flash-attention/hopper/test_flash_attn.py

import os
import math
import itertools

import pytest
import torch
import torch.nn.functional as F

from einops import rearrange, repeat
from flash_attn.layers.rotary import apply_rotary_emb

from test_util import (
    attention_ref,
    generate_qkv,
    generate_random_padding_mask,
)

from flash_attn_interface import flash_attn_func, flash_attn_varlen_func, flash_attn_combine, flash_attn_with_kvcache


DISABLE_BACKWARD = os.getenv("FLASH_ATTENTION_DISABLE_BACKWARD", "FALSE") == "TRUE"
DISABLE_SPLIT = os.getenv("FLASH_ATTENTION_DISABLE_SPLIT", "FALSE") == "TRUE"
DISABLE_PAGEDKV = os.getenv("FLASH_ATTENTION_DISABLE_PAGEDKV", "FALSE") == "TRUE"
DISABLE_APPENDKV = os.getenv("FLASH_ATTENTION_DISABLE_APPENDKV", "FALSE") == "TRUE"
DISABLE_LOCAL = os.getenv("FLASH_ATTENTION_DISABLE_LOCAL", "FALSE") == "TRUE"
DISABLE_SOFTCAP = os.getenv("FLASH_ATTENTION_DISABLE_SOFTCAP", "FALSE") == "TRUE"
DISABLE_PACKGQA = os.getenv("FLASH_ATTENTION_DISABLE_PACKGQA", "FALSE") == "TRUE"
DISABLE_FP16 = os.getenv("FLASH_ATTENTION_DISABLE_FP16", "FALSE") == "TRUE"
DISABLE_FP8 = os.getenv("FLASH_ATTENTION_DISABLE_FP8", "FALSE") == "TRUE"
DISABLE_HDIM64 = os.getenv("FLASH_ATTENTION_DISABLE_HDIM64", "FALSE") == "TRUE"
DISABLE_HDIM96 = os.getenv("FLASH_ATTENTION_DISABLE_HDIM96", "FALSE") == "TRUE"
DISABLE_HDIM128 = os.getenv("FLASH_ATTENTION_DISABLE_HDIM128", "FALSE") == "TRUE"
DISABLE_HDIM192 = os.getenv("FLASH_ATTENTION_DISABLE_HDIM192", "FALSE") == "TRUE"
DISABLE_HDIM256 = os.getenv("FLASH_ATTENTION_DISABLE_HDIM256", "FALSE") == "TRUE"

COMPILED_HDIMS = (
    []
    + ([64] if not DISABLE_HDIM64 else [])
    + ([96] if not DISABLE_HDIM96 else [])
    + ([128] if not DISABLE_HDIM128 else [])
    + ([192] if not DISABLE_HDIM192 else [])
    + ([256] if not DISABLE_HDIM256 else [])
)


