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feat(neuron,candle): req_id spans, terminal failure logs, pool-lock warnings

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Every chat completion path (single-GPU + TP, streaming + non-streaming)
now opens an `info_span!("chat", req_id=…, model=…)`. The fmt subscriber
prefixes every event with that span so `grep req_id=…` over journalctl
reconstructs one request even when dozens overlap.

Every path also emits a terminal log line on both success ("done", with
prompt_tokens/completion_tokens/finish_reason/total_ms) and failure
("failed", with full anyhow chain + total_ms). Failures used to vanish
silently — a request that hit a CUDA OOM left "starting" in the journal
and no further trace.

New `acquire_pool_lock` helper replaces the bare `tp.pool.lock().await`
in both TP paths. It warns at 2s ("still waiting on pool lock") and
re-warns every 2s thereafter, so queued requests stuck behind a
deadlocked holder are visible immediately instead of looking like idle
silence.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
rob thijssen
2026-05-26 12:25:11 +03:00
parent ea0e0f7911
commit 0a1cfcd4d0
1 changed files with 386 additions and 189 deletions
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299
crates/neuron/src/harness/candle.rs
View File

@@ -29,9 +29,12 @@ use serde_json::json;
use std::collections::HashMap;
use std::path::PathBuf;
use std::sync::Arc;
#[cfg(feature = "cuda")]
use std::time::Duration;
use std::time::{SystemTime, UNIX_EPOCH};
use tokenizers::Tokenizer;
use tokio::sync::{Mutex, RwLock, mpsc};
use tracing::Instrument;
/// In-process candle harness. Owns the loaded model registry.
pub struct CandleHarness {
@@ -351,6 +354,64 @@ fn resolve_hf_cache(explicit: Option<PathBuf>) -> Option<PathBuf> {
None
}
/// A short hex tag used to group every log line emitted on behalf of
/// one chat-completion request. Six hex digits is unique enough across
/// a 4-hour journal window (24 bits ≈ 16M values, while a busy neuron
/// sees ~10³ requests/hour) and fits cleanly inside `req_id=…` in the
/// fmt subscriber's span-prefix output.
fn new_req_id() -> String {
format!("{:06x}", unix_subsec_nanos() & 0xFFFFFF)
}
/// Threshold above which `pool.lock().await` blocking is interesting
/// enough to warn about. Healthy concurrent requests serialise behind
/// the pool in single-digit ms — anything past 2 seconds is either a
/// huge in-flight prompt or, more often, a stuck request holding the
/// lock against a poisoned CUDA context. See the 2026-05-26 4-hour
/// silence on beast where dozens of requests piled up invisibly here.
#[cfg(feature = "cuda")]
const POOL_LOCK_WARN_THRESHOLD: Duration = Duration::from_secs(2);
/// Acquire the TP pool lock, emitting a warn-level breadcrumb if the
/// wait exceeds [`POOL_LOCK_WARN_THRESHOLD`]. Wrapped in a helper so
/// the warn happens at the call site — the request whose lock-wait is
/// slow is the one that knows its prompt_len and other context.
#[cfg(feature = "cuda")]
async fn acquire_pool_lock(
pool: &tokio::sync::Mutex<super::tp::WorkerPool>,
model_id: &str,
) -> tokio::sync::MutexGuard<'_, super::tp::WorkerPool> {
let start = std::time::Instant::now();
// Tick once at the threshold so a stuck request shows up in
// journalctl even while it's still waiting. Without this the wait
// looks like silence in the log right up until the lock is freed.
tokio::pin! {
let lock = pool.lock();
}
loop {
tokio::select! {
guard = &mut lock => {
let elapsed = start.elapsed();
if elapsed >= POOL_LOCK_WARN_THRESHOLD {
tracing::warn!(
model = %model_id,
waited_ms = elapsed.as_millis(),
"TP chat_completion: pool lock acquired after long wait"
);
}
return guard;
}
_ = tokio::time::sleep(POOL_LOCK_WARN_THRESHOLD) => {
tracing::warn!(
model = %model_id,
waited_ms = start.elapsed().as_millis(),
"TP chat_completion: still waiting on pool lock"
);
}
}
}
}
/// Apply the repetition penalty (if any) to the prediction logits and
/// then sample. Centralises the prefill / generation-loop call sites
/// so they share identical sampling behaviour.
