[GDN] Eliminate GPU->CPU sync in prepare_chunk_indices during prefill (#38361)

Signed-off-by: Artem Perevedentsev <aperevedents@nvidia.com>
Signed-off-by: Vadim Gimpelson <156319763+vadiklyutiy@users.noreply.github.com>

vllm/model_executor/layers/fla/ops/chunk.py

@@ -16,7 +16,7 @@ from .chunk_scaled_dot_kkt import chunk_scaled_dot_kkt_fwd
 from .cumsum import chunk_local_cumsum
 from .l2norm import l2norm_fwd
 from .solve_tril import solve_tril
-from .utils import SUPPRESS_LEVEL, input_guard
+from .utils import FLA_CHUNK_SIZE, SUPPRESS_LEVEL, input_guard
 from .wy_fast import recompute_w_u_fwd
 
 
@@ -30,13 +30,24 @@ def chunk_gated_delta_rule_fwd(
     initial_state: torch.Tensor,
     output_final_state: bool,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
+    chunk_offsets: torch.Tensor | None = None,
 ):
-    g = chunk_local_cumsum(g, chunk_size=64, cu_seqlens=cu_seqlens)
+    g = chunk_local_cumsum(
+        g, chunk_size=FLA_CHUNK_SIZE, cu_seqlens=cu_seqlens, chunk_indices=chunk_indices
+    )
     # obtain WY representation. u is actually the new v.
     A = chunk_scaled_dot_kkt_fwd(
-        k=k, beta=beta, g=g, cu_seqlens=cu_seqlens, output_dtype=torch.float32
+        k=k,
+        beta=beta,
+        g=g,
+        cu_seqlens=cu_seqlens,
+        chunk_indices=chunk_indices,
+        output_dtype=torch.float32,
+    )
+    A = solve_tril(
+        A=A, cu_seqlens=cu_seqlens, chunk_indices=chunk_indices, output_dtype=k.dtype
     )
-    A = solve_tril(A=A, cu_seqlens=cu_seqlens, output_dtype=k.dtype)
     w, u = recompute_w_u_fwd(
         k=k,
         v=v,
@@ -44,6 +55,7 @@ def chunk_gated_delta_rule_fwd(
         A=A,
         g_cumsum=g,
         cu_seqlens=cu_seqlens,
+        chunk_indices=chunk_indices,
     )
     h, v_new, final_state = chunk_gated_delta_rule_fwd_h(
         k=k,
@@ -53,6 +65,8 @@ def chunk_gated_delta_rule_fwd(
         initial_state=initial_state,
         output_final_state=output_final_state,
         cu_seqlens=cu_seqlens,
+        chunk_indices=chunk_indices,
+        chunk_offsets=chunk_offsets,
     )
     o = chunk_fwd_o(
         q=q,
@@ -62,6 +76,7 @@ def chunk_gated_delta_rule_fwd(
         g=g,
         scale=scale,
         cu_seqlens=cu_seqlens,
+        chunk_indices=chunk_indices,
     )
     if SUPPRESS_LEVEL < 3:
         return g, o, A, final_state, None, None, None
@@ -84,6 +99,8 @@ class ChunkGatedDeltaRuleFunction(torch.autograd.Function):
         initial_state: torch.Tensor,
         output_final_state: bool,
         cu_seqlens: torch.Tensor | None = None,
+        chunk_indices: torch.Tensor | None = None,
+        chunk_offsets: torch.Tensor | None = None,
         use_qk_l2norm_in_kernel: bool = False,
     ):
         if use_qk_l2norm_in_kernel:
@@ -100,6 +117,8 @@ class ChunkGatedDeltaRuleFunction(torch.autograd.Function):
             initial_state=initial_state,
             output_final_state=output_final_state,
             cu_seqlens=cu_seqlens,
+            chunk_indices=chunk_indices,
+            chunk_offsets=chunk_offsets,
         )
         ctx.scale = scale
         ctx.use_qk_l2norm_in_kernel = use_qk_l2norm_in_kernel
@@ -117,6 +136,8 @@ def chunk_gated_delta_rule(
     initial_state: torch.Tensor = None,
     output_final_state: bool = False,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
+    chunk_offsets: torch.Tensor | None = None,
     use_qk_l2norm_in_kernel: bool = False,
 ):
     r"""
@@ -206,6 +227,8 @@ def chunk_gated_delta_rule(
         initial_state,
         output_final_state,
         cu_seqlens,
+        chunk_indices,
+        chunk_offsets,
         use_qk_l2norm_in_kernel,
     )
     return o, final_state

