[seansie0830]

Fixes #65324

🐛 Problem Description

As reported in #65324, in vLLM V1, when waking up an engine from level-2 sleep using a tagless POST /wake_up, EngineCoreProc.wake_up() clears all sleeping tags in the executor and immediately evaluates fully_awake = True, prematurely resuming the scheduler via self.resume_scheduler().

However, level-2 sleep discards model weights. If incoming requests were queued while the engine was asleep, the scheduler resumes and processes them in the race window before POST /collective_rpc {"method": "reload_weights"} completes. Consequently, queued requests execute against uninitialized/discarded weights, returning HTTP 200 with garbage tokens.

🧪 Reproduction & Verification

Tested on Qwen/Qwen2.5-0.5B-Instruct with prompt: "What is the color of the sky? Answer in a complete sentence.":

--- [Test 1] Baseline (Normal Awake Query) ---
Output: The color of the sky varies depending on the time of day and location, but generally, it appears blue or purple due to the scattering of sunlight by Earth's atmosphere.

--- [Test 2] Before Fix: Bug Trigger (Tagless wake_up -> reload_weights) ---
Output: !#Cuma!#DHCL!, 2019/12/14.
# 2019-12-14.
"""
@Bb: 3567890
Status: REPRODUCED (Output corrupted due to premature scheduling)

--- [Test 2] After Fix: Tagless wake_up -> reload_weights ---
Output: The color of the sky varies depending on the time of day and location, but generally, it appears blue or purple due to the scattering of sunlight by Earth's atmosphere.
Status: PASSED (Scheduler safely waits for weights reload)

--- [Test 3] Stepwise Wakeup (weights -> reload -> kv_cache) ---
Output: The color of the sky varies depending on the time of day and location, but generally, it appears blue or purple due to the scattering of sunlight by Earth's atmosphere.
Status: PASSED

💡 Proposed Solution

1. Added _needs_reload_weights: bool = False flag to EngineCoreProc.
2. In EngineCoreProc.sleep(level): Set _needs_reload_weights = True when level >= 2.
3. In EngineCoreProc.wake_up(): Gate fully_awake so it only evaluates to True when memory allocations are resident and weights do not require reloading:

   fully_awake = (
       not self.model_executor.is_sleeping
       and not self._needs_reload_weights
   )

4. In EngineCoreProc.collective_rpc(): Clear _needs_reload_weights upon completion of reload_weights, and call self.resume_scheduler() if all memory allocations are resident.

Alternatives Considered

We also considered refactoring sleep transitions into an explicit state machine (EngineSleepState: AWAKE, PAUSED, SLEEPING_L1, SLEEPING_L2, WAITING_WEIGHTS). However, keeping the fix scoped as a lightweight gate guard within EngineCoreProc minimizes architectural churn while correctly synchronizing the scheduler with weight restoration.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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🚀

[seansie0830]

this is my reprod and the verify test script

#!/usr/bin/env bash
# ==============================================================================
# Reproduction Script: Sleep Mode Race Condition vs Stepwise Wake
# Logs: Concise test results written to ./scratch/repro_log.txt
#       vLLM server logs streamed directly to stdout/stderr.
# ==============================================================================

set -euo pipefail

# 1. Environment variables
export VLLM_USE_FLASHINFER_SAMPLER=0
export NVCC_PREPEND_FLAGS="-allow-unsupported-compiler"
export VLLM_USE_PRECOMPILED=1
export VLLM_SERVER_DEV_MODE=1

MODEL_ID="Qwen/Qwen2.5-0.5B-Instruct"
PORT=8000
BASE_URL="http://127.0.0.1:${PORT}"
LOG_FILE="./scratch/repro_log.txt"

# Ensure log directory exists and clear previous run log
mkdir -p ./scratch
echo "=== vLLM Sleep Mode Reproduction Run ($(date)) ===" > "${LOG_FILE}"

# Helper to log both to terminal and concise log file
log_summary() {
    echo "$1"
    echo "$1" >> "${LOG_FILE}"
}

echo "[INFO] Starting vLLM server with model: ${MODEL_ID}..."
vllm serve "${MODEL_ID}" \
    --port "${PORT}" \
    --enable-sleep-mode \
    --kv-cache-memory-bytes 1g \
    --gpu-memory-utilization 0.2 \
    --enforce-eager &
SERVER_PID=$!

cleanup() {
    echo "[INFO] Terminating vLLM server (PID: ${SERVER_PID})..."
    kill -9 "${SERVER_PID}" 2>/dev/null || true
}
trap cleanup EXIT INT TERM

echo "[INFO] Waiting for server readiness..."
until curl -s "${BASE_URL}/health" > /dev/null 2>&1; do
    sleep 2
done
echo "[INFO] Server is healthy."

