llm/src/kv_router/prefill_router/execution.rs

@@ -123,7 +123,7 @@ impl PrefillRouter {
     ///
     /// If `phase_transition_permit` is provided, it is dropped immediately after routing completes,
     /// allowing subsequent `set_phase` calls to proceed. This preserves the current synchronization:
-    /// the prefill route must finish `record_worker_full` before the phase can change to Decode.
+    /// the prefill route must finish worker recording before the phase can change to Decode.
     ///
     /// Returns (PrefillResult, Option<(worker_id, dp_rank)>).
     pub(super) async fn execute_prefill(
@@ -131,7 +131,7 @@ impl PrefillRouter {
         request: SingleIn<PreprocessedRequest>,
         target_worker: Option<u64>,
         phase_transition_permit: Option<OwnedSemaphorePermit>,
-    ) -> Result<(PrefillResult, Option<(u64, u32)>), PrefillError> {
+    ) -> Result<(PrefillResult, Option<(u64, Option<u32>)>), PrefillError> {
         let router = router.ok_or(PrefillError::NotActivated)?;
         let mut prefill_response = router
             .generate_to_worker(request, target_worker)
@@ -143,7 +143,7 @@ impl PrefillRouter {
                 )
             })?;
 
-        // Release the phase barrier now that routing completed and record_worker_full already ran.
+        // Release the phase barrier now that routing completed and worker recording already ran.
         // Decode may proceed without waiting for prefill output streaming to finish.
         drop(phase_transition_permit);
 
@@ -201,8 +201,7 @@ impl PrefillRouter {
                     let dp_rank = worker_id_json
                         .get("prefill_dp_rank")
                         .and_then(|v| v.as_u64())
-                        .map(|r| r as u32)
-                        .unwrap_or(0);
+                        .map(|r| r as u32);
                     Some((worker_id, dp_rank))
                 });
         Ok((

lib/llm/src/kv_router/prefill_router/mod.rs

@@ -148,7 +148,7 @@ impl
                     link_child_context(&engine_ctx, prefill_req, request_id.as_str());
 
                 // Pass the phase barrier to the spawned task. It is released after routing
-                // completes so `record_worker_full` finishes before phase changes to Decode.
+                // completes so worker recording finishes before phase changes to Decode.
                 self.spawn_prefill_task(prefill_context, Some(worker_id), prefill_phase_barrier);
 
                 Ok(PrefillOutcome::Bootstrap(bootstrap_info))

lib/llm/src/kv_router/push_router.rs

@@ -34,8 +34,9 @@ pub struct KvPushRouter {
 /// Result of worker selection containing instance ID, dp_rank, and overlap amount.
 struct WorkerSelection {
     instance_id: u64,
-    dp_rank: u32,
-    overlap_amount: u32,
+    backend_dp_rank: Option<u32>,
+    bookkeeping_dp_rank: Option<u32>,
+    overlap_amount: Option<u32>,
 }
 
 /// Drop guard that manages the full lifecycle of a routed request:
@@ -46,6 +47,7 @@ struct WorkerSelection {
 /// `Drop` impl fires and spawns a task to call `free()`.
 struct RequestGuard {
     chooser: Arc<KvRouter>,
+    scheduler_tracked: bool,
     context_id: String,
     tracker: Option<Arc<RequestTracker>>,
     request_metrics: Arc<RouterRequestMetrics>,
@@ -70,7 +72,9 @@ impl RequestGuard {
                 .map(|d| !d.token_ids.is_empty())
                 .unwrap_or(false);
             if has_tokens {
-                if let Err(e) = self.chooser.mark_prefill_completed(&self.context_id).await {
+                if self.scheduler_tracked
+                    && let Err(e) = self.chooser.mark_prefill_completed(&self.context_id).await
+                {
                     tracing::warn!(
                         "Failed to mark prefill completed for request {}: {e}",
                         self.context_id
@@ -130,7 +134,9 @@ impl RequestGuard {
 