vllm/model_executor/layers/fla/ops/chunk_delta_h.py

@@ -14,7 +14,7 @@ from vllm.triton_utils import tl, triton
 
 from .index import prepare_chunk_indices, prepare_chunk_offsets
 from .op import exp
-from .utils import use_cuda_graph
+from .utils import FLA_CHUNK_SIZE, use_cuda_graph
 
 NUM_WARPS = [2, 4, 8, 16]
 
@@ -286,9 +286,11 @@ def chunk_gated_delta_rule_fwd_h(
     gk: torch.Tensor | None = None,
     initial_state: torch.Tensor | None = None,
     output_final_state: bool = False,
-    chunk_size: int = 64,  # SY: remove this argument and force chunk size 64?
+    chunk_size: int = FLA_CHUNK_SIZE,
     save_new_value: bool = True,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
+    chunk_offsets: torch.Tensor | None = None,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     # This kernel is slightly different from fla to support Q/K with different head numbers.
     # In fla, Q/K always have the same head number, so Hg is always equal to H.
@@ -296,20 +298,15 @@ def chunk_gated_delta_rule_fwd_h(
     H = u.shape[-2]
     BT = chunk_size
 
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, chunk_size)
-        if cu_seqlens is not None
-        else None
-    )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, chunk_size)
     # N: the actual number of sequences in the batch with either equal or variable lengths
     if cu_seqlens is None:
         N, NT, chunk_offsets = B, triton.cdiv(T, BT), None
     else:
-        N, NT, chunk_offsets = (
-            len(cu_seqlens) - 1,
-            len(chunk_indices),
-            prepare_chunk_offsets(cu_seqlens, BT),
-        )
+        N, NT = len(cu_seqlens) - 1, len(chunk_indices)
+        if chunk_offsets is None:
+            chunk_offsets = prepare_chunk_offsets(cu_seqlens, BT)
     assert K <= 256, "current kernel does not support head dimension larger than 256."
 
     h = k.new_empty(B, NT, H, V, K)

vllm/model_executor/layers/fla/ops/chunk_o.py

@@ -146,14 +146,14 @@ def chunk_fwd_o(
     g: torch.Tensor | None = None,  # cumsum of log decay
     scale: float | None = None,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
     chunk_size: int = FLA_CHUNK_SIZE,
 ) -> torch.Tensor:
     B, T, Hg, K, V = *q.shape, v.shape[-1]
     H = v.shape[-2]
     BT = chunk_size
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, BT) if cu_seqlens is not None else None
-    )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, BT)
     NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
     if scale is None:
         scale = k.shape[-1] ** -0.5

vllm/model_executor/layers/fla/ops/chunk_scaled_dot_kkt.py

@@ -14,6 +14,7 @@ from vllm.triton_utils import tl, triton
 
 from .index import prepare_chunk_indices
 from .op import exp
+from .utils import FLA_CHUNK_SIZE
 