# @pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float8_e4m3fn])
@pytest.mark.parametrize("dtype", [torch.bfloat16] + ([torch.float16] if not DISABLE_FP16 else []) + ([torch.float8_e4m3fn] if not DISABLE_FP8 else []))
# @pytest.mark.parametrize("dtype", [torch.bfloat16])
# @pytest.mark.parametrize("dtype", [torch.float8_e4m3fn])
@pytest.mark.parametrize("mha_type", ["mha", "mqa", "gqa"])
# @pytest.mark.parametrize("mha_type", ["mha"])
# @pytest.mark.parametrize("deterministic", [False, True])
@pytest.mark.parametrize("deterministic", [False])
@pytest.mark.parametrize("softcap", [0.0] + ([30.0] if not DISABLE_SOFTCAP else []))
# @pytest.mark.parametrize("softcap", [0.0])
@pytest.mark.parametrize("local", [False] + ([True] if not DISABLE_LOCAL else []))
# @pytest.mark.parametrize("local", [False])
@pytest.mark.parametrize("causal", [False, True])
# @pytest.mark.parametrize("causal", [False])
# @pytest.mark.parametrize("V_colmajor", [False, True])
@pytest.mark.parametrize("V_colmajor", [False])
# @pytest.mark.parametrize("d", [32, 64, 96, 128, 160, 192, 224, 256])
# @pytest.mark.parametrize('d', [32, 40, 64, 80, 96, 128, 160, 192, 256])
# @pytest.mark.parametrize('d', [32, 64, 96, 128, 160, 192])
# @pytest.mark.parametrize('d', [56, 80])
# @pytest.mark.parametrize("d", [64, 128, 256])
# @pytest.mark.parametrize('d', [32, 40, 64, 80, 96, 128])
# @pytest.mark.parametrize("d", [64, 96, 128, 192])
@pytest.mark.parametrize("d", COMPILED_HDIMS)
# @pytest.mark.parametrize("d", [128])
@pytest.mark.parametrize(
    "seqlen_q,seqlen_k",
    [
        (1, 1),
        (64, 128),
        (128, 192),
        (256, 256),
        (239, 1),
        (799, 3),
        (113, 203),
        (113, 128),
        (128, 217),
        (113, 211),
        (108, 256),
        (256, 512),
        (384, 256),
        (640, 128),
        (512, 256),
        (1024, 1024),
        (1023, 1024),
        (1024, 1023),
        (4096, 4096),
        (4224, 4224),
    ],
)
# @pytest.mark.parametrize('seqlen_q,seqlen_k', [(128, 128)])
def test_flash_attn_output(
        seqlen_q, seqlen_k, d, causal, local, softcap, V_colmajor, deterministic, mha_type, dtype
):
    # sink_token_length = 0 if not local else 4
    sink_token_length = 0 if not local else 0
    if V_colmajor and (seqlen_k % 16 != 0 or dtype != torch.float8_e4m3fn):
        pytest.skip("V_colmajor requires seqlen_k to be a multiple of 16 and dtype to be float8_e4m3fn")
    device = "cuda"
    # set seed
    torch.random.manual_seed(0)
    # batch_size = 40
    # nheads = 16
    batch_size = 9 if seqlen_k <= 2048 else 2
    # batch_size = 1
    nheads = 6
    # nheads = 1
    nheads_kv = nheads if mha_type == "mha" else (2 if mha_type == "gqa" else 1)
    dtype_ref = torch.bfloat16 if dtype == torch.float8_e4m3fn else dtype
    q_ref = torch.randn(batch_size, seqlen_q, nheads, d, device=device, dtype=dtype_ref)
    if softcap > 0.0:
        # Ensure the values of qk are at least within softcap range.
        q_ref = (q_ref * softcap / 4)
    q_ref = q_ref.to(dtype).to(dtype_ref).requires_grad_()
    k_ref = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref).requires_grad_()
    v_ref = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref).requires_grad_()
    # Put window_size after QKV randn so that window_size changes from test to test
    window_size = (-1, -1) if not local else torch.randint(0, seqlen_k, (2,))
    # window_size = (-1, -1) if not local else (16, 0)
    if dtype == torch.float8_e4m3fn:
        q_descale, k_descale, v_descale = [torch.rand(batch_size, nheads_kv, device=device, dtype=torch.float32) * 2 for _ in range(3)]
    else:
        q_descale, k_descale, v_descale = None, None, None
    q, k, v = [x.detach().to(dtype).requires_grad_() for x in (q_ref, k_ref, v_ref)]
    if V_colmajor:
        v = rearrange(rearrange(v.detach(), "b s h d -> b h d s").contiguous(), "b h d s -> b s h d").requires_grad_()
    out_ref, attn_ref = attention_ref(
        q_ref,
        k_ref,
        v_ref,
        None,
        None,
        causal=causal,
        q_descale=q_descale, k_descale=k_descale, v_descale=v_descale,
        window_size=window_size,
        sink_token_length=sink_token_length,
        softcap=softcap
    )
    out_pt, attn_pt = attention_ref(
        q_ref,
        k_ref,
        v_ref,
        None,
        None,
        causal=causal,
        q_descale=q_descale, k_descale=k_descale, v_descale=v_descale,
        window_size=window_size,
        sink_token_length=sink_token_length,
        softcap=softcap,
        upcast=False,
        reorder_ops=True,
        intermediate_dtype=dtype if dtype == torch.float8_e4m3fn else None,
    )

