cudnn: Add native cudnn_decode for improved cudnn decode performance (#1283)

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## 📌 Description

This PR tries to integrate cudnn decode by calling the cudnn kernels
directly instead of through the cubin path.

Also, enabled nvidia-cudnn-frontend on all platforms

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Author: Anerudhan

flashinfer/cudnn/decode.py

@@ -1,10 +1,260 @@
 import functools
+from enum import Enum
 from typing import Optional
 
 import torch
 
 from ..jit import get_cudnn_fmha_gen_module
 
+try:
+    import cudnn
+
+    CUDNN_AVAILABLE = True
+except ImportError:
+    cudnn = None
+    CUDNN_AVAILABLE = False
+
+# Global cudnn handle. need to make it per device in future
+_cudnn_handle = None
+
+
+def _create_cudnn_handle(stream: torch.cuda.Stream):
+    global _cudnn_handle
+    if _cudnn_handle is None:
+        _cudnn_handle = cudnn.create_handle()
+    cudnn.set_stream(_cudnn_handle, stream.cuda_stream)
+    return _cudnn_handle
+
+
+# Tensor ids
+class UIDs(Enum):
+    RESERVED_INVALID_UID = 0
+
+    Q_UID = 1  # Query tensor
+    K_UID = 2  # Key cache tensor
+    V_UID = 3  # Value cache tensor
+
+    ACTUAL_SEQ_LENS_Q_UID = 100  # Actual sequence lengths for query tensor
+    ACTUAL_SEQ_LENS_KV_UID = 101  # Actual sequence lengths for key/value tensor
+
+    BLOCK_TABLES_UID = 200  # Block tables tensor
+    BLOCK_TABLES_K_UID = 201  # Block tables tensor for key
+    BLOCK_TABLES_V_UID = 202  # Block tables tensor for value
+
+    RAGGED_Q_UID = 50  # Ragged query tensor
+    RAGGED_O_UID = 51  # Ragged output tensor
+    RAGGED_STATS_UID = 52  # Ragged stats tensor
+
+    O_UID = 1000  # Output tensor
+    STATS_UID = 1001  # Stats tensor
+
+
+def _sdpa_decode_key_fn(
+    q: torch.Tensor,
+    k_cache: torch.Tensor,
+    v_cache: torch.Tensor,
+    scale: float,
+    *,
+    max_sequence_kv: int,
+    block_size: Optional[int] = 1,
+    actual_seq_lens_q: Optional[torch.Tensor] = None,
+    actual_seq_lens_kv: Optional[torch.Tensor] = None,
+    block_tables: Optional[torch.Tensor] = None,
+    batch_offsets_q: Optional[torch.Tensor] = None,
+    batch_offsets_o: Optional[torch.Tensor] = None,
+):
+    return (
+        "decode",
+        max_sequence_kv,
+        tuple(q.shape),
+        tuple(k_cache.shape),
+    )
+
+
+if CUDNN_AVAILABLE:
+
+    @cudnn.jit(heur_modes=[cudnn.heur_mode.A])
+    @cudnn.graph_cache(key_fn=_sdpa_decode_key_fn)
+    def _build_decode_graph(
+        q: torch.Tensor,
+        k_cache: torch.Tensor,
+        v_cache: torch.Tensor,
+        scale: float,
+        *,
+        max_sequence_kv: int,
+        block_size: Optional[int] = 1,
+        actual_seq_lens_q: Optional[torch.Tensor] = None,
+        actual_seq_lens_kv: Optional[torch.Tensor] = None,
+        block_tables: Optional[torch.Tensor] = None,
+        batch_offsets_q: Optional[torch.Tensor] = None,
+        batch_offsets_o: Optional[torch.Tensor] = None,
+    ):
+        handle = _create_cudnn_handle(torch.cuda.current_stream())
+
+        # WAR: override batch offsets for now, as it leads to a poor performance
+        batch_offsets_q = None
+        batch_offsets_o = None
+
+        with cudnn.graph(handle) as (g, _):
+            if q.dim() == 3:
+                s_qo = 1
+                b, h_qo, d_qk = q.shape[0], q.shape[1], q.shape[2]
+            elif q.dim() == 4:
+                b, h_qo, s_qo, d_qk = (
+                    q.shape[0],
+                    q.shape[1],
+                    q.shape[2],
+                    q.shape[3],
+                )
+            else:
+                raise ValueError(f"q must have 3 or 4 dimensions, got {q.dim()}")
+
+            assert s_qo == 1, "q must have a sequence length of 1"
+            assert k_cache.dim() == 4, "k_cache must have 4 dimensions"
+
+            h_kv = k_cache.shape[1]
+            s_kv = max_sequence_kv
+            d_vo = v_cache.shape[3]
+
+            cudnn_q = g.tensor(
+                name="q",
+                dim=(b, h_qo, s_qo, d_qk),
+                stride=(h_qo * d_qk, d_qk, d_qk * h_qo, 1),
+                data_type=cudnn.data_type.BFLOAT16,
+            )
+            if batch_offsets_q is not None:
+                ragged_q = g.tensor_like(batch_offsets_q)
+                ragged_q.set_uid(UIDs.RAGGED_Q_UID.value)
+                cudnn_q.set_ragged_offset(ragged_q)
+
+            cudnn_k_cache = g.tensor_like(k_cache)
+            cudnn_v_cache = g.tensor_like(v_cache)
+
+            cudnn_q.set_uid(UIDs.Q_UID.value)
+            cudnn_k_cache.set_uid(UIDs.K_UID.value)
+            cudnn_v_cache.set_uid(UIDs.V_UID.value)
+