 
 @triton.heuristics(
@@ -103,7 +104,8 @@ def chunk_scaled_dot_kkt_fwd(
     g: torch.Tensor | None = None,
     beta: torch.Tensor | None = None,
     cu_seqlens: torch.Tensor | None = None,
-    chunk_size: int = 64,
+    chunk_indices: torch.Tensor | None = None,
+    chunk_size: int = FLA_CHUNK_SIZE,
     output_dtype: torch.dtype = torch.float32,
 ) -> torch.Tensor:
     r"""
@@ -119,6 +121,9 @@ def chunk_scaled_dot_kkt_fwd(
         cu_seqlens (torch.Tensor):
             The cumulative sequence lengths of the input tensor.
             Default: None
+        chunk_indices (torch.Tensor):
+            Pre-computed chunk indices. If None and cu_seqlens is provided,
+            computed internally. Default: None
         chunk_size (int):
             The chunk size. Default: 64.
         output_dtype (torch.dtype):
@@ -132,9 +137,8 @@ def chunk_scaled_dot_kkt_fwd(
     B, T, Hg, K = k.shape
     H = beta.shape[-1]
     BT = chunk_size
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, BT) if cu_seqlens is not None else None
-    )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, BT)
     NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
 
     A = torch.empty(B, T, H, BT, device=k.device, dtype=output_dtype)

vllm/model_executor/layers/fla/ops/cumsum.py

@@ -162,6 +162,7 @@ def chunk_local_cumsum_scalar(
     chunk_size: int,
     reverse: bool = False,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
     head_first: bool = False,
     output_dtype: torch.dtype | None = torch.float,
 ) -> torch.Tensor:
@@ -172,10 +173,9 @@ def chunk_local_cumsum_scalar(
     assert chunk_size == 2 ** (chunk_size.bit_length() - 1), (
         "chunk_size must be a power of 2"
     )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, chunk_size)
     BT = chunk_size
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, BT) if cu_seqlens is not None else None
-    )
     NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
     g_org, g = g, torch.empty_like(g, dtype=output_dtype or g.dtype)
     grid = (NT, B * H)
@@ -199,6 +199,7 @@ def chunk_local_cumsum_vector(
     chunk_size: int,
     reverse: bool = False,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
     head_first: bool = False,
     output_dtype: torch.dtype | None = torch.float,
 ) -> torch.Tensor:
@@ -206,16 +207,13 @@ def chunk_local_cumsum_vector(
         B, H, T, S = g.shape
     else:
         B, T, H, S = g.shape
-    BT = chunk_size
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, chunk_size)
-        if cu_seqlens is not None
-        else None
-    )
-    NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
     assert chunk_size == 2 ** (chunk_size.bit_length() - 1), (
         "chunk_size must be a power of 2"
     )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, chunk_size)
+    BT = chunk_size
+    NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
 
     g_org, g = g, torch.empty_like(g, dtype=output_dtype or g.dtype)
 
@@ -247,6 +245,7 @@ def chunk_local_cumsum(
     chunk_size: int,
     reverse: bool = False,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
     head_first: bool = False,
     output_dtype: torch.dtype | None = torch.float,
     **kwargs,
@@ -257,11 +256,23 @@ def chunk_local_cumsum(
         )
     if len(g.shape) == 3:
         return chunk_local_cumsum_scalar(
-            g, chunk_size, reverse, cu_seqlens, head_first, output_dtype
+            g,
+            chunk_size,
+            reverse,
+            cu_seqlens,
+            chunk_indices,
+            head_first,
+            output_dtype,
         )
     elif len(g.shape) == 4:
         return chunk_local_cumsum_vector(
-            g, chunk_size, reverse, cu_seqlens, head_first, output_dtype
+            g,
+            chunk_size,
+            reverse,
+            cu_seqlens,
+            chunk_indices,
+            head_first,
+            output_dtype,
         )
     else:
         raise ValueError(

vllm/model_executor/layers/fla/ops/kda.py

@@ -23,7 +23,7 @@ from .index import prepare_chunk_indices
 from .l2norm import l2norm_fwd
 from .op import exp, log
 from .solve_tril import solve_tril
-from .utils import is_amd
+from .utils import FLA_CHUNK_SIZE, is_amd
 