    # qk = torch.einsum('bshd,bthd->bhst', q_ref, k_ref).float()
    # m = qk.amax(-1, keepdim=True)
    # s_tmp = torch.exp((qk - m) / math.sqrt(d))
    # exp_sum = s_tmp.sum(-1)
    # qk = torch.einsum('bthd,bshd->bhts', q_ref.float() / math.sqrt(d), k_ref.float())
    # lse_ref = torch.logsumexp(qk, dim=-1)

    abs_tol = 5e-5 if softcap == 0.0 else 5e-4

    print(f"Pytorch max diff: {(out_pt - out_ref).abs().max().item()}")
    print(f"Pytorch mean diff: {(out_pt - out_ref).abs().mean().item()}")
    pack_gqa_vals = [False, True] if not DISABLE_PACKGQA else [False]
    num_splits_vals = [1, 3] if not DISABLE_SPLIT else [1]
    for pack_gqa, num_splits in itertools.product(pack_gqa_vals, num_splits_vals):
        out, lse = flash_attn_func(
            q,
            k,
            v,
            causal=causal,
            q_descale=q_descale, k_descale=k_descale, v_descale=v_descale,
            window_size=window_size,
            sink_token_length=sink_token_length,
            softcap=softcap,
            pack_gqa=pack_gqa,
            num_splits=num_splits
        )
        print(f"Output max diff: {(out - out_ref).abs().max().item()}")
        print(f"Output mean diff: {(out - out_ref).abs().mean().item()}")
        # if not causal:
        #     print(f"LSE max diff: {(lse - lse_ref).abs().max().item()}")
        # breakpoint()

        # Check that FlashAttention's numerical error is at most twice the numerical error
        # of a Pytorch implementation.
        multiple = 2 if dtype != torch.float8_e4m3fn else 3
        assert (out - out_ref).abs().max().item() <= multiple * (out_pt - out_ref).abs().max().item() + abs_tol

    if not DISABLE_BACKWARD and dtype != torch.float8_e4m3fn and not V_colmajor:
        g = torch.randn_like(out)
        do_o = ((g.float() * out.float()).sum(-1)).transpose(1, 2)
        # import flash_attn_3_cuda
        # dq, dk, dv, softmax_d, dq_accum, dk_accum, dv_accum = flash_attn_3_cuda.bwd(
        #     g,
        #     q,
        #     k,
        #     v,
        #     out,
        #     lse,
        #     None,
        #     None,
        #     None,
        #     d ** (-0.5),
        #     causal,
        #     window_size[0], window_size[1],
        #     sink_token_length,
        #     softcap,
        #     deterministic,
        #     0,  # sm_margin
        # )
        dq, dk, dv = torch.autograd.grad(out, (q, k, v), g)
        # print(f"dO_O max diff: {(softmax_d - do_o).abs().max().item()}")
        # assert (softmax_d - do_o).abs().max().item() <= 1e-5
        # assert dq_accum.abs().max().item() == 0.0

        # dS = torch.einsum('bthd,bshd->bhts', g.float(), v.float())
        # P = torch.softmax(qk, -1)
        # dP = P * (dS - do_o.transpose(1, 2).unsqueeze(1))
        # dQ = torch.einsum('bhts,bshd->bthd', dP, k.float())
        # dV = torch.einsum('bhts,bthd->bshd', P, g.float())
        # dK = torch.einsum('bhts,bthd->bshd', dP, q.float())

        # dq, dk, dv = torch.autograd.grad(out, (q, k, v), g)
        dq_ref, dk_ref, dv_ref = torch.autograd.grad(out_ref, (q_ref, k_ref, v_ref), g)
        dq_pt, dk_pt, dv_pt = torch.autograd.grad(out_pt, (q_ref, k_ref, v_ref), g)
        print(f"dQ max diff: {(dq - dq_ref).abs().max().item()}")
        print(f"dK max diff: {(dk - dk_ref).abs().max().item()}")
        print(f"dV max diff: {(dv - dv_ref).abs().max().item()}")
        print(f"dQ mean diff: {(dq - dq_ref).abs().mean().item()}")
        print(f"dK mean diff: {(dk - dk_ref).abs().mean().item()}")
        print(f"dV mean diff: {(dv - dv_ref).abs().mean().item()}")
        print(f"dQ Pytorch max diff: {(dq_pt - dq_ref).abs().max().item()}")
        print(f"dK Pytorch max diff: {(dk_pt - dk_ref).abs().max().item()}")
        print(f"dV Pytorch max diff: {(dv_pt - dv_ref).abs().max().item()}")
        print(f"dQ Pytorch mean diff: {(dq_pt - dq_ref).abs().mean().item()}")
        print(f"dK Pytorch mean diff: {(dk_pt - dk_ref).abs().mean().item()}")
        print(f"dV Pytorch mean diff: {(dv_pt - dv_ref).abs().mean().item()}")
        # breakpoint()


