flash_attn.py

from functools import lru_cache
from typing import Optional, Union

import torch
import torch.nn as nn

# try:
#     from sgl_kernel import flash_ops
# except:
#     raise ImportError("Can not import sgl_kernel. Please check your installation.")

try:
    from ._fa4_interface import flash_attn_varlen_func as flash_attn_varlen_func_v4
except ImportError:
    flash_attn_varlen_func_v4 = None


@lru_cache(maxsize=1)
def is_fa3_supported(device=None) -> bool:
    #  There some fa3 FYI
    #  FA3 can fail without a enough shared memory for a some shapes, such as higher
    #  hidden_dim or some special cases.
    #  Right now, fa3 is supported for sm80/sm87 and sm86/sm89. The main different
    #  Between sm80/sm87 and sm86/sm89 is the shared memory size. you can follow the link below for more information
    #  https://docs.nvidia.com/cuda/cuda-c-programming-guide/#shared-memory-8-x
    #  And for sgl-kernel right now, we can build fa3 on sm80/sm86/sm89/sm90a.
    #  That means if you use A100/A*0/L20/L40/L40s/4090 you can use fa3.
    return (torch.version.cuda >= "12.3") and (
        torch.cuda.get_device_capability(device)[0] == 9
        or torch.cuda.get_device_capability(device)[0] == 8
    )


def maybe_contiguous(x):
    return x.contiguous() if x is not None and x.stride(-1) != 1 else x



def flash_attn_with_kvcache(
    q,
    k_cache,
    v_cache,
    k=None,
    v=None,
    qv=None,
    rotary_cos=None,
    rotary_sin=None,
    cache_seqlens: Optional[Union[(int, torch.Tensor)]] = None,
    cache_batch_idx: Optional[torch.Tensor] = None,
    cache_leftpad: Optional[torch.Tensor] = None,
    page_table: Optional[torch.Tensor] = None,
    cu_seqlens_q: Optional[torch.Tensor] = None,
    cu_seqlens_k_new: Optional[torch.Tensor] = None,
    max_seqlen_q: Optional[int] = None,
    rotary_seqlens: Optional[torch.Tensor] = None,
    q_descale: Optional[torch.Tensor] = None,
    k_descale: Optional[torch.Tensor] = None,
    v_descale: Optional[torch.Tensor] = None,
    softmax_scale=None,
    causal=False,
    window_size=(-1, -1),  # -1 means infinite context window
    softcap=0.0,  # 0.0 means deactivated
    rotary_interleaved=True,
    scheduler_metadata=None,
    num_splits=0,  # Can be tuned for speed
    pack_gqa=None,  # Can be tuned for speed
    sm_margin=0,  # Can be tuned if some SMs are used for communication
    return_softmax_lse=False,
    sinks=None,
    ver=3,
):
    if ver == 4:
        raise NotImplementedError("haven't implemented flash_attn_with_kvcache for fa4")

    # HIP环境检测和回退
    if hasattr(torch.version, 'hip') and torch.version.hip is not None:
        # 简单PyTorch回退，处理实际的张量形状
        # q: [1, 4, 256], k_cache: [411528, 1, 1, 256], v_cache: [411528, 1, 1, 256]
        
        if softmax_scale is None:
            softmax_scale = (q.shape[-1]) ** (-0.5)
        
        # 重塑以匹配attention计算
        q_reshaped = q.unsqueeze(1)  # [1, 1, 4, 256]
        k_reshaped = k_cache.squeeze(1).squeeze(1)  # [411528, 256]  
        v_reshaped = v_cache.squeeze(1).squeeze(1)  # [411528, 256]
        
        # 简单的点积attention
        scores = torch.matmul(q, k_reshaped.T) * softmax_scale  # [1, 4, 411528]
        attn_weights = torch.softmax(scores, dim=-1)
        out = torch.matmul(attn_weights, v_reshaped)  # [1, 4, 256]
        
        if return_softmax_lse:
            softmax_lse = torch.zeros(1, 4, 1, device=q.device)
            return out, softmax_lse
        return out