 BT_LIST_AUTOTUNE = [32, 64, 128]
 NUM_WARPS_AUTOTUNE = [2, 4, 8, 16] if is_amd else [4, 8, 16, 32]
@@ -721,7 +721,7 @@ def chunk_kda_scaled_dot_kkt_fwd(
     beta: torch.Tensor | None = None,
     scale: float | None = None,
     cu_seqlens: torch.Tensor | None = None,
-    chunk_size: int = 64,
+    chunk_size: int = FLA_CHUNK_SIZE,
     output_dtype: torch.dtype = torch.float32,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     r"""
@@ -1178,7 +1178,7 @@ def chunk_kda_fwd(
     output_final_state: bool,
     cu_seqlens: torch.Tensor | None = None,
 ):
-    chunk_size = 64
+    chunk_size = FLA_CHUNK_SIZE
     g = chunk_local_cumsum(g, chunk_size=chunk_size, cu_seqlens=cu_seqlens)
     # the intra Aqk is kept in fp32
     # the computation has very marginal effect on the entire throughput
@@ -1189,6 +1189,7 @@ def chunk_kda_fwd(
         beta=beta,
         scale=scale,
         cu_seqlens=cu_seqlens,
+        chunk_size=chunk_size,
         output_dtype=torch.float32,
     )
     A = solve_tril(A=A, cu_seqlens=cu_seqlens, output_dtype=k.dtype)

vllm/model_executor/layers/fla/ops/solve_tril.py

@@ -507,6 +507,7 @@ def merge_16x16_to_64x64_inverse_kernel(
 def solve_tril(
     A: torch.Tensor,
     cu_seqlens: torch.Tensor | None = None,
+    chunk_indices: torch.Tensor | None = None,
     output_dtype: torch.dtype = torch.float,
 ) -> torch.Tensor:
     """
@@ -518,6 +519,8 @@ def solve_tril(
             [B, T, H, BT], where BT should only be 16, 32, or 64.
         cu_seqlens (torch.Tensor):
             The cumulative sequence lengths of the input tensor. Default: `None`.
+        chunk_indices (torch.Tensor):
+            Pre-computed chunk indices. Default: `None`.
         output_dtype (torch.dtype):
             The dtype of the output tensor. Default: `torch.float`.
             If `None`, the output dtype will be the same as the input dtype.
@@ -529,9 +532,8 @@ def solve_tril(
     output_dtype = A.dtype if output_dtype is None else output_dtype
 
     B, T, H, BT = A.shape
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, BT) if cu_seqlens is not None else None
-    )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, BT)
     NT = len(chunk_indices) if cu_seqlens is not None else triton.cdiv(T, BT)
 
     Ai = torch.zeros_like(A, dtype=output_dtype)

vllm/model_executor/layers/fla/ops/wy_fast.py

@@ -123,14 +123,14 @@ def recompute_w_u_fwd(
     g_cumsum: torch.Tensor,
     A: torch.Tensor,
     cu_seqlens: torch.Tensor | None,
+    chunk_indices: torch.Tensor | None = None,
 ) -> tuple[torch.Tensor, torch.Tensor]:
     B, T, Hg, K, V = *k.shape, v.shape[-1]
     H = v.shape[-2]
     BT = A.shape[-1]
 
-    chunk_indices = (
-        prepare_chunk_indices(cu_seqlens, BT) if cu_seqlens is not None else None
-    )
+    if chunk_indices is None and cu_seqlens is not None:
+        chunk_indices = prepare_chunk_indices(cu_seqlens, BT)
     NT = triton.cdiv(T, BT) if cu_seqlens is None else len(chunk_indices)
     BK = 64
     BV = 64