    if not DISABLE_BACKWARD and dtype != torch.float8_e4m3fn and not V_colmajor:
        multiple = 2
        assert (dq - dq_ref).abs().max().item() <= multiple * (dq_pt - dq_ref).abs().max().item() + abs_tol
        assert (dk - dk_ref).abs().max().item() <= multiple * (dk_pt - dk_ref).abs().max().item() + abs_tol
        assert (dv - dv_ref).abs().max().item() <= multiple * (dv_pt - dv_ref).abs().max().item() + abs_tol


# @pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float8_e4m3fn])
@pytest.mark.parametrize("dtype", [torch.bfloat16] + ([torch.float16] if not DISABLE_FP16 else []) + ([torch.float8_e4m3fn] if not DISABLE_FP8 else []))
# @pytest.mark.parametrize("dtype", [torch.bfloat16])
# @pytest.mark.parametrize("dtype", [torch.float8_e4m3fn])
@pytest.mark.parametrize("mha_type", ["mha", "mqa", "gqa"])
# @pytest.mark.parametrize("mha_type", ["mha"])
# @pytest.mark.parametrize("deterministic", [False, True])
@pytest.mark.parametrize("deterministic", [False])
@pytest.mark.parametrize("softcap", [0.0] + ([30.0] if not DISABLE_SOFTCAP else []))
# @pytest.mark.parametrize("softcap", [0.0])
@pytest.mark.parametrize("local", [False] + ([True] if not DISABLE_LOCAL else []))
# @pytest.mark.parametrize("local", [False])
@pytest.mark.parametrize("causal", [False, True])
# @pytest.mark.parametrize("causal", [False])
@pytest.mark.parametrize("add_unused_qkv", [False, True])
# @pytest.mark.parametrize("add_unused_qkv", [True])
# @pytest.mark.parametrize("d", [32, 64, 96, 128, 160, 192, 224, 256])
# @pytest.mark.parametrize('d', [32, 40, 64, 80, 96, 128, 160, 192, 256])
# @pytest.mark.parametrize('d', [32, 64, 96, 128, 160, 192])
# @pytest.mark.parametrize('d', [56, 80])
# @pytest.mark.parametrize('d', [32, 40, 64, 80, 96, 128])
# @pytest.mark.parametrize("d", [64, 96, 128])
@pytest.mark.parametrize("d", COMPILED_HDIMS)
# @pytest.mark.parametrize("d", [128])
@pytest.mark.parametrize(
    "seqlen_q,seqlen_k",
    [
        (1, 1),
        (1, 3),
        (2, 1),
        (511, 1),
        (3, 513),
        (64, 128),
        (128, 128),
        (256, 256),
        (113, 203),
        (128, 217),
        (113, 211),
        (108, 256),
        (256, 512),
        (307, 256),
        (640, 128),
        (512, 256),
        (1024, 1024),
        (1023, 1024),
        (1024, 1023),
        (2048, 2048),
    ],
)
def test_flash_attn_varlen_output(
    seqlen_q, seqlen_k, d, add_unused_qkv, causal, local, softcap, deterministic, mha_type, dtype
):
    device = "cuda"
    # set seed
    torch.random.manual_seed(seqlen_q + seqlen_k + d + int(causal) * 2 + int(local))
    # batch_size = 40
    # nheads = 16
    batch_size = 9 if seqlen_q <= 2048 else 2
    nheads = 6
    # batch_size = 2
    # nheads = 1
    nheads_kv = nheads if mha_type == "mha" else (2 if mha_type == "gqa" else 1)
    dtype_ref = torch.bfloat16 if dtype == torch.float8_e4m3fn else dtype
    q_ref = torch.randn(batch_size, seqlen_q, nheads, d, device=device, dtype=dtype_ref)
    if softcap > 0.0:
        # Ensure the values of qk are at least within softcap range.
        q_ref = (q_ref * softcap / 4).detach().requires_grad_()
    q_ref = q_ref.to(dtype).to(dtype_ref).requires_grad_()
    k_ref = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref).requires_grad_()
    v_ref = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref).requires_grad_()
    # Put window_size after QKV randn so that window_size changes from test to test
    window_size = (-1, -1) if not local else torch.randint(0, seqlen_k, (2,))
    if dtype == torch.float8_e4m3fn:
        q_descale, k_descale, v_descale = [torch.rand(batch_size, nheads_kv, device=device, dtype=torch.float32) * 2 for _ in range(3)]
    else:
        q_descale, k_descale, v_descale = None, None, None
    q, k, v = [x.detach().requires_grad_() for x in (q_ref, k_ref, v_ref)]
    query_padding_mask = generate_random_padding_mask(
        seqlen_q, batch_size, device, mode="random", zero_lengths=False
    )
    key_padding_mask = generate_random_padding_mask(
        seqlen_k, batch_size, device, mode="random", zero_lengths=True
    )

