[The-iyed]

Purpose

Fixes #58214.

CPUModelRunner._postprocess_triton installs C++ CPU fallbacks for Triton kernels when Triton is unavailable — the production path on linux/arm64 and macOS, since docker/Dockerfile.cpu builds triton-cpu only for TARGETARCH=amd64. CI's CPU lane is x86, where Triton is present, so these fallbacks are never exercised there.

Two independent defects stop them working:

1. Six patches never take effect. #56323 (aed894c19) moved the sampling and DFlash kernels behind triton_kernel_dispatcher_with_warmup, which captures the kernel on the dispatcher instance at import and launches self.kernel. Three patches were updated to assign .kernel; six were left rebinding the module-level kernel name, which nothing reads at call time any more. Before that commit these were launched by module-global lookup (expand_kernel[(batch_size,)](...) at rejection_sampler.py:597 in parent e19a3e172), so the rebinding used to work.

The dead setattr onto the dflash module is dropped too: dflash.py imports and calls the dispatcher, never that name. It was the only user of import sys in the file.

2. Two shim signatures do not bind. _launch_binding_plan maps each dispatch input to a kernel argument named name, name_ptr, NAME or NAME_PTR. _expand_kernel_impl took input_val (dispatch input is input) and _eagle_step_slot_mapping_metadata_kernel_impl took stride (dispatch input is block_table_stride, and that dispatch passes no launch kwargs). Both are renamed.

Why this is not a duplicate

• [Bugfix][CPU] Use CPU fallback for Mamba batch_memcpy without Triton #55480 fixes mamba_utils.batch_memcpy only, at its call site. Different kernel, untouched here.
• [Bugfix][CPU][Spec Decode] Write expand_kernel output at caller dtype in CPU shim #55006 / [Bug][CPU][Spec Decode]: CPU expand_kernel shim discards its output — non-greedy spec decode consumes uninitialized memory as temperature/top_k/top_p #55005 fix a dtype copy-back inside _expand_kernel_impl. Orthogonal, and that shim is currently never installed on arm64. This PR does not change its body.
• No open PR references cpu_triton_utils or _postprocess_triton patching.

Test Plan

New tests/v1/worker/test_cpu_triton_fallbacks.py parametrizes over all nine dispatchers and asserts _postprocess_triton() installs each CPU fallback. It would have caught #56323. Run on linux/arm64 with Triton absent (HAS_TRITON = False):

.venv/bin/python -m pytest tests/v1/worker/test_cpu_triton_fallbacks.py -q --noconftest

Test Result

On unmodified main — the six module-global patches fail, the three .kernel patches pass:

6 failed, 3 passed, 14 warnings in 4.09s
FAILED ...[_copy_and_expand_dflash_inputs-copy_and_expand_dflash_inputs_kernel]
FAILED ...[_eagle_step_slot_mapping_metadata-eagle_step_slot_mapping_metadata_kernel]
FAILED ...[_rejection_greedy_sample-rejection_greedy_sample_kernel]
FAILED ...[_rejection_random_sample-rejection_random_sample_kernel]
FAILED ...[_expand-expand_kernel]
FAILED ...[_sample_recovered_tokens-sample_recovered_tokens_kernel]

With this PR:

9 passed, 14 warnings in 3.80s

End-to-end, calling expand_batch_to_tokens() on the CPU path:

state	result
main	TypeError: Cannot inspect kernel parameters for function
retarget only	TypeError: _expand_kernel_impl() missing 1 required positional argument: 'input_val'
this PR	reaches the bound C++ op

Limitation, stated plainly: my native extension build has not finished, so the final call ends at '_C' object has no attribute 'expand_kernel_impl' — the Python dispatch chain is verified, the C++ kernel execution is not. I have not run a full arm64 spec-decode serving test, and I would value a reviewer with a complete arm64 build confirming that last hop. No model-output or accuracy change is expected: this only determines whether the existing C++ fallbacks are reached at all.

Notes

AI assistance was used to investigate and prepare this change. I reviewed every changed line and ran the tests above.

Happy to split the two defects into separate PRs, or to drop the first half if maintainers prefer making the dispatcher resolve its kernel dynamically so this class of breakage cannot recur.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[The-iyed]

Verified on a completed arm64 build — and a sequencing warning

My native build has finished, so I can replace the "not tested" caveat with real execution, and I found something reviewers need before merging this.

Build: vllm 0.1.dev1+gbc162b3f9 on linux/arm64, torch 2.13.0+cpu, HAS_TRITON = False, torch.ops._C.expand_kernel_impl bound.

