[XPU][Bugfix] Fix XPU crash with speculative decoding + prefix caching in Mamba models - #52186
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Signed-off-by: Swapnil Khandekar <10198332+swakhandekar@users.noreply.github.com>
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…eGPUContext
On XPU, data_ptr() values can be >= 2**63, which overflows the
element-wise int64 tensor assignment in
MambaSpecDecodeGPUContext.initialize_from_forward_context when spec
decoding and prefix caching are both enabled
("ValueError: Overflow when unpacking long long").
This test pins the uint64 bulk-creation fix:
- the uint64 dtype asserts run on any accelerator, so a revert to
int64 fails in CUDA CI as well
- on devices that allocate above 2**63 (XPU), it additionally
verifies the recorded state addresses round-trip exactly
Verified on XPU: fails on upstream/main with the original overflow
error (1 failed, 4 passed); passes with the fix (5 passed).
Co-authored-by: Claude <noreply@anthropic.com>
Signed-off-by: Swapnil Khandekar <10198332+swakhandekar@users.noreply.github.com>
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Fixes: #48059
Alternate fix: #48109
Why this PR:
Did not notice the reported bug and alternate fix when I created the PR, apologies.
The pattern this PR implements is more consistent with implementation across codebase. Re-interpretation to a negative-int64 in the alternate fix breaks the established pattern.
Purpose
Models tested:
HW Backend: XPU
Hardware: Intel ARC Pro B70
vllm version: 0.27.2rc1.dev122+g8efa13b70
If I only enable prefix caching, I see no error and vllm can process requests. If I only use spec decoding(MTP) that works fine too. But if I use both together I get following error:
Root cause:
On XPU,
tensor.data_ptr()can return memory addresses >= 2^63 — exceeding the signedint64range. TheMambaSpecDecodeGPUContextclass stores these addresses via element-wise assignment (tensor[idx] = value), which fails because PyTorch's__setitem__converts the scalar through C-level long() before any dtype check, regardless of the tensor's declared dtype.Fix applied:
Two changes, both in
vllm/v1/worker/mamba_utils.py:Test Plan
Re-build and run the same model with both
--speculative-config '{"method":"mtp","num_speculative_tokens":3}'and--enable-prefix-caching. Send a request and it should not throw following error.Logs before (updated after merge):
Test Result
Mechanism: Locally built docker image with the applied change.
vllm command:
Result: No errors
Sample logs:
AI Disclosure:
Have taken assistance of same model i.e. cyankiwi/Qwen3.6-27B-AWQ-INT4 via Claude code.
Essential Elements of an Effective PR Description Checklist
supported_models.mdandexamplesfor a new model.