+            if block_tables is not None:
+                nd_block_tables = block_tables.reshape(
+                    block_tables.shape[0], 1, block_tables.shape[1], 1
+                )
+                cudnn_k_block_tables = g.tensor_like(nd_block_tables)
+                cudnn_k_block_tables.set_uid(UIDs.BLOCK_TABLES_K_UID.value)
+
+                cudnn_v_block_tables = g.tensor_like(nd_block_tables)
+                cudnn_v_block_tables.set_uid(UIDs.BLOCK_TABLES_V_UID.value)
+
+            if actual_seq_lens_q is not None:
+                cudnn_actual_seq_lens_q = g.tensor_like(actual_seq_lens_q)
+                cudnn_actual_seq_lens_q.set_uid(UIDs.ACTUAL_SEQ_LENS_Q_UID.value)
+
+            if actual_seq_lens_kv is not None:
+                cudnn_actual_seq_lens_kv = g.tensor_like(actual_seq_lens_kv)
+                cudnn_actual_seq_lens_kv.set_uid(UIDs.ACTUAL_SEQ_LENS_KV_UID.value)
+                cudnn_actual_seq_lens_kv.set_is_pass_by_value(False)
+
+            padding_mask = actual_seq_lens_kv is not None
+
+            O, _ = g.sdpa(
+                name="sdpa",
+                q=cudnn_q,
+                k=cudnn_k_cache,
+                v=cudnn_v_cache,
+                seq_len_q=(
+                    cudnn_actual_seq_lens_q if actual_seq_lens_q is not None else None
+                ),
+                seq_len_kv=(
+                    cudnn_actual_seq_lens_kv if actual_seq_lens_kv is not None else None
+                ),
+                use_padding_mask=padding_mask,
+                is_inference=True,
+                attn_scale=scale,
+                paged_attention_k_table=cudnn_k_block_tables,
+                paged_attention_v_table=cudnn_v_block_tables,
+                paged_attention_max_seq_len_kv=max_sequence_kv,
+                compute_data_type=cudnn.data_type.FLOAT,
+            )
+
+            if batch_offsets_o is not None:
+                ragged_o = g.tensor_like(batch_offsets_o)
+                ragged_o.set_uid(UIDs.RAGGED_O_UID.value)
+                O.set_ragged_offset(ragged_o)
+
+            O.set_uid(UIDs.O_UID.value).set_output(True).set_dim(
+                [b, h_qo, s_qo, d_vo]
+            ).set_stride([d_vo * h_qo, d_vo, d_vo * h_qo, 1]).set_data_type(
+                cudnn.data_type.BFLOAT16
+            )
+
+        tensors_to_return = [cudnn_q, cudnn_k_cache, cudnn_v_cache, O]
+
+        if actual_seq_lens_q is not None:
+            tensors_to_return.append(cudnn_actual_seq_lens_q)
+        if actual_seq_lens_kv is not None:
+            tensors_to_return.append(cudnn_actual_seq_lens_kv)
+
+        return g, tensors_to_return
+
+
+def _batch_decode_with_kv_cache(
+    q: torch.Tensor,
+    k_cache: torch.Tensor,
+    v_cache: torch.Tensor,
+    scale: float,
+    workspace_buffer: torch.Tensor,
+    *,
+    max_sequence_kv: int,
+    actual_seq_lens_q: Optional[torch.Tensor] = None,
+    actual_seq_lens_kv: Optional[torch.Tensor] = None,
+    block_tables: Optional[torch.Tensor] = None,
+    block_size: Optional[int] = 1,
+    batch_offsets_q: Optional[torch.Tensor] = None,
+    batch_offsets_o: Optional[torch.Tensor] = None,
+    batch_offsets_k: Optional[torch.Tensor] = None,
+    batch_offsets_v: Optional[torch.Tensor] = None,
+    out: torch.Tensor,
+) -> torch.Tensor:
+
+    graph, tensors = _build_decode_graph(
+        q=q,
+        k_cache=k_cache,
+        v_cache=v_cache,
+        scale=scale,
+        max_sequence_kv=max_sequence_kv,
+        actual_seq_lens_q=actual_seq_lens_q,
+        actual_seq_lens_kv=actual_seq_lens_kv,
+        block_tables=block_tables,
+        block_size=block_size,
+        batch_offsets_q=batch_offsets_q if batch_offsets_q is not None else None,
+        batch_offsets_o=batch_offsets_q if batch_offsets_q is not None else None,
+    )
+
+    handle_ = _create_cudnn_handle(torch.cuda.current_stream())
+
+    var_map = {
+        UIDs.Q_UID.value: q,
+        UIDs.K_UID.value: k_cache,
+        UIDs.V_UID.value: v_cache,
+        UIDs.O_UID.value: out,
+    }
+    if actual_seq_lens_q is not None:
+        var_map[UIDs.ACTUAL_SEQ_LENS_Q_UID.value] = actual_seq_lens_q
+    if actual_seq_lens_kv is not None:
+        var_map[UIDs.ACTUAL_SEQ_LENS_KV_UID.value] = actual_seq_lens_kv
+
+    if batch_offsets_q is not None:
+        var_map[UIDs.RAGGED_Q_UID.value] = batch_offsets_q
+    if batch_offsets_o is not None:
+        var_map[UIDs.RAGGED_O_UID.value] = batch_offsets_o
+
+    if block_tables is not None:
+        var_map[UIDs.BLOCK_TABLES_K_UID.value] = block_tables
+        var_map[UIDs.BLOCK_TABLES_V_UID.value] = block_tables
+
+    graph.execute(var_map, workspace=workspace_buffer, handle=handle_)
+
+    return out
+
 