    # 原始sgl_kernel实现
    assert k_cache.stride(-1) == 1, "k_cache must have contiguous last dimension"
    assert v_cache.stride(-1) == 1, "v_cache must have contiguous last dimension"
    if softmax_scale is None:
        softmax_scale = (q.shape[-1] + (qv.shape[-1] if qv is not None else 0)) ** (-0.5)
    if cache_seqlens is not None and isinstance(cache_seqlens, int):
        cache_seqlens = torch.full(
            (k_cache.shape[0],), cache_seqlens, dtype=torch.int32, device=k_cache.device
        )
        cache_seqlens = maybe_contiguous(cache_seqlens)

    q, k_cache, k, v = [maybe_contiguous(x) for x in (q, k_cache, k, v)]
    v_cache = (
        v_cache.contiguous()
        if v_cache.stride(-1) != 1 and v_cache.stride(-3) != 1
        else v_cache
    )
    cu_seqlens_q, cu_seqlens_k_new = [
        maybe_contiguous(x) for x in (cu_seqlens_q, cu_seqlens_k_new)
    ]
    page_table, cache_batch_idx, cache_leftpad = [
        maybe_contiguous(x) for x in (page_table, cache_batch_idx, cache_leftpad)
    ]
    rotary_cos, rotary_sin = [maybe_contiguous(x) for x in (rotary_cos, rotary_sin)]
    rotary_seqlens = maybe_contiguous(rotary_seqlens)

    out, softmax_lse, *rest = torch.ops.sgl_kernel.fwd.default(
        q,
        k_cache,
        v_cache,
        k,
        v,
        qv,
        None,  # out
        cu_seqlens_q,
        None,  # cu_seqlens_k
        cu_seqlens_k_new,
        None,  # seqused_q
        cache_seqlens,
        max_seqlen_q,
        None,  # max_seqlen_k
        page_table,
        cache_batch_idx,
        cache_leftpad,
        rotary_cos,
        rotary_sin,
        rotary_seqlens,
        q_descale,
        k_descale,
        v_descale,
        softmax_scale,
        causal,
        window_size[0],
        window_size[1],
        softcap,
        rotary_interleaved,
        scheduler_metadata,
        num_splits,
        pack_gqa,
        sm_margin,
        sinks,
    )
    return (out, softmax_lse) if return_softmax_lse else out


def flash_attn_varlen_func(
    q,
    k,
    v,
    cu_seqlens_q,
    cu_seqlens_k,
    max_seqlen_q,
    max_seqlen_k,
    seqused_q=None,
    seqused_k=None,
    softmax_scale=None,
    causal=False,
    qv=None,
    q_descale=None,
    k_descale=None,
    v_descale=None,
    window_size=(-1, -1),
    softcap=0.0,
    num_splits=1,
    pack_gqa=None,
    sm_margin=0,
    return_softmax_lse=False,
    sinks=None,
    ver=3,
):
    if ver == 4:
        assert (
            flash_attn_varlen_func_v4 is not None
        ), "FA4 is not available, please check your installation."
        # Using `(-1, -1)` as no sliding window causes correctness issues for FA4.
        if window_size == (-1, -1):
            window_size = (None, None)
        return flash_attn_varlen_func_v4(
            q,
            k,
            v,
            cu_seqlens_q,
            cu_seqlens_k,
            # max_seqlen_q,
            # max_seqlen_k,
            seqused_q=seqused_q,
            seqused_k=seqused_k,
            softmax_scale=softmax_scale,
            causal=causal,
            # qv=qv,
            # q_descale=q_descale,
            # k_descale=k_descale,
            # v_descale=v_descale,
            window_size=window_size,
            softcap=softcap,
            # num_splits=num_splits,
            pack_gqa=pack_gqa,
            # sm_margin=sm_margin,
            return_softmax_lse=return_softmax_lse,
            learnable_sink=sinks,
        )

    if not is_fa3_supported():
        raise NotImplementedError(
            "flash_attn at sgl-kernel is only supported on sm90 and above"
        )

    if softmax_scale is None:
        softmax_scale = (q.shape[-1] + (qv.shape[-1] if qv is not None else 0)) ** (
            -0.5
        )

    out, softmax_lse, *rest = torch.ops.sgl_kernel.fwd.default(
        q,
        k,
        v,
        None,  # k_new
        None,  # v_new
        qv,  # qv
        None,  # out
        cu_seqlens_q,
        cu_seqlens_k,
        None,  # cu_seqlens_k_new
        seqused_q,
        seqused_k,
        max_seqlen_q,
        max_seqlen_k,
        None,  # page_table,
        None,  # kv_batch_idx
        None,  # leftpad_k
        None,  # rotary cos
        None,  # rotary sin
        None,  # seqlens_rotary
        q_descale,
        k_descale,
        v_descale,
        softmax_scale,
        causal,
        window_size[0],
        window_size[1],
        softcap,
        is_rotary_interleaved=False,
        scheduler_metadata=None,
        num_splits=num_splits,
        pack_gqa=pack_gqa,
        sm_margin=sm_margin,
        sinks=sinks,
    )

    return (out, softmax_lse, *rest) if return_softmax_lse else out