     async fn finish(&mut self) {
         self.record_metrics();
-        if let Err(e) = self.chooser.free(&self.context_id).await {
+        if self.scheduler_tracked
+            && let Err(e) = self.chooser.free(&self.context_id).await
+        {
             tracing::warn!("Failed to free request {}: {e}", self.context_id);
         }
         self.freed = true;
@@ -155,7 +161,7 @@ impl RequestGuard {
 impl Drop for RequestGuard {
     fn drop(&mut self) {
         self.record_metrics();
-        if !self.freed {
+        if !self.freed && self.scheduler_tracked {
             let chooser = self.chooser.clone();
             let context_id = self.context_id.clone();
             let Ok(handle) = tokio::runtime::Handle::try_current() else {
@@ -198,7 +204,6 @@ impl KvPushRouter {
         let routing = request.routing.as_ref();
         let lora_name = routing.and_then(|r| r.lora_name.clone());
         let priority_jump = routing.and_then(|r| r.priority_jump).unwrap_or(0.0);
-        let dp_rank = routing.and_then(|r| r.dp_rank).unwrap_or(0);
         let expected_output_tokens = routing.and_then(|r| r.expected_output_tokens);
         let allowed_worker_ids = routing.and_then(|r| r.allowed_worker_ids.clone());
         let (routing_token_ids, block_mm_infos) = request.block_mm_routing_info();
@@ -213,6 +218,10 @@ impl KvPushRouter {
             }
             RequestPhase::Aggregated => routing.and_then(|r| r.backend_instance_id),
         };
+        let requested_dp_rank = match phase {
+            RequestPhase::Prefill => routing.and_then(|r| r.prefill_dp_rank.or(r.dp_rank)),
+            RequestPhase::Decode | RequestPhase::Aggregated => routing.and_then(|r| r.dp_rank),
+        };
 
         let Some(id) = preselected_id else {
             let _nvtx_kv = dynamo_nvtx_range!("route.kv_match");
@@ -254,18 +263,23 @@ impl KvPushRouter {
 
             return Ok(WorkerSelection {
                 instance_id: best_worker.worker_id,
-                dp_rank: best_worker.dp_rank,
-                overlap_amount,
+                backend_dp_rank: Some(best_worker.dp_rank),
+                bookkeeping_dp_rank: Some(best_worker.dp_rank),
+                overlap_amount: Some(overlap_amount),
             });
         };
 
+        let backend_dp_rank =
+            requested_dp_rank.or_else(|| self.chooser.unique_dp_rank_for_worker(id));
+
         tracing::debug!(
             worker_id = id,
-            dp_rank = dp_rank,
+            dp_rank = ?backend_dp_rank,
             ?phase,
             "Routing to specified worker"
         );
 
+        let (bookkeeping_dp_rank, overlap_amount) = if let Some(dp_rank) = backend_dp_rank {
             let worker = WorkerWithDpRank::new(id, dp_rank);
             let overlap_blocks = self
                 .chooser
@@ -295,14 +309,26 @@ impl KvPushRouter {
                     request_id = %context_id,
                     worker_id = id,
                     dp_rank = dp_rank,
-                "Skipping add_request - query or handled externally"
+                    "Skipping add_request - query-only request"
                 );
             }
 
+            (Some(dp_rank), Some(overlap_blocks))
+        } else {
+            tracing::debug!(
+                request_id = %context_id,
+                worker_id = id,
+                ?phase,
+                "Routing to specified worker without resolved dp_rank; skipping scheduler bookkeeping"
+            );
+            (None, None)
+        };
+
         Ok(WorkerSelection {
             instance_id: id,
-            dp_rank,
-            overlap_amount: overlap_blocks,
+            backend_dp_rank,
+            bookkeeping_dp_rank,
+            overlap_amount,
         })
     }
 }
@@ -354,14 +380,17 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
             .await?;
         let WorkerSelection {
             instance_id,
-            dp_rank,
+            backend_dp_rank,
+            bookkeeping_dp_rank,
             overlap_amount,
         } = selection;
+        let scheduler_tracked = !is_query_only && bookkeeping_dp_rank.is_some();
 
