port cudnn API.

Author: Elfie Guo

vllm/envs.py

@@ -139,6 +139,7 @@
     VLLM_ROCM_QUICK_REDUCE_CAST_BF16_TO_FP16: bool = True
     VLLM_ROCM_QUICK_REDUCE_MAX_SIZE_BYTES_MB: Optional[int] = None
     VLLM_NIXL_ABORT_REQUEST_TIMEOUT: int = 120
+    VLLM_USE_CUDNN_PREFILL: bool = False
 
 
 def get_default_cache_root():
@@ -961,7 +962,11 @@ def get_vllm_port() -> Optional[int]:
     # consumer. This is only applicable when using NixlConnector in a
     # disaggregated decode-prefill setup.
     "VLLM_NIXL_ABORT_REQUEST_TIMEOUT":
-    lambda: int(os.getenv("VLLM_NIXL_ABORT_REQUEST_TIMEOUT", "120"))
+    lambda: int(os.getenv("VLLM_NIXL_ABORT_REQUEST_TIMEOUT", "120")),
+
+    # Controls whether or not to use cudnn prefill
+    "VLLM_USE_CUDNN_PREFILL":
+    lambda: bool(int(os.getenv("VLLM_USE_CUDNN_PREFILL", "1")))
 }
 
 # --8<-- [end:env-vars-definition]

vllm/v1/attention/backends/mla/common.py

@@ -194,6 +194,7 @@
 
 import torch
 
+import vllm.envs as envs
 from vllm import _custom_ops as ops
 from vllm.attention.backends.abstract import (AttentionBackend, AttentionLayer,
                                               AttentionMetadata,
@@ -228,6 +229,9 @@
 
 logger = init_logger(__name__)
 
+CUDNN_SUPPORTED_HEAD_DIMS = [192, 128]
+CUDNN_WORKSPACE_SIZE = 12800
+
 
 class MLACommonBackend(AttentionBackend):
 
@@ -282,11 +286,14 @@ class ChunkedContextMetadata:
         starts: torch.Tensor
         seq_tot: list[int]
         max_seq_lens: list[int]
+        seq_lens: torch.Tensor
         workspace: torch.Tensor
 
     block_table: torch.Tensor
     query_start_loc: torch.Tensor
+    query_seq_lens: torch.Tensor
     max_query_len: int
+    workspace: torch.Tensor
     chunked_context: Optional[ChunkedContextMetadata] = None
 
 
@@ -390,6 +397,12 @@ def __init__(self,
                 dtype=model_config.dtype,
                 device=runner.device,
             )
+            self.workspace = torch.empty(
+                CUDNN_WORKSPACE_SIZE * scheduler_config.max_num_seqs,
+                dtype=torch.int8,
+                device=runner.device,
+            )
+
         self.block_table = block_table
 
     def reorder_batch(self, input_batch: "InputBatch",
@@ -566,6 +579,7 @@ def build(self, common_prefix_len: int,
                     starts=chunk_starts.to(device, non_blocking=True),
                     seq_tot=chunk_seq_lens.sum(dim=1).tolist(),
                     max_seq_lens=chunk_seq_lens.max(dim=1).values.tolist(),
+                    seq_lens=chunk_seq_lens,
                     workspace=self.chunked_prefill_workspace,
                 )
 
@@ -576,6 +590,9 @@ def build(self, common_prefix_len: int,
                 block_table=block_table_tensor[reqs_start:, ...],
                 query_start_loc=prefill_query_start_loc,
                 max_query_len=max_query_len,
+                workspace=self.workspace,
+                query_seq_lens=prefill_query_start_loc[1:] -
+                prefill_query_start_loc[:-1],
                 chunked_context=chunked_context_metadata,
             )
 
@@ -663,9 +680,10 @@ def __init__(
         # v with 0s to match the qk head dim for attention backends that do
         # not support different headdims
         # We don't need to pad V if we are on a hopper system with FA3
-        self._pad_v = self.vllm_flash_attn_version is None or not (
-            self.vllm_flash_attn_version == 3
-            and current_platform.get_device_capability()[0] == 9)
+        self._pad_v = not envs.VLLM_USE_CUDNN_PREFILL and (
+            self.vllm_flash_attn_version is None
+            or not (self.vllm_flash_attn_version == 3
+                    and current_platform.get_device_capability()[0] == 9))
 
     def _flash_attn_varlen_diff_headdims(self,
                                          q,
@@ -705,6 +723,40 @@ def _flash_attn_varlen_diff_headdims(self,
             return attn_out, lse
         return attn_out
 
+    def _cudnn_varlen_func_diff_headdims(
+        self,
+        q,
+        k,
+        v,
+        scale,
+        workspace,
+        max_q_seq_lens,
+        max_kv_seq_lens,
+        seq_lens_q,
+        seq_lens_kv,
+        causal,
+        is_cuda_graph_compatible=True,
+    ):
+        from flashinfer.prefill import cudnn_batch_prefill_with_kv_cache
+
+        if not is_cuda_graph_compatible:
+            seq_lens_q = seq_lens_q.to("cpu")
+            seq_lens_kv = seq_lens_kv.to("cpu")
+        return cudnn_batch_prefill_with_kv_cache(
+            q=q,
+            k_cache=k,
+            v_cache=v,
+            scale=scale,
+            workspace_buffer=workspace,
+            max_token_per_sequence=max_q_seq_lens,
+            max_sequence_kv=max_kv_seq_lens,
+            actual_seq_lens_q=seq_lens_q.view(-1, 1, 1, 1),
+            actual_seq_lens_kv=seq_lens_kv.view(-1, 1, 1, 1),
+            causal=causal,
+            return_lse=True,
+            is_cuda_graph_compatible=is_cuda_graph_compatible,
+        )
+
     def _v_up_proj(self, x):
         # Convert from (B, N, L) to (N, B, L)
         x = x.view(-1, self.num_heads, self.kv_lora_rank).transpose(0, 1)
@@ -803,19 +855,41 @@ def _compute_prefill_context(
             k = torch.cat((k_nope, k_pe.expand((*k_nope.shape[:-1], -1))),
                           dim=-1)
 
