common.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
import torch

from vllm.distributed.parallel_state import GroupCoordinator
from vllm.triton_utils import tl, triton


@triton.jit
def _correct_attn_cp_out_kernel(
    outputs_ptr,
    new_output_ptr,
    lses_ptr,
    vlse_ptr,
    outputs_stride_B,
    outputs_stride_H,
    outputs_stride_D,
    lses_stride_N,
    lses_stride_B,
    lses_stride_H,
    lse_idx,
    HEAD_DIM: tl.constexpr,
    N_ROUNDED: tl.constexpr,
):
    """
    Apply the all-gathered lses to correct each local rank's attention
    output. we still need perform a cross-rank reduction to obtain the
    final attention output.

    Args:
        outputs_ptr (triton.PointerType):
            Pointer to input tensor of shape [ B, H, D ]
        lses_ptr (triton.PointerType):
            Pointer to input tensor of shape [ N, B, H ]
        new_output_ptr (triton.PointerType):
            Pointer to output tensor of shape [ B, H, D ]
        vlse_ptr (triton.PointerType):
            Pointer to output tensor of shape [ B, H ]
    """
    batch_idx = tl.program_id(axis=0).to(tl.int64)
    head_idx = tl.program_id(axis=1).to(tl.int64)
    d_offsets = tl.arange(0, HEAD_DIM)
    num_n_offsets = tl.arange(0, N_ROUNDED)

    # shape = [N]
    lse_offsets = (
        num_n_offsets * lses_stride_N
        + batch_idx * lses_stride_B
        + head_idx * lses_stride_H
    )

    # calc final lse
    lse = tl.load(lses_ptr + lse_offsets)
    lse = tl.where((lse != lse) | (lse == float("inf")), -float("inf"), lse)
    lse_max = tl.max(lse, axis=0)
    lse_max = tl.where(lse_max == -float("inf"), 0, lse_max)
    lse -= lse_max
    lse_exp = tl.exp(lse)
    lse_acc = tl.sum(lse_exp, axis=0)
    lse = tl.log(lse_acc)
    lse += lse_max

    lse_offsets = batch_idx * lses_stride_B + head_idx * lses_stride_H
    tl.store(vlse_ptr + lse_offsets, lse)

    # shape = [D]
    output_offsets = (
        batch_idx * outputs_stride_B
        + head_idx * outputs_stride_H
        + d_offsets * outputs_stride_D
    )

    # correct output
    lse_offset = (
        lse_idx * lses_stride_N + batch_idx * lses_stride_B + head_idx * lses_stride_H
    )
    lse_tmp = tl.load(lses_ptr + lse_offset)
    lse_finally = lse_tmp - lse
    lse_finally = tl.where(
        (lse_finally != lse_finally) | (lse_finally == float("inf")),
        -float("inf"),
        lse_finally,
    )
    factor = tl.exp(lse_finally)
    output = tl.load(outputs_ptr + output_offsets)
    output = output * factor

    tl.store(new_output_ptr + output_offsets, output)


class CPTritonContext:
    """The CPTritonContext is used to avoid recompilation of the Triton JIT."""

    def __init__(self):
        self.inner_kernel = None

    def call_kernel(self, kernel, grid, *regular_args, **const_args):
        if self.inner_kernel is None:
            self.inner_kernel = kernel[grid](*regular_args, **const_args)
        else:
            self.inner_kernel[grid](*regular_args)


def correct_attn_out(
    out: torch.Tensor, lses: torch.Tensor, cp_rank: int, ctx: CPTritonContext
) -> tuple[torch.Tensor, torch.Tensor]:
    """Correct the attention output using the all-gathered lses.

    Args:
        out: Tensor of shape [ B, H, D ]
        lses: Tensor of shape [ N, B, H ]
        cp_rank: Current rank in the context-parallel group
        ctx: Triton context to avoid recompilation

    Returns:
        Tuple of (out, lse) with corrected attention and final log-sum-exp.
    """
    if ctx is None:
        ctx = CPTritonContext()

    # --- Normalize to 3D views ---
    if out.ndim == 4 and out.shape[1] == 1:
        out = out.squeeze(1)
    assert out.ndim == 3, f"expected out [B,H,D] or [B,1,H,D], got {tuple(out.shape)}"

    if lses.ndim == 4 and lses.shape[-1] == 1:
        lses = lses.squeeze(-1)
    if lses.ndim == 4 and lses.shape[1] == 1:
        lses = lses.squeeze(1)
    assert lses.ndim == 3, (
        f"expected lses [N,B,H] (optionally with a 1-sized extra dim), "
        f"got {tuple(lses.shape)}"
    )

    B, H, D = out.shape
    N = lses.shape[0]

