[Bugfix] Platform-aware DCP error message for ROCm by renruirui1234 · Pull Request #55188 · vllm-project/vllm

renruirui1234 · 2026-09-03T17:16:51Z

Summary

Replace the DCP assert with a ValueError that includes platform-specific guidance.
On ROCm: states that no LSE-capable backend is available and DCP cannot be used.
On other platforms: suggests trying --attention-backend or disabling DCP.

Motivation

Fixes #54761. On ROCm, no attention backend returns softmax LSE during decode, making DCP impossible. The previous message suggested --attention-backend, which is unhelpful on ROCm.

Not duplicating an existing PR

No open PR addresses this issue (verified via gh pr list).

Test plan

Code review of error paths
Import of current_platform verified

AI assistance disclosure

This PR was developed with AI assistance (Trae AI). All changes have been reviewed by the human submitter.

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coderabbitai · 2026-09-03T17:17:47Z

No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between d4d703c and c524649.

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vllm/v1/worker/cp_utils.py

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📝 Summary

Summary by CodeRabbit

Bug Fixes
- Improved validation for attention configuration compatibility during decoding.
- Replaced an unclear internal assertion failure with a descriptive error message.
- Added platform-specific guidance, including backend switching or disabling the affected configuration when supported.
- On ROCm platforms, errors now clearly indicate when no compatible attention backend is available.

Walkthrough

The DCP compatibility check now imports current_platform and raises a platform-specific ValueError when the selected attention implementation cannot return softmax LSE during decode.

Changes

DCP compatibility validation

Layer / File(s)	Summary
Platform-aware DCP validation `vllm/v1/worker/cp_utils.py`	The DCP decode LSE assertion is replaced with `ValueError`. ROCm reports that no LSE-capable backend is available. Other platforms suggest changing `--attention-backend` or disabling DCP.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to c5246

DCP configurations without decode softmax-LSE support now fail with actionable platform-specific guidance, including clear ROCm unavailability messaging. No current merge-readiness risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: a platform-aware DCP error message for ROCm.
Description check	✅ Passed	The description accurately explains the DCP error change, ROCm-specific behavior, non-ROCm guidance, motivation, and scope.
Linked Issues check	✅ Passed	The change satisfies the directly linked issue's DCP objective [`#54761`] by replacing the assertion with an always-active ValueError and providing ROCm-specific guidance. The linked issue's separate KV…
Out of Scope Changes check	✅ Passed	The changes are limited to the requested DCP error handling and platform-specific guidance. No unrelated code changes are described.

Full details: Linked Issues check

Explanation

The change satisfies the directly linked issue's DCP objective [#54761] by replacing the assertion with an always-active ValueError and providing ROCm-specific guidance. The linked issue's separate KV-cache dtype objective is explicitly outside this PR's scope.

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🛠️ Fix failing CI checks 💡

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github-actions · 2026-09-03T19:05:24Z

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🚀

On ROCm, no attention backend currently returns softmax LSE during decode, making Decode Context Parallelism (DCP) impossible. The previous assertion suggested trying --attention-backend, which is unhelpful since no suitable backend exists on the platform. Replace the assert with a ValueError that includes platform-specific guidance: on ROCm it states that DCP is not available, on other platforms it suggests switching backends. Also converts assert to raise ValueError so the error survives python -O. Closes vllm-project#54761 Co-authored-by: Trae AI Assistant Signed-off-by: renrui <renrui.1234@bytedance.com>

github-actions · 2026-09-10T03:45:51Z

✅ @renruirui1234, CI is now available for this PR.

/ci run starts upstream CI; /amd-ci run starts AMD CI only.
/ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
/amd-ci retry retries failed jobs in AMD CI for the current PR head. Use /amd-ci run when the current head has no AMD CI build.
/ci cancel cancels scheduled or running CI builds for this PR branch; /amd-ci cancel does the same for AMD CI only.

tjtanaa · 2026-09-10T03:47:53Z

#55780 supports dcp with lse right now. So this PR needs to be reevaluated.

renruirui1234 requested a review from njhill as a code owner September 3, 2026 17:16

claude Bot reviewed Sep 3, 2026

View reviewed changes

mergify Bot added rocm Related to AMD ROCm bug Something isn't working labels Sep 3, 2026

github-project-automation Bot added this to AMD Sep 3, 2026

github-project-automation Bot moved this to Todo in AMD Sep 3, 2026

renruirui1234 force-pushed the fix/rocm-error-messages branch from c524649 to 5c27be3 Compare September 4, 2026 03:15

AndreasKaratzas mentioned this pull request Sep 8, 2026

[Attention] Require explicit DCP support from attention implementations #55780

Merged

tjtanaa added the ready ONLY add when PR is ready to merge/full CI is needed label Sep 10, 2026

Merge branch 'main' into fix/rocm-error-messages

e2a2246

tjtanaa removed the ready ONLY add when PR is ready to merge/full CI is needed label Sep 10, 2026

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[Bugfix] Platform-aware DCP error message for ROCm - #55188

[Bugfix] Platform-aware DCP error message for ROCm#55188
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renruirui1234 commented Sep 3, 2026

Summary

Motivation

Not duplicating an existing PR

Test plan

AI assistance disclosure

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