-            attn_output, attn_softmax_lse = \
-                self._flash_attn_varlen_diff_headdims(
-                q=q,
-                k=k,
-                v=v,
-                cu_seqlens_q=prefill_metadata.query_start_loc,
-                cu_seqlens_k=prefill_metadata.chunked_context.cu_seq_lens[i],
-                max_seqlen_q=prefill_metadata.max_query_len,
-                max_seqlen_k=prefill_metadata.chunked_context.max_seq_lens[i],
-                softmax_scale=self.scale,
-                causal=False,  # Context is unmasked
-                return_softmax_lse=True,
-            )
+            if envs.VLLM_USE_CUDNN_PREFILL and all(
+                    t.shape[-1] in CUDNN_SUPPORTED_HEAD_DIMS
+                    for t in (q, k, v)):
+                attn_output, attn_softmax_lse = (
+                    self._cudnn_varlen_func_diff_headdims(
+                        q,
+                        k,
+                        v,
+                        scale=self.scale,
+                        workspace=prefill_metadata.workspace,
+                        max_q_seq_lens=prefill_metadata.max_query_len,
+                        max_kv_seq_lens=prefill_metadata.chunked_context.
+                        max_seq_lens[i],
+                        seq_lens_q=prefill_metadata.query_seq_lens.view(
+                            -1, 1, 1, 1),
+                        seq_lens_kv=prefill_metadata.chunked_context.
+                        seq_lens[i].view(-1, 1, 1, 1),
+                        causal=False,
+                        is_cuda_graph_compatible=
+                        True,  #Indicates actual_seq_lens are on GPU or CPU.
+                    ))
+            else:
+                attn_output, attn_softmax_lse = \
+                    self._flash_attn_varlen_diff_headdims(
+                    q=q,
+                    k=k,
+                    v=v,
+                    cu_seqlens_q=prefill_metadata.query_start_loc,
+                    cu_seqlens_k=prefill_metadata.chunked_context.cu_seq_lens[i],
+                    max_seqlen_q=prefill_metadata.max_query_len,
+                    max_seqlen_k=prefill_metadata.chunked_context.max_seq_lens[i],
+                    softmax_scale=self.scale,
+                    causal=False,  # Context is unmasked
+                    return_softmax_lse=True,
+                )
 
             if output is None:
                 output = attn_output
@@ -854,18 +928,39 @@ def _forward_prefill(
 
         k = torch.cat((k_nope, k_pe.expand((*k_nope.shape[:-1], -1))), dim=-1)
 
-        output = self._flash_attn_varlen_diff_headdims(
-            q=q,
-            k=k,
-            v=v,
-            cu_seqlens_q=attn_metadata.prefill.query_start_loc,
-            cu_seqlens_k=attn_metadata.prefill.query_start_loc,
-            max_seqlen_q=attn_metadata.prefill.max_query_len,
-            max_seqlen_k=attn_metadata.prefill.max_query_len,
-            softmax_scale=self.scale,
-            causal=True,
-            return_softmax_lse=has_context,
-        )
+        if envs.VLLM_USE_CUDNN_PREFILL and all(
+                t.shape[-1] in CUDNN_SUPPORTED_HEAD_DIMS for t in (q, k, v)):
+            output = self._cudnn_varlen_func_diff_headdims(
+                q,
+                k,
+                v,
+                scale=self.scale,
+                workspace=attn_metadata.prefill.workspace,
+                max_q_seq_lens=attn_metadata.prefill.max_query_len,
+                max_kv_seq_lens=attn_metadata.prefill.max_query_len,
+                seq_lens_q=attn_metadata.prefill.query_seq_lens.view(
+                    -1, 1, 1, 1),
+                seq_lens_kv=attn_metadata.prefill.query_seq_lens.view(
+                    -1, 1, 1, 1),
+                causal=True,
+                is_cuda_graph_compatible=
+                True,  #Indicates actual_seq_lens are on GPU or CPU.
+            )
+            if not has_context:
+                output = output[0]
+        else:
+            output = self._flash_attn_varlen_diff_headdims(
+                q=q,
+                k=k,
+                v=v,
+                cu_seqlens_q=attn_metadata.prefill.query_start_loc,
+                cu_seqlens_k=attn_metadata.prefill.query_start_loc,
+                max_seqlen_q=attn_metadata.prefill.max_query_len,
+                max_seqlen_k=attn_metadata.prefill.max_query_len,
+                softmax_scale=self.scale,
+                causal=True,
+                return_softmax_lse=has_context,
+            )
 
         if has_context:
             suffix_output, suffix_lse = output