REQUEST_PAYLOAD=$(cat <<'EOF'
{
  "model": "Qwen/Qwen2.5-0.5B-Instruct",
  "messages": [
    {"role": "user", "content": "What is the color of the sky? Answer in a complete sentence."}
  ],
  "max_tokens": 48,
  "temperature": 0
}
EOF
)

# ------------------------------------------------------------------------------
# Test 1: Normal Baseline
# ------------------------------------------------------------------------------
log_summary ""
log_summary "--- [Test 1] Baseline (Normal Awake Query) ---"
curl -s "${BASE_URL}/v1/chat/completions" \
    -H "Content-Type: application/json" \
    -d "${REQUEST_PAYLOAD}" > ./scratch/baseline.json

BASELINE_TEXT=$(jq -r '.choices[0].message.content' ./scratch/baseline.json 2>/dev/null || echo "")
log_summary "Output: ${BASELINE_TEXT}"

# ------------------------------------------------------------------------------
# Test 2: Bug Reproduction (Tagless wake_up before reload_weights)
# ------------------------------------------------------------------------------
log_summary ""
log_summary "--- [Test 2] Bug Trigger (Tagless wake_up -> reload_weights) ---"
# Put to sleep level 2
curl -s -X POST "${BASE_URL}/sleep?level=2&mode=wait" > /dev/null

# Queue request in background
curl -s "${BASE_URL}/v1/chat/completions" \
    -H "Content-Type: application/json" \
    -d "${REQUEST_PAYLOAD}" > ./scratch/bug_queued.json &
CURL_PID_BUG=$!
sleep 1

# Premature tagless wake_up resumes scheduler immediately
curl -s -X POST "${BASE_URL}/wake_up" > /dev/null

# Weights reload occurs after scheduler has already scheduled the request
curl -s -X POST "${BASE_URL}/collective_rpc" \
    -H "Content-Type: application/json" \
    -d '{"method":"reload_weights"}' > /dev/null

wait "${CURL_PID_BUG}"
BUG_TEXT=$(jq -r '.choices[0].message.content' ./scratch/bug_queued.json 2>/dev/null || echo "")
log_summary "Output: ${BUG_TEXT}"

if [ "${BASELINE_TEXT}" != "${BUG_TEXT}" ]; then
    log_summary "Status: REPRODUCED (Output corrupted due to premature scheduling)"
else
    log_summary "Status: NOT REPRODUCED (Output unexpectedly matched)"
fi

# ------------------------------------------------------------------------------
# Test 3: Safe Stepwise Wakeup (wake weights -> reload -> wake kv_cache)
# ------------------------------------------------------------------------------
log_summary ""
log_summary "--- [Test 3] Stepwise Wakeup (weights -> reload -> kv_cache) ---"
# Put to sleep level 2
curl -s -X POST "${BASE_URL}/sleep?level=2&mode=wait" > /dev/null

# Queue request in background
curl -s "${BASE_URL}/v1/chat/completions" \
    -H "Content-Type: application/json" \
    -d "${REQUEST_PAYLOAD}" > ./scratch/stepwise_queued.json &
CURL_PID_STEPWISE=$!
sleep 1

# Wake weights only (status stays sleeping, request continues waiting)
curl -s -X POST "${BASE_URL}/wake_up?tags=weights" > /dev/null

# Reload discarded weights
curl -s -X POST "${BASE_URL}/collective_rpc" \
    -H "Content-Type: application/json" \
    -d '{"method":"reload_weights"}' > /dev/null

# Wake kv_cache (resumes scheduler safely with valid weights)
curl -s -X POST "${BASE_URL}/wake_up?tags=kv_cache" > /dev/null

wait "${CURL_PID_STEPWISE}"
STEPWISE_TEXT=$(jq -r '.choices[0].message.content' ./scratch/stepwise_queued.json 2>/dev/null || echo "")
log_summary "Output: ${STEPWISE_TEXT}"

if [ "${BASELINE_TEXT}" = "${STEPWISE_TEXT}" ]; then
    log_summary "Status: PASSED (Output correctly matches baseline)"
else
    log_summary "Status: FAILED (Output did not match baseline)"
fi

log_summary ""
log_summary "=== Summary complete. Check details above. ==="

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @seansie0830.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[LinzeShi]

With VLLM_ENABLE_V1_MULTIPROCESSING=0, a level-1 sleep followed by wake-up hits AttributeError because this flag is only initialized in the process-based engine. Please initialize it in EngineCore.__init__ so the in-process engine gets it too.

[seansie0830]

Thanks for pointing this out! I have moved _needs_reload_weights directly into EngineCore.init
at commit 92ed279
so it supports both multiprocessing and in-process execution.

Verified with VLLM_ENABLE_V1_MULTIPROCESSING=0 bash ./scratch/reprod.sh , output correctly.

Could you please take another look when you have a moment? Thanks!

[seansie0830]

I've rebased the branch onto the latest main and moved _needs_reload_weights directly into EngineCore to resolve the conflict and support in-process execution.

Range diff: https://github.com/vllm-project/vllm/compare/92ed279722d4896ca568ac2c02f503cc730638cd..fcb2a0b420047e2055b5183d218bc2010d567f9f

This is my first time contributing to a large-scale repository, so please feel free to point out if anything looks off or if additional adjustments are needed. Thanks again for your guidance!