[Bugfix] Add Multiple of 16 block_size to triton fallback on rocm Attention to support qwen3_5 (#35923)

Signed-off-by: JartX <sagformas@epdcenter.es>
Co-authored-by: akaratza <akaratza@amd.com>
Co-authored-by: TJian <tunjian.tan@embeddedllm.com>

docs/design/attention_backends.md

@@ -173,7 +173,7 @@ Priority is **1 = highest** (tried first).
 | `FLEX_ATTENTION` | | fp16, bf16, fp32 | `auto`, `bfloat16` | Any | Any | ❌ | ✅ | ❌ | Decoder, Encoder Only | Any |
 | `ROCM_AITER_FA` | | fp16, bf16 | `auto`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | 16, 32 | 64, 128, 256 | ❌ | ❌ | ❌ | Decoder, Enc-Dec | N/A |
 | `ROCM_AITER_UNIFIED_ATTN` | | fp16, bf16 | `auto` | %16 | Any | ✅ | ✅ | ❌ | All | N/A |
-| `ROCM_ATTN` | | fp16, bf16, fp32 | `auto`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | 16, 32, 544 | 32, 64, 80, 96, 128, 160, 192, 224, 256 | ✅ | ✅ | ❌ | All | N/A |
+| `ROCM_ATTN` | | fp16, bf16, fp32 | `auto`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | %16 | 32, 64, 80, 96, 128, 160, 192, 224, 256 | ✅ | ✅ | ❌ | All | N/A |
 | `TREE_ATTN` | | fp16, bf16 | `auto` | %16 | 32, 64, 96, 128, 160, 192, 224, 256 | ❌ | ❌ | ❌ | Decoder | Any |
 | `TRITON_ATTN` | | fp16, bf16, fp32 | `auto`, `bfloat16`, `fp8`, `fp8_e4m3`, `fp8_e5m2` | %16 | Any | ✅ | ✅ | ❌ | All | Any |
 

vllm/v1/attention/backends/rocm_attn.py

@@ -174,25 +174,15 @@ class RocmAttentionBackend(AttentionBackend):
 
     @staticmethod
     def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
-        # ROCM paged attention kernel only supports block sizes 16 and 32
+        # ROCM paged attention native C++ kernel only supports block sizes 16 and 32
         # due to shared memory (LDS) constraints on AMD GPUs.
         # See csrc/rocm/attention.cu CALL_CUSTOM_LAUNCHER_BLK macro.
-
+        # However, vLLM allows support for any multiple of 16 via the Triton path.
-        # However, The limitations in [16, 32] are reasonable for a native C++ kernel,
+        # As addressed in PR: https://github.com/vllm-project/vllm/pull/31380,
-        # but vLLM should allow support for non-standard sizes via the Triton path,
+        # non-standard models (like qwen3-next with block_size 544, or qwen3_5
-        # as addressed in this PR: https://github.com/vllm-project/vllm/pull/31380,
+        # with 784 and 1056) are dynamically routed to our optimized Triton kernel
-        # where the Triton kernel under rocm_atten does not support inference
+        # in `do_kv_cache_update`.
-        # for a non-standard qwen3-next model with a block_size of 544.
+        return [MultipleOf(16)]
-        # We have fixed the Triton kernel so that the standard model uses the original
-        # bit-addressing logic, while the non-standard model
-        # uses our optimized kernel logic.
-        return [16, 32, 544]
-
-    @classmethod
-    def supports_block_size(cls, block_size: int | None) -> bool:
-        if block_size is None:
-            return True
-        return block_size in (16, 32, 544)
 
     @classmethod
     def get_supported_head_sizes(cls) -> list[int]:
@@ -463,11 +453,9 @@ class RocmAttentionImpl(AttentionImpl):
         # Get the actual block_size from value_cache
         # value_cache shape: [num_blocks, num_heads, head_size, block_size]
         block_size = value_cache.shape[3]
-        # Determine if it is a power of 2
-        is_pow2 = block_size > 0 and (block_size & (block_size - 1) == 0)
 
-        if is_pow2:
+        if block_size in (16, 32):
-            # Normal 16, 32, 64, etc., use vLLM native HIP C++ logic
+            # Normal 16, 32, use vLLM native HIP C++ logic
             PagedAttention.write_to_paged_cache(
                 key,
                 value,
@@ -479,7 +467,7 @@ class RocmAttentionImpl(AttentionImpl):
                 layer._v_scale,
             )
         else:
-            # Case B: Non-standard blocks (e.g., 544 in Qwen3),
+            # Case B: Non-standard blocks (e.g., 64, 128, 544 in Qwen3Next or Qwen3.5 ),
             # force using our modified Triton logic
             triton_reshape_and_cache_flash(
                 key,