 def cudnn_batch_decode_with_kv_cache(
     q: torch.Tensor,
@@ -37,7 +287,6 @@ def cudnn_batch_decode_with_kv_cache(
         is_cuda_graph_compatible: Whether the decode operation is compatible with CUDA graph
         batch_offsets: Optional batch offsets tensor of shape (batch_size,) on GPU
         out: Optional pre-allocated output tensor
-        lse: Optional pre-allocated tensor for log-sum-exp values if return_lse is True else returns None
         batch_offsets_q: Optional batch offsets for query tensor of shape (batch_size,) on GPU
         batch_offsets_o: Optional batch offsets for output tensor of shape (batch_size,) on GPU
         batch_offsets_k: Optional batch offsets for key tensor of shape (batch_size,) on GPU
@@ -53,30 +302,51 @@ def cudnn_batch_decode_with_kv_cache(
     """
 
     bs = q.shape[0]
-    s_q = 1
     h_qo = q.shape[1]
     d_vo = v_cache.shape[3]
 
     if out is None:
         out = torch.empty(bs, h_qo, d_vo, device=q.device, dtype=q.dtype)
 
-    actual_seq_lens_kv_gpu = actual_seq_lens_kv.to(q.device, non_blocking=True)
+    if not CUDNN_AVAILABLE:
+        actual_seq_lens_kv_gpu = actual_seq_lens_kv.to(q.device, non_blocking=True)
 
