fix(gdn): Align prefill warmup with real prefill path by ibrahim1023 · Pull Request #39169 · vllm-project/vllm

ibrahim1023 · 2026-04-07T09:31:40Z

Purpose

Fixes #39163 by aligning GDN prefill warmup with the real prefill path.

Real GDN prefills build q/k/v/g/beta via fused_post_conv_prep and call chunk_gated_delta_rule(..., use_qk_l2norm_in_kernel=False). The previous warmup path did not mirror that contract, which could leave first-request work deferred to the first real prefill. This PR updates the warmup path and adds a regression test.

Duplicate-work check:

Ran gh issue view 39163 --repo vllm-project/vllm --comments
Ran gh pr list --repo vllm-project/vllm --state open --search '39163 in:body'
Ran gh pr list --repo vllm-project/vllm --state open --search 'GDN prefill warmup first request Qwen3.5 Blackwell'
I did not find an open PR already addressing this fix

AI assistance:

This PR was prepared with AI assistance. The submitting human reviewed the changed lines and is responsible for the change end-to-end.

Test Plan

python -m compileall vllm/model_executor/layers/mamba/gdn_linear_attn.py tests/model_executor/test_gdn_linear_attn.py
.venv/bin/python -m pytest --noconftest tests/model_executor/test_gdn_linear_attn.py -v
.venv/bin/pre-commit run --files vllm/model_executor/layers/mamba/gdn_linear_attn.py tests/model_executor/test_gdn_linear_attn.py

Test Result

compileall passed.

Targeted pytest:

1 passed

Changed-file pre-commit:

ruff check Passed
ruff format Passed
typos Passed
mypy-local Passed
all remaining relevant file checks Passed

Notes:

I did not reproduce the original Blackwell/Qwen3.5 issue end-to-end on this machine because the environment does not expose the target GPU stack.
pre-commit run --all-files still has unrelated repo-baseline/local-environment issues on this machine, but the checks for the files changed in this PR pass.

Essential Elements of an Effective PR Description Checklist

The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
The test plan, such as providing test command.
The test results, such as pasting the results comparison before and after, or e2e results
(Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
(Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

github-actions · 2026-04-07T09:31:51Z

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Code Review

This pull request updates the _warmup_prefill_kernels method in GatedDeltaNetAttention to more accurately mirror the actual prefill path by using fused_post_conv_prep to generate input tensors and disabling in-kernel L2 normalization. A new unit test was added to verify that the warmup logic correctly adheres to the prefill contract. I have no feedback to provide.

vadiklyutiy · 2026-04-07T09:49:57Z

@ibrahim1023 Thank you for fixing it. I also saw that the first request for Qwen3.5 takes really long.

@arpera you touched this code recently. Pls review.

arpera · 2026-04-07T10:39:00Z

@ibrahim1023 thank you for a fix! Could you please run server before and after your change with TRITON_PRINT_AUTOTUNING=1 env var set and then run benchmark? This env var forces vLLM server to log Triton autotuning events. I would like to see how many kernels we autotune before and after your change in real inference.
If you didn't catch my idea, please, feel free to ask questions, I'll help you to understand.

ibrahim1023 · 2026-04-07T13:24:13Z

Hey,

I can’t run the requested TRITON_PRINT_AUTOTUNING=1 before/after benchmark on my side because I don’t have access to a CUDA-capable environment. My current machine falls back to CPU and reports Triton not installed or not compatible and missing vllm._C, so the result would not be meaningful for Triton autotuning validation.

What I was able to validate locally is the code-path change itself: the warmup path now matches the real GDN prefill contract, I added a regression test for that contract, and the targeted local checks pass.

If you’d like, I can still help narrow the benchmark command or interpret the logs if someone can run the before/after comparison on a proper GPU machine.

arpera · 2026-04-08T14:32:11Z

I admit that this patch removes Triton's autotuning during inference time. I checked this patch with TRITON_PRINT_AUTOTUNING=1 on B200 on model nvidia/Qwen3.5-397B-A17B-NVFP4 with empty Triton cache, and there is no messages about autotuning in the server's log during inference.

LGTM

chaunceyjiang · 2026-04-09T10:57:25Z

vllm serve /mnt/data3/models/Qwen/Qwen3.5-27B --enable-auto-tool-choice --tool-call-parser qwen3_coder

mian

[Batch Test] Sending 10 requests with same prefix...
Common prefix: You are a helpful assistant. ...
--------------------------------------------------
  Request 1: 1.111s
  Request 2: 0.497s
  Request 3: 0.489s
  Request 4: 0.494s
  Request 5: 0.490s
  Request 6: 0.493s
  Request 7: 0.491s
  Request 8: 0.490s
  Request 9: 0.492s
  Request 10: 0.488s
--------------------------------------------------
Average time: 0.554s
First request: 1.111s
Last request: 0.488s

this pr

[Batch Test] Sending 10 requests with same prefix...
Common prefix: You are a helpful assistant. ...
--------------------------------------------------
  Request 1: 0.746s
  Request 2: 0.503s
  Request 3: 0.497s
  Request 4: 0.501s
  Request 5: 0.497s
  Request 6: 0.504s
  Request 7: 0.497s
  Request 8: 0.500s
  Request 9: 0.503s
  Request 10: 0.502s
--------------------------------------------------
Average time: 0.525s
First request: 0.746s
Last request: 0.502s

chaunceyjiang · 2026-04-09T11:02:38Z

/cc @ZJY0516 PTAL.

