[Bugfix] Keep the frontend profiler on one thread, export off it - #54322
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`AsyncLLM.start_profile` / `stop_profile` dispatch the frontend profiler
lifecycle with `asyncio.to_thread`. Kineto records only the thread that called
`start()` and binds its client there, so this is broken on every platform, not
only the ones the linked issues were filed against:
- started from a worker thread, the profiler records nothing. A trace file is
still written, so the failure is silent: `--profiler-config
'{"profiler":"torch"}'` yields an AsyncLLM trace with no Kineto activity.
- `asyncio.to_thread` uses a thread pool, so `start` and `stop` can land on
different threads. When they do, the process segfaults. That is the crash
reported in vllm-project#39603 and vllm-project/vllm-ascend#14974, and it explains why it
looks intermittent.
Measured with 20000 recorded spans, each case in its own process:
| start / stop thread | spans recorded | cpu_op |
| --- | --- | --- |
| worker pool (today) | 0/20000 | 0 |
| one dedicated thread | 0/20000 | 0 |
| event loop | 20000/20000 | 100003 |
A dedicated thread would have fixed the affinity without touching the loop, but
Kineto records only the starting thread, so the lifecycle has to run there.
Exporting the trace does not: it serializes and compresses every recorded event
and has no thread affinity. Leaving it armed as `on_trace_ready` would move that
cost onto the loop along with `stop()`. So hold the handler and call it through
`asyncio.to_thread` after `stop()` returns.
On an H20 serving Qwen3-0.6B with the default profiler config, ~91k frontend
events, polling `/v1/models` every 20 ms across `/stop_profile`:
| | stock | lifecycle inline only | this change |
| --- | --- | --- | --- |
| frontend work on the event loop | 0 s | 2.045 s | 0.232 s |
| max event-loop stall | 246 ms | 2064 ms | 380 ms |
| frontend trace events | 91542 | 91958 | 91959 |
Concurrency with the EngineCore request, exception propagation and
`profile_prefix` handling are unchanged.
Fixes vllm-project#39603
Assisted-by: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Zihan Lin <linzihan.ai@gmail.com>
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Hello, I tried applying that fix to vLLM 0.26.0 and using vllm-ascend v0.26.0rc to collect profiles, but I'm still encountering the same issue |
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Duplicate check
I searched #39603 and open PRs for the affected profiler paths. No PR fixes this bug. #51678 handles repeated-stop idempotency, while #51816 offloads worker-profiler export.
Purpose
Fixes #39603. Also reported in vllm-project/vllm-ascend#14974.
AsyncLLM.start_profileandstop_profilecurrently useasyncio.to_thread. Kineto records frontend activity only on the thread that calledstart(); starting on a worker loses Kineto events, and stopping on a different pool thread can segfault.This PR keeps
start()andstop()on the event-loop thread, then exports the completed trace throughasyncio.to_thread. The existingasyncio.gather, EngineCore coordination, exception behavior, andprofile_prefixhandling are preserved.Test plan and results
Result: 5 passed; all pre-commit hooks passed.
Collection test with 20,000 spans:
cpu_opThe thread behavior was also reproduced with 2,000 spans on CPU, CUDA, and Ascend; mixed loop/worker lifecycle calls segfault.
H20 HTTP profiling test, Qwen3-0.6B, default profiler configuration, approximately 91k frontend events:
The split preserves the collected trace while moving serialization and compression off the event loop.
Independently confirmed at the HTTP layer on a live server (current nightly plus this patch): all 959
/healthprobes issued during a 48.6 s/stop_profilesucceeded, max latency 210 ms.AI assistance disclosure
Developed with AI assistance (Claude). I reviewed every changed line, ran the tests above, and can defend the change end to end. Model evaluation is not applicable because this only changes profiler lifecycle threading.
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