    def _gen_unused_masks(padding_mask, add_unused, max_seq_len, bs, device):
        if add_unused:
            another_mask = generate_random_padding_mask(max_seq_len, bs, device)
            attn_mask = torch.logical_and(padding_mask, another_mask)
            unused_mask = torch.logical_xor(
                torch.logical_or(padding_mask, another_mask), attn_mask
            )
        else:
            attn_mask = padding_mask
            unused_mask = None
        return attn_mask, unused_mask

    query_padding_mask, query_unused_mask = _gen_unused_masks(
        query_padding_mask, add_unused_qkv, seqlen_q, batch_size, q.device
    )
    key_padding_mask, key_unused_mask = _gen_unused_masks(
        key_padding_mask, add_unused_qkv, seqlen_k, batch_size, k.device
    )

    (
        q_unpad,
        k_unpad,
        v_unpad,
        cu_seqlens_q,
        cu_seqlens_k,
        seqused_q,
        seqused_k,
        max_seqlen_q,
        max_seqlen_k,
        q,
        k,
        v,
        output_pad_fn,
        dq_pad_fn,
        dk_pad_fn,
    ) = generate_qkv(q, k, v, query_padding_mask, key_padding_mask, kvpacked=False,
                     query_unused_mask=query_unused_mask, key_unused_mask=key_unused_mask)
    q_unpad, k_unpad, v_unpad = [x.detach().to(dtype).requires_grad_() for x in (q_unpad, k_unpad, v_unpad)]
    out_ref, attn_ref = attention_ref(
        q_ref,
        k_ref,
        v_ref,
        query_padding_mask,
        key_padding_mask,
        causal=causal,
        q_descale=q_descale, k_descale=k_descale, v_descale=v_descale,
        window_size=window_size,
        softcap=softcap
    )
    out_pt, attn_pt = attention_ref(
        q_ref,
        k_ref,
        v_ref,
        query_padding_mask,
        key_padding_mask,
        causal=causal,
        q_descale=q_descale, k_descale=k_descale, v_descale=v_descale,
        window_size=window_size,
        softcap=softcap,
        upcast=False,
        reorder_ops=True,
        intermediate_dtype=dtype if dtype == torch.float8_e4m3fn else None,
    )


    print(f"Pytorch max diff: {(out_pt - out_ref).abs().max().item()}")
    print(f"Pytorch mean diff: {(out_pt - out_ref).abs().mean().item()}")

    if query_unused_mask is not None:
        q_zero_masking = rearrange(query_unused_mask, "b s -> b s 1 1")

    abs_tol = 5e-5 if softcap == 0.0 else 5e-4

    pack_gqa_vals = [False, True] if not DISABLE_PACKGQA else [False]
    num_splits_vals = [1, 3] if not DISABLE_SPLIT else [1]
    for pack_gqa, num_splits in itertools.product(pack_gqa_vals, num_splits_vals):
        out_unpad, lse = flash_attn_varlen_func(
            q_unpad,
            k_unpad,
            v_unpad,
            cu_seqlens_q,
            cu_seqlens_k,
            seqused_q, seqused_k,
            max_seqlen_q,
            max_seqlen_k,
            causal=causal,
            q_descale=q_descale,
            k_descale=k_descale, v_descale=v_descale,
            window_size=window_size,
            softcap=softcap,
        )
        out = output_pad_fn(out_unpad)
        if query_unused_mask is not None:
            out.masked_fill_(q_zero_masking, 0.0)
        print(f"Output max diff: {(out - out_ref).abs().max().item()}")
        print(f"Output mean diff: {(out - out_ref).abs().mean().item()}")
        # if not causal:
        #     print(f"LSE max diff: {(lse - lse_ref).abs().max().item()}")
        # breakpoint()