         // In approximate mode (use_kv_events=false), record the routing decision
         // so the indexer can track cache state based on routing decisions.
         // This covers both pre-selected workers and find_best_match selections.
         if !is_query_only && !self.chooser.kv_router_config().use_kv_events {
+            if let Some(dp_rank) = bookkeeping_dp_rank {
                 let lora_name = request.routing.as_ref().and_then(|r| r.lora_name.clone());
                 let (routing_token_ids, block_mm_infos) = request.block_mm_routing_info();
                 let worker = WorkerWithDpRank::new(instance_id, dp_rank);
@@ -387,6 +416,13 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
                         "Failed to record routing decision in approximate mode"
                     );
                 }
+            } else {
+                tracing::debug!(
+                    request_id = %context_id,
+                    worker_id = instance_id,
+                    "Skipping approximate-mode routing decision for unresolved dp_rank"
+                );
+            }
         }
 
         // Record routing metrics on tracker and observe ISL + prefill start.
@@ -395,12 +431,14 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
         if let Some(ref tracker) = request.tracker {
             let (routing_token_ids, _) = request.block_mm_routing_info();
             let isl_blocks = routing_token_ids.len().div_ceil(block_size);
+            if let Some(overlap_amount) = overlap_amount {
                 tracker.record_kv_hit(overlap_amount, isl_blocks);
+            }
             tracker.record_isl(
                 routing_token_ids.len(),
-                overlap_amount as usize * block_size,
+                overlap_amount.map(|overlap| overlap as usize * block_size),
             );
-            tracker.record_worker_full(instance_id, dp_rank, self.chooser.worker_type());
+            tracker.record_worker(instance_id, backend_dp_rank, self.chooser.worker_type());
             tracker.record_router_queue_depth(self.chooser.pending_count());
             if let Some(hit_rate) = tracker.kv_hit_rate() {
                 request_metrics.kv_hit_rate.observe(hit_rate);
@@ -444,7 +482,7 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
         let tracker = request.tracker.clone();
 
         let (mut backend_input, context) = request.into_parts();
-        backend_input.routing_mut().dp_rank = Some(dp_rank);
+        backend_input.routing_mut().dp_rank = backend_dp_rank;
         let updated_request = context.map(|_| backend_input);
 
         // Record prefill start right before pushing to backend (OnceLock: first call wins).
@@ -460,8 +498,8 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
                 "kv_router.route_request",
                 request_id = %context_id,
                 worker_id = instance_id,
-                dp_rank = dp_rank,
-                overlap_blocks = overlap_amount,
+                dp_rank = ?backend_dp_rank,
+                overlap_blocks = ?overlap_amount,
                 phase = ?phase,
             ))
             .await?;
@@ -471,6 +509,7 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
         let wrapped_stream = Box::pin(async_stream::stream! {
             let mut guard = RequestGuard {
                 chooser: chooser.clone(),
+                scheduler_tracked,
                 context_id: context_id.clone(),
                 tracker: tracker.clone(),
                 request_metrics: request_metrics.clone(),
@@ -479,7 +518,7 @@ impl AsyncEngine<SingleIn<PreprocessedRequest>, ManyOut<Annotated<LLMEngineOutpu
                 freed: false,
                 prefill_marked: false,
                 first_token_recorded: false,
-                track_output_blocks,
+                track_output_blocks: scheduler_tracked && track_output_blocks,
                 current_total_blocks: isl_tokens.div_ceil(block_size),
                 isl_tokens,
                 block_size,

lib/llm/src/protocols/common/timing.rs

@@ -8,7 +8,7 @@
 
 use std::sync::Arc;
 use std::sync::OnceLock;
-use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
+use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 
 use parking_lot::Mutex;
@@ -22,16 +22,12 @@ use crate::http::service::metrics::{
 };
 use crate::protocols::openai::nvext::WorkerIdInfo;
 