-    run_func = get_cudnn_fmha_gen_module().decode
-    run_func(
-        max_sequence_kv,
-        q,
-        k_cache,
-        v_cache,
-        scale,
-        workspace_buffer,
-        actual_seq_lens_kv,
-        actual_seq_lens_kv_gpu,
-        block_tables,
-        out,
-        batch_offsets_q,
-        batch_offsets_o,
-        is_cuda_graph_compatible,
-    )
+        run_func = get_cudnn_fmha_gen_module().decode
+        run_func(
+            max_sequence_kv,
+            q,
+            k_cache,
+            v_cache,
+            scale,
+            workspace_buffer,
+            actual_seq_lens_kv,
+            actual_seq_lens_kv_gpu,
+            block_tables,
+            out,
+            batch_offsets_q,
+            batch_offsets_o,
+            is_cuda_graph_compatible,
+        )
+    else:
+        actual_seq_lens_q = torch.ones(
+            (bs, 1, 1, 1), device=q.device, dtype=torch.int32
+        )
+        block_size = k_cache.shape[2]
+
+        _batch_decode_with_kv_cache(
+            q=q,
+            k_cache=k_cache,
+            v_cache=v_cache,
+            scale=scale,
+            workspace_buffer=workspace_buffer,
+            max_sequence_kv=max_sequence_kv,
+            actual_seq_lens_q=actual_seq_lens_q,
+            actual_seq_lens_kv=actual_seq_lens_kv,
+            block_tables=block_tables,
+            batch_offsets_q=batch_offsets_q,
+            batch_offsets_o=batch_offsets_o,
+            block_size=block_size,
+            out=out,
+        )
 
     return out

setup.py

@@ -62,7 +62,7 @@ def generate_build_meta(aot_build_meta: dict) -> None:
     "einops",
     "nvidia-nvshmem-cu12",
     "nvidia-cudnn-cu12",
-    'nvidia-cudnn-frontend; platform_machine == "x86_64" or platform_machine == "AMD64"',
+    "nvidia-cudnn-frontend>=1.13.0",
 ]
 generate_build_meta({})
 

tests/test_cudnn_decode.py

@@ -7,12 +7,12 @@
 import flashinfer
 
 
-@pytest.mark.parametrize("batch_size", [4, 8, 17, 64])
-@pytest.mark.parametrize("s_kv", [8, 40, 1024])
-@pytest.mark.parametrize("page_size", [1, 8])
-@pytest.mark.parametrize("num_kv_heads", [4])
-@pytest.mark.parametrize("num_qo_heads", [4, 32])
-@pytest.mark.parametrize("is_cuda_graph_compatible", [False, True])
+@pytest.mark.parametrize("batch_size", [8, 16, 32])
+@pytest.mark.parametrize("s_kv", [512, 8192])
+@pytest.mark.parametrize("page_size", [16])
+@pytest.mark.parametrize("num_kv_heads", [8])
+@pytest.mark.parametrize("num_qo_heads", [32])
+@pytest.mark.parametrize("is_cuda_graph_compatible", [True, False])
 def test_cudnn_decode(
     batch_size,
     s_kv,
@@ -79,7 +79,11 @@ def test_cudnn_decode(
 
     # Actual sequence lengths (should be randomized across batches. )
     actual_seq_lens_kv = torch.randint(
-        0, s_kv, (batch_size, 1, 1, 1), dtype=torch.int32
+        0, s_kv + 1, (batch_size, 1, 1, 1), dtype=torch.int32, device=device
+    )
+
+    ragged_q = torch.arange(0, batch_size + 1, device=device) * (
+        num_qo_heads * head_dim
     )
 
     workspace_buffer_size = math.ceil(
@@ -106,6 +110,8 @@ def test_cudnn_decode(
         actual_seq_lens_kv=actual_seq_lens_kv,
         block_tables=block_tables,
         is_cuda_graph_compatible=is_cuda_graph_compatible,
+        batch_offsets_q=ragged_q,
+        batch_offsets_o=ragged_q,
     )
 
     actual_seq_lens_kv_device = actual_seq_lens_kv.to(device)