vllm/model_executor/layers/mamba/gdn_linear_attn.py

@@ -163,6 +163,8 @@ class ChunkGatedDeltaRule(CustomOp):
         initial_state: torch.Tensor,
         output_final_state: bool,
         cu_seqlens: torch.Tensor | None = None,
+        chunk_indices: torch.Tensor | None = None,
+        chunk_offsets: torch.Tensor | None = None,
         use_qk_l2norm_in_kernel: bool = True,
     ):
         return fi_chunk_gated_delta_rule(
@@ -187,6 +189,8 @@ class ChunkGatedDeltaRule(CustomOp):
         initial_state: torch.Tensor,
         output_final_state: bool,
         cu_seqlens: torch.Tensor | None = None,
+        chunk_indices: torch.Tensor | None = None,
+        chunk_offsets: torch.Tensor | None = None,
         use_qk_l2norm_in_kernel: bool = True,
     ):
         return fla_chunk_gated_delta_rule(
@@ -198,6 +202,8 @@ class ChunkGatedDeltaRule(CustomOp):
             initial_state=initial_state,
             output_final_state=output_final_state,
             cu_seqlens=cu_seqlens,
+            chunk_indices=chunk_indices,
+            chunk_offsets=chunk_offsets,
             use_qk_l2norm_in_kernel=use_qk_l2norm_in_kernel,
         )
 
@@ -959,6 +965,8 @@ class GatedDeltaNetAttention(PluggableLayer, MambaBase):
                 initial_state=initial_state,
                 output_final_state=True,
                 cu_seqlens=non_spec_query_start_loc,
+                chunk_indices=attn_metadata.chunk_indices,
+                chunk_offsets=attn_metadata.chunk_offsets,
                 use_qk_l2norm_in_kernel=False,
             )
             # Init cache

vllm/v1/attention/backends/gdn_attn.py

@@ -63,6 +63,10 @@ class GDNAttentionMetadata:
 
     num_accepted_tokens: torch.Tensor | None = None  # shape: [batch,]
 
+    # Pre-computed FLA chunk metadata (avoids GPU->CPU sync in prepare_chunk_indices)
+    chunk_indices: torch.Tensor | None = None
+    chunk_offsets: torch.Tensor | None = None
+
     # The following attributes are for triton implementation of causal_conv1d
     nums_dict: dict | None = None
     batch_ptr: torch.Tensor | None = None
@@ -305,6 +309,26 @@ class GDNAttentionMetadataBuilder(AttentionMetadataBuilder[GDNAttentionMetadata]
             assert num_accepted_tokens is not None
             num_accepted_tokens = num_accepted_tokens[spec_sequence_masks]
 
+        chunk_indices: torch.Tensor | None = None
+        chunk_offsets: torch.Tensor | None = None
+        if num_prefills > 0:
+            # Only prefill batches use FLA chunk ops.
+            # Pre-compute on CPU and async-copy to GPU to avoid
+            # GPU→CPU sync (.tolist()) in prepare_chunk_indices.
+            from vllm.model_executor.layers.fla.ops.index import (
+                prepare_chunk_indices,
+                prepare_chunk_offsets,
+            )
+            from vllm.model_executor.layers.fla.ops.utils import FLA_CHUNK_SIZE
+
+            gpu_device = query_start_loc.device
+            chunk_indices = prepare_chunk_indices(
+                non_spec_query_start_loc_cpu, FLA_CHUNK_SIZE
+            ).to(device=gpu_device, non_blocking=True)
+            chunk_offsets = prepare_chunk_offsets(
+                non_spec_query_start_loc_cpu, FLA_CHUNK_SIZE
+            ).to(device=gpu_device, non_blocking=True)
+
         if num_prefills > 0:
             has_initial_state = context_lens_tensor > 0
             if spec_sequence_masks is not None:
@@ -405,6 +429,8 @@ class GDNAttentionMetadataBuilder(AttentionMetadataBuilder[GDNAttentionMetadata]
             num_spec_decode_tokens=num_spec_decode_tokens,
             num_actual_tokens=m.num_actual_tokens,
             has_initial_state=has_initial_state,
+            chunk_indices=chunk_indices,
+            chunk_offsets=chunk_offsets,
             spec_query_start_loc=spec_query_start_loc,
             non_spec_query_start_loc=non_spec_query_start_loc,
             spec_state_indices_tensor=spec_state_indices_tensor,