From e6f35d3c69b23b2ed35afebde31a0567c40bc661 Mon Sep 17 00:00:00 2001
From: Wentao Ye <44945378+yewentao256@users.noreply.github.com>
Date: Fri, 21 Aug 2026 15:11:45 -0400
Subject: [PATCH] [DSv4 Perf] Adaptive topk width for dsv4, making #50004 back
 (#52823)

Signed-off-by: yewentao256 <zhyanwentao@126.com>
---
 .../kernels/attention/test_flashmla_sparse.py | 57 +++++++++++++++++++
 vllm/models/deepseek_v4/sparse_mla.py         | 24 +++++++-
 2 files changed, 78 insertions(+), 3 deletions(-)

diff --git a/tests/kernels/attention/test_flashmla_sparse.py b/tests/kernels/attention/test_flashmla_sparse.py
index eb953faa44..e180b035c5 100644
--- a/tests/kernels/attention/test_flashmla_sparse.py
+++ b/tests/kernels/attention/test_flashmla_sparse.py
@@ -4,6 +4,63 @@ import pytest
 import torch
 
 
+def test_deepseek_v4_c128a_adaptive_width_has_capture_stable_stride():
+    from vllm.models.deepseek_v4.sparse_mla import build_c128a_topk_metadata
+
+    device = torch.device("cuda")
+    capacity_width = 512
+    global_decode_buffer = torch.empty(
+        (2, capacity_width), dtype=torch.int32, device=device
+    )
+    prefill_buffer = torch.empty_like(global_decode_buffer)
+    kwargs = dict(
+        positions=torch.tensor([255, 511, 383, 639], device=device),
+        compress_ratio=128,
+        num_decode_tokens=2,
+        token_to_req_indices=torch.tensor(
+            [0, 1, 0, 1], dtype=torch.int32, device=device
+        ),
+        block_table=torch.tensor([[3], [5]], dtype=torch.int32, device=device),
+        block_size=capacity_width,
+        slot_mapping=torch.arange(4, dtype=torch.int64, device=device),
+        global_decode_buffer=global_decode_buffer,
+        decode_lens_buffer=torch.empty(2, dtype=torch.int32, device=device),
+        prefill_buffer=prefill_buffer,
+    )
+    captured_decode, _, captured_prefill = build_c128a_topk_metadata(
+        max_compressed_tokens=256,
+        **kwargs,
+    )
+    assert captured_decode.shape == captured_prefill.shape == (2, 256)
+    assert captured_decode.stride(0) == captured_prefill.stride(0) == capacity_width
+
+    captured_rows = torch.empty((4, 4), dtype=torch.int32, device=device)
+    captured_rows[:2].copy_(captured_decode[:, :4])
+    captured_rows[2:].copy_(captured_prefill[:, :4])
+    torch.accelerator.synchronize()
+    graph = torch.cuda.CUDAGraph()
+    with torch.cuda.graph(graph):
+        captured_rows[:2].copy_(captured_decode[:, :4])
+        captured_rows[2:].copy_(captured_prefill[:, :4])
+
+    global_decode_buffer.fill_(-99)
+    prefill_buffer.fill_(-99)
+    build_c128a_topk_metadata(
+        max_compressed_tokens=128,
+        **kwargs,
+    )
+    graph.replay()
+
+    assert captured_rows.cpu().tolist() == [
+        [1536, 1537, -1, -1],
+        [2560, 2561, 2562, 2563],
+        [0, 1, 2, -1],
+        [0, 1, 2, 3],
+    ]
+    assert torch.all(global_decode_buffer[:, 128:] == -99)
+    assert torch.all(prefill_buffer[:, 128:] == -99)
+
+
 def test_sparse_flashmla_metadata_smoke():
     import vllm.v1.attention.ops.flashmla as fm
 
diff --git a/vllm/models/deepseek_v4/sparse_mla.py b/vllm/models/deepseek_v4/sparse_mla.py
index 0475e4c96d..30e386df32 100644
--- a/vllm/models/deepseek_v4/sparse_mla.py
+++ b/vllm/models/deepseek_v4/sparse_mla.py
@@ -257,6 +257,15 @@ class DeepseekV4SparseMLAMetadataBuilder(
         assert cm.positions is not None, (
             "positions is required for C128A metadata build"
         )
+        active_topk_width = min(
+            max(
+                triton.next_power_of_2(max(cm.max_seq_len // self.compress_ratio, 1)),
+                _C128A_TOPK_ALIGNMENT,
+            ),
+            self.c128a_max_compressed,
+        )
+        assert active_topk_width >= cm.max_seq_len // self.compress_ratio
+        assert active_topk_width % _C128A_TOPK_ALIGNMENT == 0
         block_size = self.kv_cache_spec.block_size // self.compress_ratio
         global_decode, decode_lens, prefill_local = build_c128a_topk_metadata(
             cm.positions[:num_total],
@@ -269,7 +278,7 @@ class DeepseekV4SparseMLAMetadataBuilder(
             self.c128a_global_decode_buffer,
             self.c128a_decode_lens_buffer,
             self.c128a_prefill_buffer,
-            max_compressed_tokens=self.c128a_max_compressed,
+            max_compressed_tokens=active_topk_width,
         )
 
         result: dict[str, torch.Tensor | None] = {}
@@ -320,10 +329,19 @@ def build_c128a_topk_metadata(
     """
     num_tokens = positions.shape[0]
     num_prefill_tokens = num_tokens - num_decode_tokens
+    assert max_compressed_tokens % _C128A_TOPK_ALIGNMENT == 0
+    assert (
+        0
+        < max_compressed_tokens
+        <= min(global_decode_buffer.shape[1], prefill_buffer.shape[1])
+    )
+    assert global_decode_buffer.stride(-1) == prefill_buffer.stride(-1) == 1
 
-    global_decode = global_decode_buffer[:num_decode_tokens]
+    global_decode = global_decode_buffer[:num_decode_tokens, :max_compressed_tokens]
     decode_lens = decode_lens_buffer[:num_decode_tokens]
-    prefill_local = prefill_buffer[:num_prefill_tokens]
+    prefill_local = prefill_buffer[:num_prefill_tokens, :max_compressed_tokens]
+    assert global_decode.stride(0) == global_decode_buffer.stride(0)
+    assert prefill_local.stride(0) == prefill_buffer.stride(0)
 
     if num_tokens == 0:
         return global_decode, decode_lens, prefill_local