        # Check that FlashAttention's numerical error is at most twice the numerical error
        # of a Pytorch implementation.
        assert (out - out_ref).abs().max().item() <= 2 * (out_pt - out_ref).abs().max().item() + abs_tol


    if not DISABLE_BACKWARD and dtype != torch.float8_e4m3fn:
        g_unpad = torch.randn_like(out_unpad)
        do_o = ((g_unpad.float() * out_unpad.float()).sum(-1)).transpose(-1, -2)
        # import flash_attn_3_cuda
        # dq_unpad, dk_unpad, dv_unpad, softmax_d, dq_accum, lse_log2 = flash_attn_3_cuda.bwd_varlen(
        #     g_unpad,
        #     q_unpad,
        #     k_unpad,
        #     v_unpad,
        #     out_unpad,
        #     lse,
        #     None,
        #     None,
        #     None,
        #     cu_seqlens_q,
        #     cu_seqlens_k,
        #     None, None,
        #     max_seqlen_q,
        #     max_seqlen_k,
        #     d ** (-0.5),
        #     causal,
        #     window_size[0], window_size[1],
        #     softcap,
        #     deterministic,
        #     0,  # sm_margin
        # )
        dq_unpad, dk_unpad, dv_unpad = torch.autograd.grad(out_unpad, (q_unpad, k_unpad, v_unpad), g_unpad)
        dq = dq_pad_fn(dq_unpad)
        dk = dk_pad_fn(dk_unpad)
        dv = dk_pad_fn(dv_unpad)
        if key_unused_mask is not None:
            k_zero_masking = rearrange(key_unused_mask, "b s -> b s 1 1")
            dk.masked_fill_(k_zero_masking, 0.0)
            dv.masked_fill_(k_zero_masking, 0.0)
        if query_unused_mask is not None:
            dq.masked_fill_(q_zero_masking, 0.0)
        # print(f"dO_O max diff: {(softmax_d - do_o).abs().max().item()}")
        # assert (softmax_d - do_o).abs().max().item() <= 1e-5
        # assert dq_accum.abs().max().item() == 0.0
        g = output_pad_fn(g_unpad)

        # qk = torch.einsum('bthd,bshd->bhts', q / (d ** 0.5), k).float()
        # qk = torch.masked_fill(qk, rearrange(~key_padding_mask, "b s -> b 1 1 s"), float("-inf"))
        # dS = torch.einsum('bthd,bshd->bhts', g.float(), v.float())
        # P = torch.softmax(qk, -1)
        # dP = P * (dS - (g.float() * out.float()).sum(-1).transpose(1, 2).unsqueeze(-1))
        # dQ = torch.einsum('bhts,bshd->bthd', dP, k.float())
        # dV = torch.einsum('bhts,bthd->bshd', P, g.float())
        # dK = torch.einsum('bhts,bthd->bshd', dP, q.float())


        # dq, dk, dv = torch.autograd.grad(out, (q, k, v), g)
        dq_ref, dk_ref, dv_ref = torch.autograd.grad(out_ref, (q_ref, k_ref, v_ref), g)
        dq_pt, dk_pt, dv_pt = torch.autograd.grad(out_pt, (q_ref, k_ref, v_ref), g)
        print(f"dQ max diff: {(dq - dq_ref).abs().max().item()}")
        print(f"dK max diff: {(dk - dk_ref).abs().max().item()}")
        print(f"dV max diff: {(dv - dv_ref).abs().max().item()}")
        print(f"dQ mean diff: {(dq - dq_ref).abs().mean().item()}")
        print(f"dK mean diff: {(dk - dk_ref).abs().mean().item()}")
        print(f"dV mean diff: {(dv - dv_ref).abs().mean().item()}")
        print(f"dQ Pytorch max diff: {(dq_pt - dq_ref).abs().max().item()}")
        print(f"dK Pytorch max diff: {(dk_pt - dk_ref).abs().max().item()}")
        print(f"dV Pytorch max diff: {(dv_pt - dv_ref).abs().max().item()}")
        print(f"dQ Pytorch mean diff: {(dq_pt - dq_ref).abs().mean().item()}")
        print(f"dK Pytorch mean diff: {(dk_pt - dk_ref).abs().mean().item()}")
        print(f"dV Pytorch mean diff: {(dv_pt - dv_ref).abs().mean().item()}")
        # breakpoint()