@@ -746,6 +807,18 @@ impl CandleHarness {
}
};
// Span every line of this request with a short req_id +
// model so `grep req_id=…` over the journal can reconstruct
// one request even when dozens overlap. Add a terminal log
// line on both success and failure — the single-GPU path
// used to log nothing on either side, so a failing request
// looked exactly like an idle neuron.
let req_id = new_req_id();
let model_id = request.model.clone();
let span = tracing::info_span!("chat", req_id = %req_id, model = %model_id);
let req_start = std::time::Instant::now();
let result = async {
let prompt = format_qwen3_prompt(&request.messages);
let encoding = loaded
@@ -765,9 +838,17 @@ impl CandleHarness {
.token_to_id("<|im_end|>")
.or_else(|| loaded.tokenizer.token_to_id("<|endoftext|>"));
tracing::info!(
prompt_len,
max_new,
temperature,
?top_p,
?eos_id,
"chat_completion: starting"
);
let arch_arc = Arc::clone(&loaded.arch);
let device = loaded.device.clone();
let model_id = request.model.clone();
let (generated_ids, finish_reason) =
tokio::task::spawn_blocking(move || -> Result<(Vec<u32>, String)> {
@@ -784,7 +865,9 @@ impl CandleHarness {
)
})
.await
.map_err(|e| InferenceError::Other(anyhow::anyhow!("inference task panicked: {e}")))?
.map_err(|e| {
InferenceError::Other(anyhow::anyhow!("inference task panicked: {e}"))
})?
.map_err(InferenceError::Other)?;
let completion_text = loaded
@@ -798,11 +881,19 @@ impl CandleHarness {
total_tokens: (prompt_len + generated_ids.len()) as u64,
};
Ok(ChatCompletionResponse {
tracing::info!(
prompt_tokens = prompt_len,
completion_tokens = generated_ids.len(),
finish_reason = %finish_reason,
total_ms = req_start.elapsed().as_millis(),
"chat_completion: done"
);
Ok::<_, InferenceError>(ChatCompletionResponse {
id: format!("chatcmpl-{:x}", unix_subsec_nanos()),
object: "chat.completion".into(),
created: unix_now_secs(),
model: model_id,
model: request.model.clone(),
choices: vec![ChatCompletionChoice {
index: 0,
message: ChatMessage {
@@ -817,6 +908,19 @@ impl CandleHarness {
extra: serde_json::Value::Object(Default::default()),
})
}
.instrument(span.clone())
.await;
if let Err(ref e) = result {
let _g = span.enter();
tracing::error!(
error = %format!("{e:#}"),
total_ms = req_start.elapsed().as_millis(),
"chat_completion: failed"
);
}
result
}
/// Run a streaming chat completion against a loaded model.
///
@@ -903,9 +1007,30 @@ impl CandleHarness {
.await
.map_err(|_| InferenceError::Other(anyhow::anyhow!("client disconnected")))?;
// Span context — spawn_blocking detaches from the async
// executor so we capture the span explicitly and re-enter it
// inside the closure to keep the req_id on every emitted line.