ZJY0516 · 2026-04-09T11:09:17Z

@@ -0,0 +1,82 @@
+# SPDX-License-Identifier: Apache-2.0


Could you please explain what is this for?

Yes. This repo enforces SPDX headers in pre-commit via the check-spdx-header hook, so the line was added to satisfy the project’s required license-header format for Python files.

More specifically, the hook expects the standard SPDX header block used in the repo, not just an arbitrary comment.

I mean, what is this test supposed to test

This test checks that the warmup path behaves like a real prefill call.

The idea is simple: if warmup uses the same setup and kernel call as real inference, the first real request should not have to do that work again.

please remove this test file. I think we need a more general approach

ZJY0516 · 2026-04-09T11:20:13Z

I think we need a more general warmup approach for this in the long term. @vadiklyutiy @arpera

arpera · 2026-04-09T11:25:41Z

Yes, I also think so. As a some kind of a partial solution I suggest to set TRITON_PRINT_AUTOTUNING=1 by default in vLLM after server is up and ready to process queries. Then we will see in the server log warnings about autotuning and more rapidly react on such performance critical issues. What do you think about it @ZJY0516?

ZJY0516 · 2026-04-09T11:33:39Z

Yes, I also think so. As a some kind of a partial solution I suggest to set TRITON_PRINT_AUTOTUNING=1 by default in vLLM after server is up and ready to process queries. Then we will see in the server log warnings about autotuning and more rapidly react on such performance critical issues. What do you think about it @ZJY0516?

Yeah, sounds great. My only concern is whether this might cause log spamming for some necessary recompilation scenarios, though I'm not sure if those scenarios really exist.

Actually, what I mean is a more general way to warmup, not log

arpera · 2026-04-09T11:38:23Z

Autotuning print in the server log in inference time can be done only once, because if there is at least one such a warning message in the log then it's worth investigating this issue closer. So, I think we won't spam server logs.

ZJY0516 · 2026-04-09T11:43:45Z

@@ -735,7 +749,7 @@ def _warmup_prefill_kernels(self, mixed_qkv: torch.Tensor) -> None:
                initial_state=state,
                output_final_state=True,
                cu_seqlens=cu_seqlens,
-                use_qk_l2norm_in_kernel=True,
+                use_qk_l2norm_in_kernel=False,


why change this?

Because warmup is for the prefill path, and the real prefill call here uses use_qk_l2norm_in_kernel=False. Leaving it as True means warmup does not match the actual inference path we are trying to prepare.

ZJY0516 · 2026-04-09T11:44:33Z

@@ -0,0 +1,82 @@
+# SPDX-License-Identifier: Apache-2.0


please remove this test file. I think we need a more general approach

ZJY0516 · 2026-04-09T11:50:17Z

please also fix DCO

Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

…9169) Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

ibrahim1023 requested review from ZJY0516, tdoublep and vadiklyutiy as code owners April 7, 2026 09:31

gemini-code-assist Bot reviewed Apr 7, 2026

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ZJY0516 reviewed Apr 9, 2026

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ZJY0516 added the ready ONLY add when PR is ready to merge/full CI is needed label Apr 9, 2026

ZJY0516 approved these changes Apr 9, 2026

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fix(gdn): Align prefill warmup with real prefill path

3756e8e

Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

ibrahim1023 force-pushed the fix-39163-gdn-prefill-warmup branch from e1bc283 to 3756e8e Compare April 9, 2026 11:55

Merge branch 'main' into fix-39163-gdn-prefill-warmup

2652fc6

vadiklyutiy enabled auto-merge (squash) April 9, 2026 23:32

vadiklyutiy merged commit 9853a3c into vllm-project:main Apr 10, 2026
57 checks passed

wojciech-wais pushed a commit to wojciech-wais/vllm that referenced this pull request Apr 13, 2026

fix(gdn): Align prefill warmup with real prefill path (vllm-project#3…

8f9dace

…9169) Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

arpera mentioned this pull request Apr 17, 2026

[Feature] Add Triton kernel JIT compilation monitor for inference #40137

Merged

4 tasks

whk-lab pushed a commit to whk-lab/vllm that referenced this pull request Apr 23, 2026

fix(gdn): Align prefill warmup with real prefill path (vllm-project#3…

a7d1718

…9169) Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

mystous pushed a commit to mystous/vllm_hybrid that referenced this pull request May 10, 2026

fix(gdn): Align prefill warmup with real prefill path (vllm-project#3…

2119354

…9169) Signed-off-by: Ibrahim Arshad <38925737+ibrahim1023@users.noreply.github.com>

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