-/// Sentinel value indicating no worker ID has been set.
-/// We use 0 as the sentinel since valid worker IDs are non-zero lease IDs from etcd.
-const NO_WORKER_ID: u64 = 0;
-const NO_DP_RANK: u32 = u32::MAX;
-
 /// Worker type constants for Prometheus metric labels.
 /// These are stored in RequestTracker at routing time to avoid costly MDC lookups
 /// when updating per-worker metrics (TTFT, ITL).
 pub const WORKER_TYPE_PREFILL: &str = "prefill";
 pub const WORKER_TYPE_DECODE: &str = "decode";
+const UNSET_DP_RANK_LABEL: &str = "none";
 
 /// Phase of the request in disaggregated serving.
 ///
@@ -81,8 +77,8 @@ impl std::fmt::Display for RequestPhase {
 /// phase's final finish naturally overwrites the prefill phase's earlier finish.
 /// `phase` also uses a Mutex since it transitions across phases.
 ///
-/// **`AtomicU64`/`AtomicU32`:** Used for frequently updated counters (`osl_tokens`)
-/// and worker IDs/ranks where `OnceLock`'s heap overhead is unnecessary.
+/// **`AtomicU64`:** Used for frequently updated counters (`osl_tokens`) and
+/// accumulated detokenize timing, where lock-free updates are beneficial.
 #[derive(Debug)]
 pub struct RequestTracker {
     /// When the request was received (monotonic clock for duration calculations)
@@ -118,19 +114,17 @@ pub struct RequestTracker {
     /// Output sequence length in tokens - updated atomically as tokens stream back
     osl_tokens: AtomicU64,
 
-    /// Prefill worker ID (for disaggregated serving).
-    /// Uses atomic with compare-exchange for set-once semantics.
-    /// Value of 0 (NO_WORKER_ID) means not yet set.
-    prefill_worker_id: AtomicU64,
+    /// Prefill worker ID (for disaggregated serving) - set once when known.
+    prefill_worker_id: OnceLock<u64>,
 
-    /// Prefill DP rank. Value of u32::MAX (NO_DP_RANK) means not yet set.
-    prefill_dp_rank: AtomicU32,
+    /// Prefill DP rank - set once when known.
+    prefill_dp_rank: OnceLock<u32>,
 
-    /// Decode worker ID. Value of 0 (NO_WORKER_ID) means not yet set.
-    decode_worker_id: AtomicU64,
+    /// Decode worker ID - set once when known.
+    decode_worker_id: OnceLock<u64>,
 
-    /// Decode DP rank. Value of u32::MAX (NO_DP_RANK) means not yet set.
-    decode_dp_rank: AtomicU32,
+    /// Decode DP rank - set once when known.
+    decode_dp_rank: OnceLock<u32>,
 
     /// Worker type for the prefill worker ("prefill" or "decode").
     /// Stored at routing time to avoid MDC lookup when updating Prometheus metrics.
@@ -149,7 +143,7 @@ pub struct RequestTracker {
     /// Semaphore for coordinating phase transitions.
     /// Acquiring a permit blocks subsequent set_phase calls until the permit is dropped.
     /// This prevents race conditions in the bootstrap optimization path where prefill
-    /// runs in background and needs to complete record_worker_full before phase changes.
+    /// runs in background and needs to complete worker recording before phase changes.
     phase_semaphore: Arc<Semaphore>,
 