1. The regression test, now executed against the compiled extension

Unmodified main:

6 failed, 3 passed, 14 warnings in 3.40s

With this PR:

9 passed, 14 warnings in 3.16s

2. ⚠️ This PR alone turns a loud failure into a silent one

Installing the fallbacks makes _expand_kernel_impl reachable — and it is the shim reported in #55005 as discarding its output. I confirmed that directly with a sentinel buffer (no new_empty guesswork):

expand_kernel (output buffer dtype round-trip):
  PASS  expand out dtype=int64
  FAIL  expand out dtype=int32     got [-1,-1,-1,-1,-1,-1]  want [7,7,5,5,5,9]
  FAIL  expand out dtype=float32   got [-1,-1,-1,-1,-1,-1]  want [7,7,5,5,5,9]

The sentinel survives untouched, so the caller's buffer is provably never written for non-int64 dtypes.

End to end with this PR applied, on the real production call:

expand_batch_to_tokens(temperature=[0.7, 0.5, 0.9]) over 6 tokens
  expected: [0.7, 0.7, 0.5, 0.5, 0.5, 0.9]
  trial 0:  [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
  trial 1:  [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
  trial 2:  [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

Those zeros are uninitialized new_empty pages consumed as temperature, so logits.div_(temperature) divides by zero.

So merging this alone would take arm64 CPU spec decode from a loud TypeError at launch to silently sampling from corrupted logits — which is worse. #55006 (or an equivalent copy-back in _expand_kernel_impl) should land first or together with this.

I have deliberately not touched _expand_kernel_impl here, since #55006 owns it and duplicating it would violate the contribution policy. Happy to rebase on top of it, or to fold the one-line copy-back in if you would rather have a single self-contained change — your call.

3. What passes

rejection_greedy_sample's shim is correct. Against a reference ported from the Triton source, including dtype round-trip and is_greedy masking:

rejection_greedy_sample (2-D [batch, max_spec_len+1]):
  PASS  greedy out dtype=int64
  PASS  greedy out dtype=int32
  PASS  greedy is_greedy=[T,F]

4. Correction to my own earlier report

In the issue I first wrote the failure as TypeError: 'function' object is not subscriptable. On a real build it is:

TypeError: Cannot inspect kernel parameters for function

It fails earlier, during kernel argument inspection, and never reaches the kernel[grid] subscript. Already corrected in #58214.

Still not tested

A full arm64 spec-decode serving run with a real draft/target model pair. I verified the kernel-level and dispatch-level behaviour; I have not measured accepted-token rates end to end.

AI assistance was used for this investigation; I ran every command shown above.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @The-iyed.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[The-iyed]

Superseded by #43048 ([Feature] Triton kernel dispatcher), merged today. Closing.

#43048 removes CPUModelRunner._postprocess_triton() and the module-global
monkey-patching entirely, and registers the CPU fallbacks through
CpuPlatform.register_triton_kernel_overrides() →
vllm.triton_utils.dispatcher.register_kernels(). That registration names every
one of the six kernels this PR was retargeting:

vllm.v1.spec_decode.utils.eagle_step_slot_mapping_metadata_kernel
vllm.v1.spec_decode.utils.copy_and_expand_dflash_inputs_kernel
vllm.v1.sample.rejection_sampler.rejection_greedy_sample_kernel
vllm.v1.sample.rejection_sampler.rejection_random_sample_kernel
vllm.v1.sample.rejection_sampler.expand_kernel
vllm.v1.sample.rejection_sampler.sample_recovered_tokens_kernel

So the failure this PR fixed — patches rebinding a module global the dispatcher
no longer reads, leaving the fallbacks uninstalled on HAS_TRITON = False
builds — is fixed structurally, and the code this diff touches no longer exists.
A rebase would mean re-adding lines to a deleted function; there is nothing left
to port. The shim-signature rename is also unnecessary now: KernelOverride
sets _forward_by_name only when the impl signature matches the kernel's
arg_names, and otherwise forwards positionally in arg_names order, so
_expand_kernel_impl(output, input_val, ...) binds correctly against a kernel
whose argument is named input.

One thing that outlives this PR, for anyone landing on it from the CPU side:
_expand_kernel_impl still wraps its output in _ensure_int64(output)
(vllm/utils/cpu_triton_utils.py), so for a non-int64 caller buffer the C++
kernel writes into a temporary that is never copied back. That is #55005, fixed
by #55006. Before #43048 that shim was unreachable on CPU because the fallbacks
were never installed; now that they register correctly, the dtype bug is on the
live path rather than latent.

Thanks @wangxiyuan — the dispatcher is a much better fix than retargeting the
patches one by one.