[GDN] Support GDN packed decode (#20627)

python/sglang/srt/layers/attention/fla/fused_recurrent.py

@@ -181,6 +181,227 @@ def fused_recurrent_gated_delta_rule_fwd(
     return o, final_state
 
 
+# Adapted from vllm project.
+@triton.jit
+def fused_recurrent_gated_delta_rule_packed_decode_kernel(
+    mixed_qkv,
+    a,
+    b,
+    A_log,
+    dt_bias,
+    o,
+    h0,
+    ht,
+    ssm_state_indices,
+    scale,
+    stride_mixed_qkv_tok: tl.constexpr,
+    stride_a_tok: tl.constexpr,
+    stride_b_tok: tl.constexpr,
+    stride_init_state_token: tl.constexpr,
+    stride_final_state_token: tl.constexpr,
+    stride_indices_seq: tl.constexpr,
+    H: tl.constexpr,
+    HV: tl.constexpr,
+    K: tl.constexpr,
+    V: tl.constexpr,
+    BK: tl.constexpr,
+    BV: tl.constexpr,
+    SOFTPLUS_THRESHOLD: tl.constexpr,
+    USE_QK_L2NORM_IN_KERNEL: tl.constexpr,
+):
+    i_v, i_nh = tl.program_id(0), tl.program_id(1)
+    i_n, i_hv = i_nh // HV, i_nh % HV
+    i_h = i_hv // (HV // H)
+
+    o_k = tl.arange(0, BK)
+    o_v = i_v * BV + tl.arange(0, BV)
+    mask_k = o_k < K
+    mask_v = o_v < V
+    mask_h = mask_v[:, None] & mask_k[None, :]
+
+    state_idx = tl.load(ssm_state_indices + i_n * stride_indices_seq).to(tl.int64)
+    p_o = o + (i_n * HV + i_hv) * V + o_v
+
+    if state_idx < 0:
+        zero = tl.zeros([BV], dtype=tl.float32).to(p_o.dtype.element_ty)
+        tl.store(p_o, zero, mask=mask_v)
+        return
+
+    p_h0 = h0 + state_idx * stride_init_state_token
+    p_h0 = p_h0 + i_hv * V * K + o_v[:, None] * K + o_k[None, :]
+    b_h = tl.load(p_h0, mask=mask_h, other=0).to(tl.float32)
+
+    p_mixed = mixed_qkv + i_n * stride_mixed_qkv_tok
+    q_off = i_h * K + o_k
+    k_off = (H * K) + i_h * K + o_k
+    v_off = (2 * H * K) + i_hv * V + o_v
+    b_q = tl.load(p_mixed + q_off, mask=mask_k, other=0).to(tl.float32)
+    b_k = tl.load(p_mixed + k_off, mask=mask_k, other=0).to(tl.float32)
+    b_v = tl.load(p_mixed + v_off, mask=mask_v, other=0).to(tl.float32)
+
+    if USE_QK_L2NORM_IN_KERNEL:
+        b_q = b_q / tl.sqrt(tl.sum(b_q * b_q) + 1e-6)
+        b_k = b_k / tl.sqrt(tl.sum(b_k * b_k) + 1e-6)
+    b_q = b_q * scale
+
+    a_val = tl.load(a + i_n * stride_a_tok + i_hv).to(tl.float32)
+    b_val = tl.load(b + i_n * stride_b_tok + i_hv).to(tl.float32)
+    A_log_val = tl.load(A_log + i_hv).to(tl.float32)
+    dt_bias_val = tl.load(dt_bias + i_hv).to(tl.float32)
+    x = a_val + dt_bias_val
+    softplus_x = tl.where(x <= SOFTPLUS_THRESHOLD, tl.log(1.0 + tl.exp(x)), x)
+    g_val = -tl.exp(A_log_val) * softplus_x
+    beta_val = tl.sigmoid(b_val).to(b.dtype.element_ty).to(tl.float32)
+
+    b_h *= exp(g_val)
+    b_v -= tl.sum(b_h * b_k[None, :], 1)
+    b_v *= beta_val
+    b_h += b_v[:, None] * b_k[None, :]
+    b_o = tl.sum(b_h * b_q[None, :], 1)
+    tl.store(p_o, b_o.to(p_o.dtype.element_ty), mask=mask_v)
+
+    p_ht = ht + state_idx * stride_final_state_token
+    p_ht = p_ht + i_hv * V * K + o_v[:, None] * K + o_k[None, :]
+    tl.store(p_ht, b_h.to(p_ht.dtype.element_ty), mask=mask_h)
+
+
+def fused_recurrent_gated_delta_rule_packed_decode(
+    mixed_qkv: torch.Tensor,
+    a: torch.Tensor,
+    b: torch.Tensor,
+    A_log: torch.Tensor,
+    dt_bias: torch.Tensor,
+    scale: float,
+    initial_state: torch.Tensor,
+    out: torch.Tensor,
+    ssm_state_indices: torch.Tensor,
+    use_qk_l2norm_in_kernel: bool = False,
+) -> tuple[torch.Tensor, torch.Tensor]:
+    if mixed_qkv.ndim != 2:
+        raise ValueError(
+            f"`mixed_qkv` must be a 2D tensor (got ndim={mixed_qkv.ndim})."
+        )
+    if mixed_qkv.stride(-1) != 1:
+        raise ValueError("`mixed_qkv` must be contiguous in the last dim.")
+    if a.ndim != 2 or b.ndim != 2:
+        raise ValueError(
+            f"`a` and `b` must be 2D tensors (got a.ndim={a.ndim}, b.ndim={b.ndim})."
+        )
+    if a.stride(-1) != 1 or b.stride(-1) != 1:
+        raise ValueError("`a`/`b` must be contiguous in the last dim.")
+    if A_log.ndim != 1 or dt_bias.ndim != 1:
+        raise ValueError("`A_log`/`dt_bias` must be 1D tensors.")
