[Bugfix][Attention] Zero the null KV block after CUDA graph capture

[sumsliu]

Purpose

Fixes #57156.

CUDA graph capture runs dummy batches whose block table is all zeros, so capture can leave
uninitialized values in block 0 of the KV cache. Attention kernels that deliberately gather
block 0 for masked or padded entries rely on the softmax mask to discard the result. That only
holds while the gathered data is finite: a non-finite value survives masking as 0 * NaN == NaN
in the value accumulation and poisons every real token in the batch.

Block 0 is the reserved null block (BlockPool.null_block, popped at init and never handed to a
request), so restoring it to zeros once after capture holds for the lifetime of the process.

Root cause

Observed on DeepSeek-V4.1-Flash with the FlashInfer SM120 sparse-MLA backend. In
sparse_mla_sm120/decode_dsv4_kernel.cuh, the scale load guards on the index:

// line 272
if (idx_raw >= 0) { scale_word = __ldg(...); }

while the NoPE/RoPE data load clamps instead:

// line 299
const int idx = (idx_raw >= 0) ? idx_raw : 0;

The intent is documented at line 431: "IO already gathered slot 0 into smem with idx clamped —
masking to -inf kills it in softmax". Masking sets the logit to -1e30 so softmax yields
p ≈ 0, but the PV accumulation still computes p * v, and in MLA the NoPE tensor is both K
and V. So non-finite data in slot 0 reaches acc_nope regardless of the mask.

Measured contents of block 0 immediately before the first real request, same build:

Mode	slot 0 NoPE	slot 0 RoPE
eager	0.0	0.0
PIECEWISE	NaN	~3.2e35

Eager never captures, so block 0 stays zeroed and the bug does not appear.

This is not specific to one backend: any kernel that gathers a sentinel block and relies on
masking has the same precondition. #57094 fixes a sibling case in Triton MLA decode where CUDA
graph padding produces 0/0.

Test Plan

Verified on 8x DGX Spark (GB10, sm_121), TP8 + EP8, DeepSeek-V4.1-Flash, with the per-call
workaround disabled so this change is the only thing preventing the NaN:

• single stream, 1024-token generation, three prompts
• concurrency 1 / 16 / 64, 1024-token input, 128-token output

Test Result

All completions correct, no NaN anywhere in the outputs or logs.

Check	Before	After
"The capital of France is"	""	" Paris."
Single stream	NaN	69 / 40 / 81 tok/s
Concurrency 64	NaN	244 tok/s

Speeds match the per-call workaround, confirming the once-after-capture placement is sufficient
and cheaper. Single-stream throughput with CUDA graphs enabled is 32.6 tok/s against 17.2 tok/s
under --enforce-eager.

Notes

I verified that block 0 holds non-finite data after capture and that zeroing it there fixes the
output, but I did not trace which specific write produces it — get_dummy_slot_mappings fills
with PAD_SLOT_ID and the fused SWA insert kernel does skip slot_id < 0, so the polluting
write is elsewhere. Restoring the invariant after capture is correct regardless, but if a
maintainer would rather fix the write itself I am happy to rework this.

---

AI assistance disclosure

Per AGENTS.md, this section covers
the required disclosures for AI-assisted contributions.

AI assistance was used. The investigation, root-cause analysis and patch were produced with
Claude Code. The submitter reviewed every changed line and reproduced the failure and the fix on
the hardware described below.

Not a duplicate. Searched before opening:

gh api "search/issues?q=repo:vllm-project/vllm+DeepSeekV41+NaN"
gh api "search/issues?q=repo:vllm-project/vllm+null_block+NaN"
gh api "search/issues?q=repo:vllm-project/vllm+flashinfer+sparse+mla+NaN"

The closest existing work is #57094, which fixes CUDA-graph padding NaN in Triton MLA decode
where stage 2 computes 0/0 for seq_len=0. That is a different backend and a different
mechanism; this PR addresses a clamped index reading a poisoned null block, where the value
accumulation computes 0 * NaN. #55636 is a sibling warmup-metadata bug on the same hardware but
surfaces as an illegal address rather than NaN. #57028 touches the same SM120 path but covers page
geometry, not the null block; the NaN reproduces independently of it.

Test commands and results. On 8x DGX Spark (GB10, sm_121), TP8 + EP8, DeepSeek-V4.1-Flash,
CUDA graphs enabled, with no other workaround in place so this change is the only thing preventing
the NaN:

# correctness
curl -s "$API/v1/completions" -d '{"model":"deepseek-v4.1-flash","prompt":"The capital of France is","max_tokens":4,"temperature":0}'
# before: ""        after: " Paris."

# single stream, 1024-token generation, three prompts
python3 bench.py single --rounds 3

# concurrency sweep, 1024-token input, 128-token output
python3 bench.py batch --concurrency 1,16,64 --in-len 1024 --out-len 128 --seed-offset 42

Model evaluation. This change affects serving output, so results are included rather than
waiting to be asked:

Check	Before	After
"The capital of France is"	""	" Paris."
Single stream (3 prompts)	NaN	69 / 40 / 81 tok/s
Concurrency 1 / 16 / 64	NaN	65 / 164 / 244 tok/s
NaN occurrences in output or logs	every token	0

Long-context retrieval was verified with the earlier per-call variant of this fix (zeroing block 0
before every kernel call), not with this once-after-capture version: a passcode buried at the 60%
mark is retrieved correctly at 131K, 262K, 524K and 1,040,139 input tokens, each with a unique
random prefix so the prefix cache is not hit. The two variants restore the same invariant, but I
have not re-run long context against this exact change.

Style: pre-commit run --all-files clean; all added lines are within the 88-character limit.

[claude]

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[mergify]

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

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

[sumsliu]

Following up on the root-cause finding in #57156 (comment): the compressor ring derives its slot mapping from the block table, so dummy/capture runs with zeroed block tables can write fp32 ring state into the null block.

#58560 addresses that write directly by mapping null-block ring slots to PAD. This PR restores the null block to zero after capture as an additional defensive measure. The independent GB10 validation reported in #58560 (comment) also supports the ring fix for #57156.

Would maintainers prefer to retain the post-capture zeroing as extra protection, or focus validation on #58560? If you'd like to keep this PR, could a maintainer enable upstream CI?

[sumsliu]

Closing in favor of #58560 (49e0f47). It prevents dummy/capture compressor-ring writes to block 0 and extends the metadata test to cover real requests, null-block requests, and padding.

There is no separate reproducer in this PR that warrants retaining the post-capture clearing. Current main has not been rerun on the 8×GB10 setup as part of this review; that end-to-end check remains open in #57156.