let req_id = new_req_id();
let span = tracing::info_span!("chat_stream", req_id = %req_id, model = %model_id);
let prompt_len = prompt_tokens.len();
let req_start = std::time::Instant::now();
let span_for_starting = span.clone();
let span_for_task = span.clone();
{
let _g = span_for_starting.enter();
tracing::info!(
prompt_len,
max_new,
temperature,
?top_p,
?eos_id,
"chat_completion (stream): starting"
);
}
tokio::task::spawn_blocking(move || {
let _g = span_for_task.enter();
let mut guard = arch_arc.blocking_lock();
if let Err(e) = run_inference_streaming(
match run_inference_streaming(
&mut guard,
&device,
&tokenizer,
@@ -920,7 +1045,17 @@ impl CandleHarness {
&model_id,
&tx,
) {
tracing::warn!(model = %model_id, error = %e, "streaming inference failed");
Ok(()) => tracing::info!(
prompt_tokens = prompt_len,
total_ms = req_start.elapsed().as_millis(),
"chat_completion (stream): done"
),
Err(e) => tracing::error!(
error = %format!("{e:#}"),
prompt_tokens = prompt_len,
total_ms = req_start.elapsed().as_millis(),
"chat_completion (stream): failed"
),
}
});
@@ -1187,13 +1322,32 @@ impl CandleHarness {
tp: Arc<TpLoadedModel>,
request: ChatCompletionRequest,
) -> Result<ChatCompletionResponse, InferenceError> {
let handle = tokio::spawn(chat_completion_tp_inner(tp, request));
match handle.await {
Ok(result) => result,
// Tag every line of this request with a short req_id so a
// grep over journalctl reconstructs one request even when
// dozens are queued and interleaved. The span prefix is added
// by the fmt subscriber to every event emitted within the
// instrumented future, including events from `WorkerPool::*`
// since those run on the leader's task.
let req_id = new_req_id();
let model_id = request.model.clone();
let span = tracing::info_span!("tp_chat", req_id = %req_id, model = %model_id);
let req_start = std::time::Instant::now();
let handle = tokio::spawn(chat_completion_tp_inner(tp, request).instrument(span.clone()));
let result = match handle.await {
Ok(r) => r,
Err(join_err) => Err(InferenceError::Other(anyhow::anyhow!(
"TP inference task panicked or was cancelled: {join_err}"
))),
};
if let Err(ref e) = result {
let _g = span.enter();
tracing::error!(
error = %format!("{e:#}"),
total_ms = req_start.elapsed().as_millis(),
"TP chat_completion: failed"
);
}
result
}
/// Streaming counterpart to `chat_completion_tp`. Same per-step
@@ -1263,14 +1417,41 @@ impl CandleHarness {
// The orchestration task. Holds the pool lock for the lifetime
// of this inference; concurrent requests against the same TP
// model serialise behind it.
//
// Tagged with the same req_id span as the non-streaming path
// so the journal can be reconstructed regardless of which API
// surface the client hit.
let req_id = new_req_id();
let span = tracing::info_span!(
"tp_chat_stream",
req_id = %req_id,
model = %model_id
);
let req_start = std::time::Instant::now();
tracing::info!(
parent: &span,
prompt_len,
max_new,
temperature,
?top_p,
?eos_id,
"TP chat_completion (stream): starting"
);
let tp_for_task = Arc::clone(&tp);
tokio::spawn(async move {
let mut pool = tp_for_task.pool.lock().await;
tokio::spawn(
async move {
let mut failure: Option<String> = None;
let mut pool = acquire_pool_lock(&tp_for_task.pool, &model_id).await;
let leader_arc = tp_for_task.leader_model.clone();
let mut all_tokens: Vec<u32> = Vec::new();
let mut decoded_prefix = String::new();
let mut finish_reason = "length".to_string();
'work: {
if let Err(e) = pool.clear_kv_cache(&model_id, leader_arc.clone()).await {
tracing::warn!(model = %model_id, error = %e, "TP stream: clear_kv_cache failed");
return;
failure = Some(format!("clear_kv_cache: {e:#}"));
break 'work;
}
let mut logits_processor = {
@@ -1285,10 +1466,6 @@ impl CandleHarness {
LogitsProcessor::from_sampling(seed, sampling)
};
let mut all_tokens: Vec<u32> = Vec::new();
let mut decoded_prefix = String::new();
let mut finish_reason = "length".to_string();
// Prefill — every rank embeds the prompt, offset = 0.