+    if A_log.stride(0) != 1 or dt_bias.stride(0) != 1:
+        raise ValueError("`A_log`/`dt_bias` must be contiguous.")
+    if ssm_state_indices.ndim != 1:
+        raise ValueError(
+            f"`ssm_state_indices` must be 1D for packed decode (got ndim={ssm_state_indices.ndim})."
+        )
+    if not out.is_contiguous():
+        raise ValueError("`out` must be contiguous.")
+
+    dev = mixed_qkv.device
+    if any(
+        t.device != dev
+        for t in (a, b, A_log, dt_bias, initial_state, out, ssm_state_indices)
+    ):
+        raise ValueError("All inputs must be on the same device.")
+
+    B = mixed_qkv.shape[0]
+    if a.shape[0] != B or b.shape[0] != B:
+        raise ValueError(
+            "Mismatched batch sizes: "
+            f"mixed_qkv.shape[0]={B}, a.shape[0]={a.shape[0]}, b.shape[0]={b.shape[0]}."
+        )
+    if ssm_state_indices.shape[0] != B:
+        raise ValueError(
+            f"`ssm_state_indices` must have shape [B] (got {tuple(ssm_state_indices.shape)}; expected ({B},))."
+        )
+
+    if initial_state.ndim != 4:
+        raise ValueError(
+            f"`initial_state` must be a 4D tensor (got ndim={initial_state.ndim})."
+        )
+    if initial_state.stride(-1) != 1:
+        raise ValueError("`initial_state` must be contiguous in the last dim.")
+    HV, V, K = initial_state.shape[-3:]
+    if a.shape[1] != HV or b.shape[1] != HV:
+        raise ValueError(
+            f"`a`/`b` must have shape [B, HV] with HV={HV} (got a.shape={tuple(a.shape)}, b.shape={tuple(b.shape)})."
+        )
+    if A_log.numel() != HV or dt_bias.numel() != HV:
+        raise ValueError(
+            f"`A_log` and `dt_bias` must have {HV} elements (got A_log.numel()={A_log.numel()}, dt_bias.numel()={dt_bias.numel()})."
+        )
+    if out.shape != (B, 1, HV, V):
+        raise ValueError(
+            f"`out` must have shape {(B, 1, HV, V)} (got out.shape={tuple(out.shape)})."
+        )
+
+    qkv_dim = mixed_qkv.shape[1]
+    qk_dim = qkv_dim - HV * V
+    if qk_dim <= 0 or qk_dim % 2 != 0:
+        raise ValueError(
+            f"Invalid packed `mixed_qkv` last dim={qkv_dim} for HV={HV}, V={V}."
+        )
+    q_dim = qk_dim // 2
+    if q_dim % K != 0:
+        raise ValueError(f"Invalid packed Q size {q_dim}: must be divisible by K={K}.")
+    H = q_dim // K
+    if H <= 0 or HV % H != 0:
+        raise ValueError(
+            f"Invalid head config inferred from mixed_qkv: H={H}, HV={HV}."
+        )
+
+    BK = triton.next_power_of_2(K)
+    if triton.cdiv(K, BK) != 1:
+        raise ValueError(
+            f"Packed decode kernel only supports NK=1 (got K={K}, BK={BK})."
+        )
+    BV = min(triton.next_power_of_2(V), 32)
+    num_stages = 3
+    num_warps = 1
+
+    stride_mixed_qkv_tok = mixed_qkv.stride(0)
+    stride_a_tok = a.stride(0)
+    stride_b_tok = b.stride(0)
+    stride_init_state_token = initial_state.stride(0)
+    stride_final_state_token = initial_state.stride(0)
+    stride_indices_seq = ssm_state_indices.stride(0)
+
+    NV = triton.cdiv(V, BV)
+    grid = (NV, B * HV)
+    fused_recurrent_gated_delta_rule_packed_decode_kernel[grid](
+        mixed_qkv=mixed_qkv,
+        a=a,
+        b=b,
+        A_log=A_log,
+        dt_bias=dt_bias,
+        o=out,
+        h0=initial_state,
+        ht=initial_state,
+        ssm_state_indices=ssm_state_indices,
+        scale=scale,
+        stride_mixed_qkv_tok=stride_mixed_qkv_tok,
+        stride_a_tok=stride_a_tok,
+        stride_b_tok=stride_b_tok,
+        stride_init_state_token=stride_init_state_token,
+        stride_final_state_token=stride_final_state_token,
+        stride_indices_seq=stride_indices_seq,
+        H=H,
+        HV=HV,
+        K=K,
+        V=V,
+        BK=BK,
+        BV=BV,
+        SOFTPLUS_THRESHOLD=20.0,
+        USE_QK_L2NORM_IN_KERNEL=use_qk_l2norm_in_kernel,
+        num_warps=num_warps,
+        num_stages=num_stages,
+    )
+    return out, initial_state
+
+
 class FusedRecurrentFunction(torch.autograd.Function):
 
