[Bugfix][MLA][DCP] Round chunked-prefill workspace and max_context_chunk to satisfy DCP divisibility asserts

[jin-amd]

Purpose

What is the bug like?
On 8× MI300X with --decode-context-parallel-size 8 --enable-expert-parallel, our Kimi-K2.5-W4A8 engine intermittently died with all 8 DCP ranks hitting assert max_context_chunk % self.dcp_world_size == 0 in mla_attention.py:1942 simultaneously. The crash is triggered by their workload pattern — a shared prefix-cached prefix plus many concurrent short tails — which drives num_prefills_with_context_cpu to values like {3,5,6,7,9,10,11,13}, where chunked_prefill_workspace_size // n lands on integers that are not multiples of 8. It only reproduces on the ROCm path because there aot_schedule=False, so the existing round_down(max_context_chunk, page_size) step that would have masked the bad divisor on CUDA never runs.

Two divisibility asserts in vllm/model_executor/layers/attention/mla_attention.py can fire when --decode-context-parallel-size > 1:

1. Init-time (line 1617): assert self.chunked_prefill_workspace_size % self.dcp_world_size == 0
2. Per-step (line 1942, in MLACommonMetadataBuilder.build): assert max_context_chunk % self.dcp_world_size == 0

The init-time case is the regression gemini-code-assist flagged on #25478 (the max(workspace_size, max_num_seqs * block_size) clamp) but never fixed. The per-step case is reproducibly hit on ROCm — where aot_schedule=False so the page-size round-down doesn't run — by shared-prefix-cached prefix + many concurrent short tails (same crash family as #28476; #28526 only fixed the adjacent reorg_kvcache shape assert).

Concretely: with chunked_prefill_workspace_size=65536, dcp_world_size=8, no page-size round-down, and num_prefills_with_context_cpu ∈ {3,5,6,7,9,10,11,13}, the floor-divide produces 21845 / 13107 / 10922 / 9362 / 7281 / 6553 / 5957 / 5041 — none divisible by 8 — and the per-step assert kills all DCP ranks.

This PR makes both asserts provably unreachable with two surgical edits, both rounding down only — no allocation grows; worst-case shrink is lcm(page_size, dcp_world_size) − 1 slots (≤ 0.1% on a 64K workspace):

1. determine_chunked_prefill_workspace_size: round the returned size down to a multiple of dcp_world_size when DCP > 1.
2. New _align_max_context_chunk_for_dcp static helper, called from build() after the existing page-size round_down. Rounds to lcm(page_size, dcp_world_size), or just dcp_world_size when aot_schedule=False.

Test Plan

Customer-shaped repro that drives num_prefills_with_context_cpu into the bad-divisor range. Run command:

VLLM_ROCM_USE_AITER=1 VLLM_ROCM_USE_AITER_FP4BMM=0 VLLM_ROCM_USE_AITER_MLA=0 \
VLLM_ATTENTION_BACKEND=TRITON_MLA VLLM_ROCM_USE_AITER_FUSION_SHARED_EXPERTS=0 \
vllm serve amd/Kimi-K2.5-W4A8 \
    --tensor-parallel-size 8 --decode-context-parallel-size 8 \
    --enable-expert-parallel --trust-remote-code \
    --gpu-memory-utilization 0.9

Stress driver (1× MI300X, 8 GPUs): prime an 80K shared prefix, then 6 rounds × 16 concurrent partial-prefills (<80K prefix> + <unique 64-tok tail>, total 96 requests).

Test Result

Un-patched upstream (ROCm aot_schedule=False path): reliably crashes within the first few rounds with all 8 DCP ranks asserting on mla_attention.py:1942 simultaneously, engine dead.

With this PR: 96 / 96 requests returned 200, /health stayed 200 throughout, no engine restarts. Earlier instrumented runs counted 80 distinct events where the new lcm(page_size, dcp_world_size) rounding actually changed max_context_chunk — i.e. 80 places where un-patched code would have asserted — with num_prefills_with_context_cpu observed in 3..13.

Performance neutrality — back-to-back c=32 saturation sweep (random dataset, --ignore-eos, 128 prompts, identical seeds) on three workloads:

Workload (in / out)	Throughput Δ	TPOT Δ	P99 ITL Δ
1024 / 1024	within ±1%	within ±1%	within ±1%
1024 / 8192	within ±1%	within ±1%	within ±1%
8192 / 1024	within ±1%	within ±1%	within ±1%

The round-down (≤ 7 slots out of ~13K for max_context_chunk, ≤ 7 of 65 536 for the workspace) has no measurable cost.

---

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[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request introduces alignment logic for Decode Context Parallelism (DCP) within the MLA attention implementation. It ensures that the chunked prefill workspace size and the maximum context chunk size are divisible by the DCP world size to satisfy downstream assertions and prevent runtime errors. Specifically, it adds rounding logic to the workspace size determination and introduces a helper method to align context chunks based on the least common multiple of the page size and DCP world size. I have no feedback to provide as there were no review comments to evaluate.

[mergify]

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

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

[jin-amd]

Closing this as obsolete — both divisibility issues have since been fixed upstream by the
MLA chunked-context refactor, and rebasing onto current main yields an empty diff.

Per-step assert (assert max_context_chunk % self.dcp_world_size == 0, the one
reproducibly hit on ROCm): removed in c4f5cd6 (#47327). Chunk sizing then moved out of
MLACommonMetadataBuilder.build into the module-level build_mla_chunked_context_metadata
by #50613, which also deleted the chunked_prefill_workspace_size // num_prefills_with_context_cpu floor-divide that produced the bad divisors in the first
place. max_context_chunk is now round_down(chunked_prefill_workspace_size, chunk_alignment) where, under DCP, chunk_alignment = lcm(block_size_or_1, dcp_virtual_block_size) and dcp_virtual_block_size = cp_kv_cache_interleave_size * dcp_world_size — so divisibility by dcp_world_size is now structural rather than
asserted. This covers the ROCm case this PR targeted: with align_chunk_to_block=False the
alignment is lcm(1, dcp_virtual_block_size), still a multiple of dcp_world_size, so the
"aot_schedule=False skips the page-size round-down" hole is closed.

Init-time assert (assert self.chunked_prefill_workspace_size % self.dcp_world_size == 0):
still present, but now provably satisfied. determine_chunked_prefill_workspace_size ends by
delegating to align_mla_chunked_context_workspace_size (added in 63ac04a, #50484), which
rounds the workspace up to lcm(block_size, dcp_world_size * cp_kv_cache_interleave_size)
when DCP > 1. Separately, the max(workspace_size, max_num_seqs * block_size) clamp from
#25478 that could knock the size back off alignment was removed by #50613, so there is no
longer a post-hoc step that can break divisibility.

Verified by rebasing this branch onto main: all three conflict hunks resolve in favor of
upstream, and the resulting tree is identical to main. Nothing from this PR is still
needed — the upstream fix is the more thorough one.