    # Strides after we normalized shapes to 3-D views.  The kernel computes
    # offsets for `vlse_ptr` using lses_stride_B/H, so the output buffer must
    # have the same B/H stride layout as a slice of `lses`.
    o_sB, o_sH, o_sD = out.stride()
    l_sN, l_sB, l_sH = lses.stride()

    # Allocate LSE with the same B/H strides as `lses` so writes land correctly
    # even when `lses` is a non-contiguous view (e.g., 4-D to 3-D squeeze).
    lse = torch.empty_strided(
        (B, H), (l_sB, l_sH), device=lses.device, dtype=lses.dtype
    )

    # Kernel launch config
    grid = (B, H, 1)

    regular_args = (
        out,
        out,
        lses,
        lse,
        o_sB,
        o_sH,
        o_sD,
        l_sN,
        l_sB,
        l_sH,
        cp_rank,
    )
    const_args = {"HEAD_DIM": D, "N_ROUNDED": N}

    ctx.call_kernel(_correct_attn_cp_out_kernel, grid, *regular_args, **const_args)
    return out, lse


def cp_lse_ag_out_rs(
    cp_attn_out: torch.Tensor,
    cp_attn_lse: torch.Tensor,
    cp_group: GroupCoordinator,
    ctx: CPTritonContext = None,
    return_lse=False,
):
    """
    cp_attn_out: [ B, H, D ]
    cp_attn_lse: [ B, H ]
    """
    if cp_group.world_size == 1:
        return cp_attn_out

    if ctx is None:
        ctx = CPTritonContext()

    lses = torch.empty(
        (cp_group.world_size,) + cp_attn_lse.shape,
        dtype=cp_attn_lse.dtype,
        device=cp_attn_lse.device,
    )

    cp_attn_lse = cp_attn_lse.contiguous()
    lses = cp_group.all_gather(cp_attn_lse, dim=0).view_as(lses)
    out, lse = correct_attn_out(cp_attn_out, lses, cp_group.rank_in_group, ctx)
    assert out.is_contiguous()
    out = cp_group.reduce_scatter(out, dim=1)

    if return_lse:
        cp_num_heads = lse.shape[1] // cp_group.world_size
        cp_rank = cp_group.rank_in_group
        lse = lse[:, cp_num_heads * cp_rank : cp_num_heads * (cp_rank + 1)]
        return out, lse
    return out


@triton.jit
def _pack_seq_kernel(
    x_ptr,  # [N, D]
    out_ptr,  # [B, Lmax, D]
    lengths_ptr,  # *i32, [B]
    N: tl.constexpr,
    D: tl.constexpr,
    Lmax: tl.constexpr,
    PAD_VALUE: tl.constexpr,
    BLOCK_T: tl.constexpr,  # timesteps per program
    BLOCK_D: tl.constexpr,  # features per program
):
    pid_b = tl.program_id(0)  # batch id
    pid_t = tl.program_id(1)  # block over time dimension
    pid_d = tl.program_id(2)  # block over feature dimension
    off_t = pid_t * BLOCK_T + tl.arange(0, BLOCK_T)  # [BLOCK_T]
    off_d = pid_d * BLOCK_D + tl.arange(0, BLOCK_D)  # [BLOCK_D]

    # Compute start index and sequence length from cumulative lengths
    in_start = 0
    for i in range(pid_b):
        in_start += tl.load(lengths_ptr + i)
    seq_len = tl.load(lengths_ptr + pid_b)

    # valid time positions for this block
    t_mask = off_t < Lmax

    # compute input row indices for valid (b, t)
    in_row = in_start + off_t
    valid_row = (off_t < seq_len) & t_mask

    # Pointers
    # x_ptr: row-major [N, D]
    x_row_ptr = x_ptr + in_row[:, None] * D + off_d[None, :]

    # out_ptr: row-major [B, Lmax, D]
    out_row_ptr = out_ptr + (pid_b * Lmax + off_t)[:, None] * D + off_d[None, :]

    # Initialize with PAD (cast will occur as needed based on out_ptr dtype)
    d_mask = off_d[None, :] < D
    pad_vals = tl.full([BLOCK_T, BLOCK_D], PAD_VALUE, tl.float32)
    tl.store(out_row_ptr, pad_vals, mask=t_mask[:, None] & d_mask)

    # Load & write only where within seq_len
    x_vals = tl.load(x_row_ptr, mask=valid_row[:, None] & d_mask)
    tl.store(out_row_ptr, x_vals, mask=valid_row[:, None] & d_mask)


def pack_seq_triton(
    x: torch.Tensor,
    lengths: torch.Tensor,
    pad_value: float = -float("inf"),
    block_t: int = 64,
    block_d: int = 64,
) -> torch.Tensor:
    """
    Pack sequences of different lengths into a batched tensor.