     @staticmethod

python/sglang/srt/layers/attention/linear/gdn_backend.py

@@ -1,4 +1,4 @@
-from typing import Tuple, Union
+from typing import Optional, Tuple, Union
 
 import torch
 
@@ -111,10 +111,48 @@ class GDNKernelDispatcher:
         else:
             self.verify_kernel = triton_kernel
 
+        self.supports_packed_decode = getattr(
+            self.decode_kernel, "supports_packed_decode", False
+        )
+
         rank0_log(
             f"GDN kernel dispatcher: decode={self.decode_kernel.__class__.__name__}, "
             f"extend={self.extend_kernel.__class__.__name__}, "
-            f"verify={self.verify_kernel.__class__.__name__}"
+            f"verify={self.verify_kernel.__class__.__name__} "
+            f"packed_decode={self.supports_packed_decode}"
+        )
+
+    def packed_decode(
+        self,
+        mixed_qkv: torch.Tensor,
+        a: torch.Tensor,
+        b: torch.Tensor,
+        *,
+        A_log: torch.Tensor,
+        dt_bias: torch.Tensor,
+        scale: float,
+        ssm_states: torch.Tensor,
+        cache_indices: torch.Tensor,
+        num_v_heads: int,
+        head_v_dim: int,
+        **kwargs,
+    ) -> Optional[torch.Tensor]:
+        """Attempt packed decode. Returns output tensor or None if
+        the decode kernel does not support packed decode."""
+        if not self.supports_packed_decode:
+            return None
+        return self.decode_kernel.packed_decode(
+            mixed_qkv,
+            a,
+            b,
+            A_log=A_log,
+            dt_bias=dt_bias,
+            scale=scale,
+            ssm_states=ssm_states,
+            cache_indices=cache_indices,
+            num_v_heads=num_v_heads,
+            head_v_dim=head_v_dim,
+            **kwargs,
         )
 
     def decode(
@@ -243,6 +281,26 @@ class GDNAttnBackend(MambaAttnBackendBase):
             conv_state_indices=cache_indices,
         )
 
