[Bugfix][Core] shm_broadcast: bound reader wait and recheck ring buffer (lossy wakeup channel)

[chaeminlim-mb]

Purpose

Fix a permanent EngineCore -> Worker MessageQueue freeze in shm_broadcast when the reader parks on the wakeup notify channel after the producer has already written data into the shared-memory ring buffer.

Problem

The SHM MessageQueue uses shared-memory metadata as the authoritative data path and a ZMQ ping only as a wakeup hint. In P/D warmup on an 8k1k ladder at concurrency 6, production runs froze before first token: the ring buffer contained work, but the reader was parked indefinitely waiting for a wakeup ping that never arrived.

Root cause

The wakeup channel is lossy by design: the writer uses a ZMQ PUB socket with SNDHWM=1, and readers use SUB with CONFLATE. That is correct for avoiding notification backlog, but it also means a ping can be silently dropped. If that drop happens while a reader is in the indefinite idle wait, the reader never rechecks the ring-buffer written flag even though SHM already contains data.

Fix mechanics

• Keep the existing idle wait path: readers still park on the poller instead of busy-polling.
• Cap indefinite reader waits at 5 seconds so the existing acquire loop periodically rechecks the authoritative SHM metadata.
• Preserve VLLM_RINGBUFFER_WARNING_INTERVAL behavior for warning-enabled waits: they still wake at the configured warning cadence and emit the existing long-wait log.

This has zero steady-state cost on the normal notify path; the 5-second timeout fires only on idle/lost-notify paths (~0.2 wakeups/s while parked).

Related

• [Bug] Fix shm_broadcast PyCFunction descriptor corruption under JIT loads #40303 also touches vllm/distributed/device_communicators/shm_broadcast.py, but fixes PyCFunction descriptor corruption under JIT loads. This PR changes only the reader wait bound and does not touch JIT loading or descriptor state.
• [Bugfix] Fix ZMQ port TOCTOU race in shm_broadcast.py #44495 and [Bugfix] Fix ZMQ port bind race condition in shm_broadcast #31367 also touch shm_broadcast.py, but address ZMQ port bind/TOCTOU setup races. This PR does not change port allocation, bind/connect ordering, or startup handshakes.

Impact (Accuracy / Performance / Stability)

• [Stability] Before: the EngineCore→Worker SHM wakeup ping channel is lossy by design (zmq PUB SNDHWM=1 + CONFLATE, no handshake); one lost ping while the reader was parked = permanent whole-engine freeze (hit in production at warmup, 8k1k @ c6 — froze before first token, kill required). After: reader wakes every 5s and rechecks the authoritative ring buffer → a lost ping self-heals in ≤5s; zero steady-state overhead (timeout path only fires on loss).

Test Plan

Environment for observed failure: P/D warmup, 8k1k ladder, concurrency 6.

# Syntax validation for the changed file
python3 -m py_compile vllm/distributed/device_communicators/shm_broadcast.py

# Upstream unit coverage for MessageQueue/shm_broadcast behavior
python3 -m pytest tests/distributed/test_shm_broadcast.py

Loss-injection reasoning: because PUB + SNDHWM=1 and SUB + CONFLATE can drop a wakeup ping without handshake, the broken path is equivalent to a reader entering poller.poll(None) after the writer has set the ring-buffer written flag. With this change, the poller wait returns after at most 5 seconds, acquire_read() loops, rereads metadata, and consumes the already-written SHM block without requiring a second ping.

Test Result

Before

Observed in production P/D warmup (8k1k ladder, concurrency 6): engine froze before first token. Mechanism: dropped wakeup ping while the reader was parked indefinitely; ring buffer held data but the reader never woke to recheck it.

After

$ python3 -m py_compile vllm/distributed/device_communicators/shm_broadcast.py
# passed (no output)

$ python3 -m pytest tests/distributed/test_shm_broadcast.py
ImportError while loading conftest '/mnt/shared_mango/amdsow-deliveries/vllm-pr-workspace/wt-pr3/tests/conftest.py'.
tests/conftest.py:31: in <module>
    import torch
E   ModuleNotFoundError: No module named 'torch'

The local checkout used to prepare this PR does not have torch installed, so the upstream distributed unit could not be executed here. The changed file compiles successfully. On the loss path, the 5-second timeout self-heals by forcing the existing ring-buffer metadata recheck; on the normal notify path it adds no steady-state wakeups.

Blast radius

graphify query on MessageQueue shows tests/distributed/test_shm_broadcast.py, tests/distributed/test_mq_connect_ip.py, MessageQueue.create_from_process_group*, and the V1 multiprocess/Ray executor message-queue initialization/worker RPC paths. The code diff is limited to the bounded reader wait state/constant/docstring in shm_broadcast.py.

Checklist

• Purpose explains the user-visible hang and root cause.
• Test plan includes exact commands.
• Test results include observed local command output.
• No documentation update needed: this changes internal synchronization behavior only.

AI assistance

This PR was prepared with AI assistance (Claude); the submitter reviewed every changed line and ran the reported tests.

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[chaeminlim-mb]

Withdrawn by author pending internal regression validation on current main; will be resubmitted afterwards.