    Args:
        x: [N, ...] - input tensor where N is total number of tokens
        lengths: [B] - sequence lengths for each batch
        pad_value: value to use for padding
        block_t: block size for time dimension
        block_d: block size for feature dimension

    Returns:
        packed: [B, Lmax, ...] - packed tensor
    """

    # Handle multi-dimensional input by reshaping to (N, -1)
    original_shape = x.shape
    if len(original_shape) > 2:
        N = original_shape[0]
        x_reshaped = x.reshape(N, -1)
        D = x_reshaped.shape[1]
    else:
        N, D = x.shape
        x_reshaped = x

    B = lengths.numel()
    Lmax = int(lengths.max().item())

    # Starts are computed inside the kernel from lengths

    out = torch.empty((B, Lmax, D), device=x.device, dtype=x.dtype)

    grid = (B, triton.cdiv(Lmax, block_t), triton.cdiv(D, block_d))
    _pack_seq_kernel[grid](
        x_reshaped,
        out,
        lengths.int(),
        N,
        D,
        Lmax,
        PAD_VALUE=float(pad_value),
        BLOCK_T=block_t,
        BLOCK_D=block_d,
        num_warps=4,
        num_stages=2,
    )

    # Reshape output back to original dimensions (except first dimension)
    if len(original_shape) > 2:
        output_shape = (B, Lmax) + original_shape[1:]
        out = out.reshape(output_shape)

    return out


@triton.jit
def _unpack_seq_triton_kernel(
    packed_ptr,  # [B, Lmax, D]
    out_ptr,  # [N, D]
    lengths_ptr,  # *i32, [B]
    B: tl.constexpr,
    Lmax: tl.constexpr,
    D: tl.constexpr,
    BLOCK_T: tl.constexpr,  # timesteps per program
    BLOCK_D: tl.constexpr,  # features per program
):
    pid_b = tl.program_id(0)  # batch id
    pid_t = tl.program_id(1)  # block over time dimension
    pid_d = tl.program_id(2)  # block over feature dimension
    off_t = pid_t * BLOCK_T + tl.arange(0, BLOCK_T)  # [BLOCK_T]
    off_d = pid_d * BLOCK_D + tl.arange(0, BLOCK_D)  # [BLOCK_D]

    # bounds: compute start from cumulative lengths
    in_start = 0
    for i in range(pid_b):
        in_start += tl.load(lengths_ptr + i)
    seq_len = tl.load(lengths_ptr + pid_b)

    # valid time positions for this block
    t_mask = off_t < Lmax
    valid_row = (off_t < seq_len) & t_mask

    # compute output row indices for valid (b, t)
    out_row = in_start + off_t

    # Pointers
    # packed_ptr: row-major [B, Lmax, D]
    packed_row_ptr = packed_ptr + (pid_b * Lmax + off_t)[:, None] * D + off_d[None, :]

    # out_ptr: row-major [N, D]
    out_row_ptr = out_ptr + out_row[:, None] * D + off_d[None, :]

    # Load from packed tensor and store to output
    d_mask = off_d[None, :] < D
    packed_vals = tl.load(packed_row_ptr, mask=valid_row[:, None] & d_mask)
    tl.store(out_row_ptr, packed_vals, mask=valid_row[:, None] & d_mask)


def unpack_seq_triton(
    packed_tensor: torch.Tensor,
    lengths: torch.Tensor,
    block_t: int = 64,
    block_d: int = 64,
) -> torch.Tensor:
    """
    Unpack a packed decode query tensor back to the original format.
    Efficient Triton implementation.

    Args:
        packed_tensor: [B, Lmax, ...] - packed tensor from pack_seq_triton
        lengths: [B] - sequence lengths for each batch
        block_t: block size for time dimension
        block_d: block size for feature dimension

    Returns:
        unpacked_tensor: [N, ...] where N = sum(lengths)
    """

    # Handle multi-dimensional input by reshaping to (B, Lmax, -1)
    original_shape = packed_tensor.shape
    if len(original_shape) > 3:
        B, Lmax = original_shape[:2]
        packed_reshaped = packed_tensor.reshape(B, Lmax, -1)
        D = packed_reshaped.shape[2]
    else:
        B, Lmax, D = packed_tensor.shape
        packed_reshaped = packed_tensor

    # Calculate total number of elements
    N = int(lengths.sum().item())

    out = torch.empty((N, D), device=packed_tensor.device, dtype=packed_tensor.dtype)

    grid = (B, triton.cdiv(Lmax, block_t), triton.cdiv(D, block_d))
    _unpack_seq_triton_kernel[grid](
        packed_reshaped,
        out,
        lengths.int(),
        B,
        Lmax,
        D,
        BLOCK_T=block_t,
        BLOCK_D=block_d,
        num_warps=4,
        num_stages=2,
    )

    # Reshape output back to original dimensions (except first dimension)
    if len(original_shape) > 3:
        output_shape = (N,) + original_shape[2:]
        out = out.reshape(output_shape)

    return out