let logits = match pool
.generate_step(&model_id, leader_arc.clone(), prompt_tokens.clone(), 0)
@@ -1296,19 +1473,16 @@ impl CandleHarness {
{
Ok(l) => l,
Err(e) => {
tracing::warn!(model = %model_id, error = %e, "TP stream: prefill failed");
return;
failure = Some(format!("prefill: {e:#}"));
break 'work;
}
};
let mut next_token = match sample_with_penalty(
&logits,
&all_tokens,
&mut logits_processor,
) {
let mut next_token =
match sample_with_penalty(&logits, &all_tokens, &mut logits_processor) {
Ok(t) => t,
Err(e) => {
tracing::warn!(model = %model_id, error = %e, "TP stream: prefill sample failed");
return;
failure = Some(format!("prefill sample: {e:#}"));
break 'work;
}
};
@@ -1327,7 +1501,9 @@ impl CandleHarness {
)
.await
{
return;
// Client gone — treat as normal stream end,
// not a failure. No log spam.
break 'work;
}
for index in 0..max_new.saturating_sub(1) {
@@ -1342,24 +1518,19 @@ impl CandleHarness {
{
Ok(l) => l,
Err(e) => {
tracing::warn!(
model = %model_id,
error = %e,
"TP stream: decode step failed"
);
return;
failure = Some(format!("decode step {index}: {e:#}"));
break 'work;
}
};
next_token =
match sample_with_penalty(&logits, &all_tokens, &mut logits_processor) {
next_token = match sample_with_penalty(
&logits,
&all_tokens,
&mut logits_processor,
) {
Ok(t) => t,
Err(e) => {
tracing::warn!(
model = %model_id,
error = %e,
"TP stream: decode sample failed"
);
return;
failure = Some(format!("decode sample {index}: {e:#}"));
break 'work;
}
};
if Some(next_token) == eos_id {
@@ -1378,12 +1549,38 @@ impl CandleHarness {
)
.await
{
return;
break 'work;
}
}
}
}
// Final chunk carrying finish_reason.
// One terminal line per request, success or failure. The
// success branch was previously implicit (the SSE final
// chunk went out and the spawned task just ended); now
// there's always a log line for the operator.
if let Some(err) = &failure {
tracing::error!(
error = %err,
completion_tokens = all_tokens.len(),
total_ms = req_start.elapsed().as_millis(),
"TP chat_completion (stream): failed"
);
} else {
tracing::info!(
prompt_tokens = prompt_len,
completion_tokens = all_tokens.len(),
finish_reason = %finish_reason,
total_ms = req_start.elapsed().as_millis(),
"TP chat_completion (stream): done"
);
}
// Final chunk carrying finish_reason — only on the success
// path. On failure we drop the channel so the client sees
// the SSE stream end abruptly (matches pre-change behaviour
// when the failed-path early-returned without final chunk).
if failure.is_none() {
let final_chunk = ChatCompletionChunk {
id: id.clone(),
object: "chat.completion.chunk".into(),
@@ -1399,7 +1596,10 @@ impl CandleHarness {
extra: serde_json::Value::Object(Default::default()),
};
let _ = tx.send(final_chunk).await;
});
}
}
.instrument(span),
);
Ok(rx)
}
@@ -1455,13 +1655,10 @@ async fn chat_completion_tp_inner(
// leader_model's own Mutex is acquired step-by-step inside
// pool.generate_step (so spawn_blocking can grab it without
// holding the pool lock across the blocking_lock call).
let lock_start = std::time::Instant::now();
let mut pool = tp.pool.lock().await;
tracing::debug!(
model = %model_id,
elapsed_ms = lock_start.elapsed().as_millis(),
"TP chat_completion: pool lock acquired"
);
// `acquire_pool_lock` warns periodically while we wait so a
// stuck holder doesn't make the queueing requests look like
// silence in the journal.
let mut pool = acquire_pool_lock(&tp.pool, &model_id).await;
let leader_arc = tp.leader_model.clone();
// Reset every rank's KV cache so this request doesn't attend