     /// How long it took to tokenize the input
@@ -185,10 +179,10 @@ impl RequestTracker {
             isl_tokens: OnceLock::new(),
             cached_tokens: OnceLock::new(),
             osl_tokens: AtomicU64::new(0),
-            prefill_worker_id: AtomicU64::new(NO_WORKER_ID),
-            prefill_dp_rank: AtomicU32::new(NO_DP_RANK),
-            decode_worker_id: AtomicU64::new(NO_WORKER_ID),
-            decode_dp_rank: AtomicU32::new(NO_DP_RANK),
+            prefill_worker_id: OnceLock::new(),
+            prefill_dp_rank: OnceLock::new(),
+            decode_worker_id: OnceLock::new(),
+            decode_dp_rank: OnceLock::new(),
             prefill_worker_type: OnceLock::new(),
             decode_worker_type: OnceLock::new(),
             phase: Mutex::new(RequestPhase::Aggregated),
@@ -220,11 +214,13 @@ impl RequestTracker {
         overlap_set && isl_set
     }
 
-    /// Record input sequence length in tokens and cached token count.
-    pub fn record_isl(&self, isl_tokens: usize, cached_tokens: usize) {
+    /// Record input sequence length in tokens and cached token count when known.
+    pub fn record_isl(&self, isl_tokens: usize, cached_tokens: Option<usize>) {
         let _ = self.isl_tokens.set(isl_tokens);
+        if let Some(cached_tokens) = cached_tokens {
             let _ = self.cached_tokens.set(cached_tokens);
         }
+    }
 
     pub fn isl_tokens(&self) -> Option<usize> {
         self.isl_tokens.get().copied()
@@ -321,31 +317,96 @@ impl RequestTracker {
         *self.phase.lock()
     }
 