+        # Skip split + reshape + separate gating kernel by consuming
+        # the packed mixed_qkv directly in a single fused Triton kernel.
+        if self.kernel_dispatcher.supports_packed_decode:
+            core_attn_out = self.kernel_dispatcher.packed_decode(
+                mixed_qkv=mixed_qkv,
+                a=a,
+                b=b,
+                A_log=layer.A_log,
+                dt_bias=layer.dt_bias,
+                scale=layer.head_k_dim**-0.5,
+                ssm_states=ssm_states,
+                cache_indices=cache_indices,
+                num_v_heads=layer.num_v_heads,
+                head_v_dim=layer.head_v_dim,
+            )
+            self._track_mamba_state_decode(
+                forward_batch, conv_states, ssm_states, cache_indices
+            )
+            return core_attn_out
+
         query, key, value = torch.split(
             mixed_qkv,
             [layer.q_dim, layer.k_dim, layer.v_dim],

python/sglang/srt/layers/attention/linear/kernels/gdn_triton.py

@@ -7,6 +7,9 @@ from sglang.srt.utils import is_cpu, is_npu
 
 if not is_cpu():
     from sglang.srt.layers.attention.fla.chunk import chunk_gated_delta_rule
+    from sglang.srt.layers.attention.fla.fused_recurrent import (
+        fused_recurrent_gated_delta_rule_packed_decode,
+    )
     from sglang.srt.layers.attention.fla.fused_sigmoid_gating_recurrent import (
         fused_sigmoid_gating_delta_rule_update,
     )
@@ -31,6 +34,63 @@ elif is_cpu():
 class TritonGDNKernel(LinearAttnKernelBase):
     """Triton-based kernel for GDN (Gated Delta Network) linear attention."""
 
+    supports_packed_decode: bool = not is_cpu() and not is_npu()
+
+    def packed_decode(
+        self,
+        mixed_qkv: torch.Tensor,
+        a: torch.Tensor,
+        b: torch.Tensor,
+        *,
+        A_log: torch.Tensor,
+        dt_bias: torch.Tensor,
+        scale: float,
+        ssm_states: torch.Tensor,
+        cache_indices: torch.Tensor,
+        num_v_heads: int,
+        head_v_dim: int,
+        **kwargs,
+    ) -> torch.Tensor:
+        """Packed decode fast path: fuse QKV extraction + gating + recurrent
+        update into a single Triton kernel, eliminating intermediate tensors
+        and extra kernel launches.
+
+        Args:
+            mixed_qkv: [B, qkv_dim] packed projection output after conv1d.
+            a, b: [B, HV] gating inputs.
+            A_log: [HV] log-space decay parameter.
+            dt_bias: [HV] time-step bias.
+            scale: attention scale factor (typically head_k_dim ** -0.5).
+            ssm_states: [num_slots, HV, V, K] full state pool.
+            cache_indices: [B] per-request state slot indices.
+            num_v_heads: number of value heads (after TP sharding).
+            head_v_dim: dimension per value head.
+
+        Returns:
+            output tensor of shape [1, B, HV, V] matching the existing
+            decode kernel output layout.
+        """
+        B = mixed_qkv.shape[0]
+        # Packed kernel expects output shape [B, 1, HV, V]
+        out = mixed_qkv.new_empty(B, 1, num_v_heads, head_v_dim)
+
+        fused_recurrent_gated_delta_rule_packed_decode(
+            mixed_qkv=mixed_qkv,
+            a=a,
+            b=b,
+            A_log=A_log,
+            dt_bias=dt_bias,
+            scale=scale,
+            initial_state=ssm_states,
+            out=out,
+            ssm_state_indices=cache_indices,
+            use_qk_l2norm_in_kernel=True,
+        )
+
+        # Convert [B, 1, HV, V] → [1, B, HV, V] to match existing output
+        # layout. transpose() returns a view — zero cost.
+        return out.transpose(0, 1)
+
     def decode(
         self,
         q: torch.Tensor,