[Bugfix] Remove duplicate size_k divisibility check in get_moe_wna16_block_config

[varjoranta]

Purpose

`get_moe_wna16_block_config` in `vllm/model_executor/layers/fused_moe/fused_moe.py` checks `size_k % (block_size_k * 2) == 0` on two adjacent lines inside the same `if`. The duplicate clause is a no-op.

```python
if (
num_m_blocks <= 16
and size_k % (block_size_k * 2) == 0
and size_k % (block_size_k * 2) == 0 # <-- this line
and block_size_k <= 512
and num_blocks >= 512
):
```

Note for reviewer

Looking at surrounding code, `size_n` is also in scope and is the other dimension that `block_size_k` divides (line 1172: `num_k_blocks = size_n // block_size_k`). It's plausible the second clause was meant to read `size_n % (block_size_k * 2) == 0` — which would make the `block_size_k` doubling path safer on shapes where `size_n` isn't aligned to the larger block size.

Keeping this PR to the literal dedupe rather than changing semantics. Happy to submit a follow-up PR promoting the second clause to `size_n`-based if that was the original intent.

Test plan

• No existing tests exercise this exact path, but the change is literally removing a duplicate condition — the resulting expression is logically equivalent.

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request removes a redundant duplicate condition in the get_moe_wna16_block_config function within fused_moe.py. The reviewer suggests that rather than just deleting the line, it should be replaced with a check on size_n to address a latent bug in how num_blocks is calculated when block_size_k is adjusted.

[gemini-code-assist]

Instead of just removing this duplicate line, I suggest replacing it with a check on size_n, as you speculated in the PR description. This would fix a latent bug in the surrounding logic.

Currently, when block_size_k is doubled (line 1190), num_blocks is only divided by 2 (line 1191). However, num_blocks depends on block_size_k through both num_n_blocks (size_k // block_size_k) and num_k_blocks (size_n // block_size_k). If both size_k and size_n are divisible by the new block_size_k, then num_blocks should be divided by 4.

By adding the check for size_n, you can then correctly update num_blocks //= 4 on line 1191. Since I cannot suggest a change outside the diff, I'm only suggesting the change for this line.

Suggested change

	and size_k % (block_size_k * 2) == 0
	and size_n % (block_size_k * 2) == 0

[varjoranta]

Thanks @gemini-code-assist — I flagged that possibility in the PR description but wanted to keep this PR as a literal no-op. Since you raised it, I took a closer look at the whole function and there are actually a few related oddities worth flagging for a potential follow-up:

1. The duplicate clause (this PR's target). Second size_k % (block_size_k * 2) == 0 was almost certainly meant to be size_n % (block_size_k * 2) == 0 — symmetric alignment check for the other dimension block_size_k tiles on line 1171.

2. Stale-value bug on line 1181:

if size_k % 256 == 0 and num_blocks >= 256 and block_size_k < 256:
    block_size_k = 256
    num_blocks = num_blocks // (256 // block_size_k)  # block_size_k already = 256

block_size_k was just set to 256 on the prior line, so 256 // block_size_k == 1 and the division is a no-op. The intent was presumably to compute the ratio before the reassignment.

3. num_blocks //= 2 vs //= 4 (gemini's point, line 1191). Follows from num_n_blocks and num_k_blocks both depending on block_size_k per lines 1170-1171 — but see point 4.

4. num_n_blocks //= 2 on line 1195 looks orphaned. Fires when block_size_n doubles, but num_n_blocks is defined on line 1170 as size_k // block_size_k — doesn't involve block_size_n or size_n. This only "makes sense" if lines 1170-1171 have swapped assignments (intended: num_n_blocks = size_n // block_size_n, num_k_blocks = size_k // block_size_k). Under that reading, //= 2 on line 1191 is correct and gemini's //= 4 is wrong. The two interpretations are mutually exclusive.

5. Minor — line 1174 uses / instead of //, so num_m_blocks (and the downstream num_blocks) is a float. Inconsistent with everything else treating these as ints.

Blast radius

All of 2–5 only affect callers that don't already have BLOCK_SIZE_N/BLOCK_SIZE_K in a tuned config (line 1151 early-returns otherwise), so users with benchmark_moe.py tuned JSON are unaffected. Default-config MoE wna16 CUDA users (GPTQ/AWQ int4 MoE) could see different block choices post-fix — direction unclear without a tuning smoke.

Proposal

Keep this PR as the narrow no-op dedupe so it can land without a perf discussion. Happy to open a follow-up covering:

• Issue 1 (promote duplicate to size_n)
• Issue 2 (stale-value fix)
• Issue 5 (/ → //)
• Either issue 3 or the 1170-1171 swap from issue 4, whichever reflects the kernel's actual dim convention — that one needs @JinZhenlin or @mgoin to adjudicate

…with a short MoE wna16 tuning smoke to guard against regressions. Want me to spin that up?

[gemini-code-assist]

Thanks for the detailed breakdown of these issues, @varjoranta. Your analysis of the potential logic errors and the orphaned num_n_blocks calculation is very helpful. Given the complexity and the potential impact on MoE tuning, I agree that keeping this PR as a narrow no-op fix is the right approach to ensure it lands safely. Please go ahead and open a follow-up PR to address the issues you've identified (2–5), and feel free to tag the relevant maintainers for their input on the dimension convention in issue 4.

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