-    /// Record worker ID, DP rank, and worker type based on the current phase.
-    ///
-    /// Each slot is written exactly once by `KvPushRouter::generate()`:
-    /// - Prefill phase: stores as prefill worker
-    /// - Decode phase: stores as decode worker
-    /// - Aggregated phase: stores as both prefill and decode worker
-    pub fn record_worker_full(&self, instance_id: u64, dp_rank: u32, worker_type: &'static str) {
-        match self.phase() {
-            RequestPhase::Prefill => {
-                self.prefill_worker_id.store(instance_id, Ordering::Relaxed);
-                self.prefill_dp_rank.store(dp_rank, Ordering::Relaxed);
-                let _ = self.prefill_worker_type.set(worker_type);
+    fn record_once_u64(slot: &OnceLock<u64>, value: u64, field_name: &'static str) {
+        if let Some(existing) = slot.get() {
+            if *existing != value {
+                tracing::error!(
+                    field = field_name,
+                    existing = *existing,
+                    new = value,
+                    "Conflicting request tracker write"
+                );
+            }
+            return;
         }
-            RequestPhase::Decode => {
-                self.decode_worker_id.store(instance_id, Ordering::Relaxed);
-                self.decode_dp_rank.store(dp_rank, Ordering::Relaxed);
-                let _ = self.decode_worker_type.set(worker_type);
+        let _ = slot.set(value);
+    }
+
+    fn record_once_u32(slot: &OnceLock<u32>, value: u32, field_name: &'static str) {
+        if let Some(existing) = slot.get() {
+            if *existing != value {
+                tracing::error!(
+                    field = field_name,
+                    existing = *existing,
+                    new = value,
+                    "Conflicting request tracker write"
+                );
+            }
+            return;
+        }
+        let _ = slot.set(value);
+    }
+
+    fn record_once_worker_type(
+        slot: &OnceLock<&'static str>,
+        value: &'static str,
+        field_name: &'static str,
+    ) {
+        if let Some(existing) = slot.get() {
+            if *existing != value {
+                tracing::error!(
+                    field = field_name,
+                    existing = *existing,
+                    new = value,
+                    "Conflicting request tracker write"
+                );
             }
+            return;
+        }
+        let _ = slot.set(value);
+    }
+
+    fn record_prefill_worker(
+        &self,
+        instance_id: u64,
+        dp_rank: Option<u32>,
+        worker_type: &'static str,
+    ) {
+        Self::record_once_u64(&self.prefill_worker_id, instance_id, "prefill_worker_id");
+        if let Some(rank) = dp_rank {
+            Self::record_once_u32(&self.prefill_dp_rank, rank, "prefill_dp_rank");
+        }
+        Self::record_once_worker_type(
+            &self.prefill_worker_type,
+            worker_type,
+            "prefill_worker_type",
+        );
+    }
+
+    fn record_decode_worker(
+        &self,
+        instance_id: u64,
+        dp_rank: Option<u32>,
+        worker_type: &'static str,
+    ) {
+        Self::record_once_u64(&self.decode_worker_id, instance_id, "decode_worker_id");
+        if let Some(rank) = dp_rank {
+            Self::record_once_u32(&self.decode_dp_rank, rank, "decode_dp_rank");
+        }
+        Self::record_once_worker_type(&self.decode_worker_type, worker_type, "decode_worker_type");
+    }
+
+    /// Record worker ID, optional DP rank, and worker type based on the current phase.
+    ///
+    /// Worker ID and type are recorded as soon as they are known. DP rank is recorded only
+    /// when it is concrete, allowing the unresolved rank to remain unset until later.
+    pub fn record_worker(&self, instance_id: u64, dp_rank: Option<u32>, worker_type: &'static str) {
+        match self.phase() {
+            RequestPhase::Prefill => self.record_prefill_worker(instance_id, dp_rank, worker_type),
+            RequestPhase::Decode => self.record_decode_worker(instance_id, dp_rank, worker_type),
             RequestPhase::Aggregated => {
-                self.prefill_worker_id.store(instance_id, Ordering::Relaxed);
-                self.prefill_dp_rank.store(dp_rank, Ordering::Relaxed);
-                let _ = self.prefill_worker_type.set(worker_type);
-                self.decode_worker_id.store(instance_id, Ordering::Relaxed);
-                self.decode_dp_rank.store(dp_rank, Ordering::Relaxed);
-                let _ = self.decode_worker_type.set(worker_type);
+                self.record_prefill_worker(instance_id, dp_rank, worker_type);
+                self.record_decode_worker(instance_id, dp_rank, worker_type);
             }
         }
     }
@@ -415,26 +476,22 @@ impl RequestTracker {
 
     /// Get the decode worker ID if recorded.
     pub fn decode_worker_id(&self) -> Option<u64> {
-        let id = self.decode_worker_id.load(Ordering::SeqCst);
-        if id == NO_WORKER_ID { None } else { Some(id) }
+        self.decode_worker_id.get().copied()
     }
 
     /// Get the decode DP rank if recorded.
     pub fn decode_dp_rank(&self) -> Option<u32> {
-        let rank = self.decode_dp_rank.load(Ordering::SeqCst);
-        if rank == NO_DP_RANK { None } else { Some(rank) }
+        self.decode_dp_rank.get().copied()
     }
 
     /// Get the prefill worker ID if recorded.
     pub fn prefill_worker_id(&self) -> Option<u64> {
-        let id = self.prefill_worker_id.load(Ordering::SeqCst);
-        if id == NO_WORKER_ID { None } else { Some(id) }
+        self.prefill_worker_id.get().copied()
     }
 
     /// Get the prefill DP rank if recorded.
     pub fn prefill_dp_rank(&self) -> Option<u32> {
-        let rank = self.prefill_dp_rank.load(Ordering::SeqCst);
-        if rank == NO_DP_RANK { None } else { Some(rank) }
+        self.prefill_dp_rank.get().copied()
     }
 
     /// Get the prefill worker type if recorded.
@@ -456,7 +513,7 @@ impl RequestTracker {
         let worker_id_str = worker_id.to_string();
         let dp_rank_str = self
             .prefill_dp_rank()
-            .map_or("0".to_string(), |r| r.to_string());
+            .map_or(UNSET_DP_RANK_LABEL.to_string(), |r| r.to_string());
         let worker_type = self.prefill_worker_type().unwrap_or(WORKER_TYPE_PREFILL);
         let labels = &[worker_id_str.as_str(), dp_rank_str.as_str(), worker_type];
 
@@ -481,7 +538,7 @@ impl RequestTracker {
         let worker_id_str = worker_id.to_string();
         let dp_rank_str = self
             .decode_dp_rank()
-            .map_or("0".to_string(), |r| r.to_string());
+            .map_or(UNSET_DP_RANK_LABEL.to_string(), |r| r.to_string());
         let worker_type = self.decode_worker_type().unwrap_or(WORKER_TYPE_DECODE);
         let labels = &[worker_id_str.as_str(), dp_rank_str.as_str(), worker_type];
 
@@ -555,7 +612,7 @@ mod tests {
     fn test_record_isl_osl() {
         let tracker = RequestTracker::new();
 
-        tracker.record_isl(512, 256);
+        tracker.record_isl(512, Some(256));
         assert_eq!(tracker.isl_tokens(), Some(512));
         assert_eq!(tracker.cached_tokens(), Some(256));
 
@@ -659,7 +716,7 @@ mod tests {
     fn test_observe_first_token_gauges_no_panic_without_worker() {
         let tracker = RequestTracker::new();
         tracker.record_first_token();
-        tracker.record_isl(100, 50);
+        tracker.record_isl(100, Some(50));
         // No worker recorded — should return early without panic
         tracker.observe_first_token_gauges();
     }
@@ -677,10 +734,10 @@ mod tests {
     #[test]
     fn test_observe_first_token_gauges_with_worker() {
         let tracker = RequestTracker::new();
-        tracker.record_worker_full(42, 0, WORKER_TYPE_PREFILL);
+        tracker.record_worker(42, Some(0), WORKER_TYPE_PREFILL);
         thread::sleep(Duration::from_millis(5));
         tracker.record_first_token();
-        tracker.record_isl(256, 128);
+        tracker.record_isl(256, Some(128));
 
         tracker.observe_first_token_gauges();
 
@@ -702,7 +759,7 @@ mod tests {
     #[test]
     fn test_observe_finish_gauges_with_worker() {
         let tracker = RequestTracker::new();
-        tracker.record_worker_full(99, 1, WORKER_TYPE_DECODE);
+        tracker.record_worker(99, Some(1), WORKER_TYPE_DECODE);
         tracker.record_first_token();
         thread::sleep(Duration::from_millis(10));
         tracker.record_osl(5);

lib/llm/src/protocols/openai/nvext.rs

@@ -13,6 +13,7 @@ pub const HEADER_WORKER_INSTANCE_ID: &str = "x-worker-instance-id";
 pub const HEADER_PREFILL_INSTANCE_ID: &str = "x-prefill-instance-id";
 pub const HEADER_DP_RANK: &str = "x-dp-rank";
 pub const HEADER_PREFILL_DP_RANK: &str = "x-prefill-dp-rank";
+const UNSET_DP_RANK_SENTINEL: u32 = u32::MAX;
 
 /// Apply routing overrides from HTTP headers to nvext.
 ///
@@ -44,6 +45,7 @@ pub fn apply_header_routing_overrides(nvext: Option<NvExt>, headers: &HeaderMap)
         .get(HEADER_PREFILL_DP_RANK)
         .and_then(|v| v.to_str().ok())
         .and_then(|s| s.parse::<u32>().ok());
+    let prefill_dp_rank = prefill_dp_rank.filter(|rank| *rank != UNSET_DP_RANK_SENTINEL);
 
     if worker_id.is_none() && prefill_id.is_none() && dp_rank.is_none() && prefill_dp_rank.is_none()
     {