    if not DISABLE_BACKWARD and dtype != torch.float8_e4m3fn:
        multiple = 2
        assert (dq - dq_ref).abs().max().item() <= multiple * (dq_pt - dq_ref).abs().max().item() + abs_tol
        assert (dk - dk_ref).abs().max().item() <= multiple * (dk_pt - dk_ref).abs().max().item() + abs_tol
        assert (dv - dv_ref).abs().max().item() <= multiple * (dv_pt - dv_ref).abs().max().item() + abs_tol


# @pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float8_e4m3fn])
@pytest.mark.parametrize("dtype", [torch.bfloat16] + ([torch.float8_e4m3fn] if not DISABLE_FP8 else []))
# @pytest.mark.parametrize("dtype", [torch.bfloat16])
# @pytest.mark.parametrize("dtype", [torch.float8_e4m3fn])
@pytest.mark.parametrize("num_splits", [1] + ([0] if not DISABLE_SPLIT else []))
# @pytest.mark.parametrize("num_splits", [1])
@pytest.mark.parametrize("mha_type", ["mha", "mqa", "gqa"])
# @pytest.mark.parametrize("mha_type", ["mha"])
@pytest.mark.parametrize("new_kv", [False] + ([True] if not DISABLE_APPENDKV else []))
# @pytest.mark.parametrize("new_kv", [True])
# @pytest.mark.parametrize("local", [False, True])
@pytest.mark.parametrize("causal,local", [(False, False), (True, False)] + ([(False, True)] if not DISABLE_LOCAL else []))
# @pytest.mark.parametrize("causal,local", [(False, False), (True, False)])
# @pytest.mark.parametrize("causal,local", [(False, False)])
@pytest.mark.parametrize("seqlen_new_eq_seqlen_q", [True, False] if not DISABLE_APPENDKV else [True])
# @pytest.mark.parametrize("seqlen_new_eq_seqlen_q", [True])
@pytest.mark.parametrize("rotary_interleaved", [False, True] if not DISABLE_APPENDKV else [False])
# @pytest.mark.parametrize("rotary_interleaved", [True])
@pytest.mark.parametrize("rotary_fraction", [0.0, 0.5, 1.0] if not DISABLE_APPENDKV else [0.0])
# @pytest.mark.parametrize("rotary_fraction", [0.0])
@pytest.mark.parametrize("page_size", [None] + ([1, 4, 128] if not DISABLE_PAGEDKV else []))
# @pytest.mark.parametrize("page_size", [None])
@pytest.mark.parametrize("has_leftpad", [False, True])
# @pytest.mark.parametrize("has_leftpad", [False])
@pytest.mark.parametrize("has_batch_idx", [False, True])
# @pytest.mark.parametrize("has_batch_idx", [False])
@pytest.mark.parametrize("varlen_q", [False, True])
# @pytest.mark.parametrize("varlen_q", [True])
# @pytest.mark.parametrize("d", [32, 59, 64, 80, 128, 256])
# @pytest.mark.parametrize("d", [32, 64, 96, 128, 160, 192, 224, 256])
# @pytest.mark.parametrize('d', [32, 40, 64, 80, 96, 128, 160, 192])
# @pytest.mark.parametrize('d', [56, 80])
@pytest.mark.parametrize("d", [128])
@pytest.mark.parametrize(
    "seqlen_q,seqlen_k",
    [
        (1, 128),
        (1, 339),
        (3, 1024),
        (64, 800),
        (64, 256),
        (3, 799),
        (64, 2048),
        (16, 20000),
        (1, 128 * 1024),
        (16, 128 * 1024),
        (128, 128),
        (256, 512),  # To test appending KV with more than 1 block
        (2048, 3577),  # Enough tile to test persistent scheduler
    ],
)
# @pytest.mark.parametrize('seqlen_q,seqlen_k', [(256, 128)])
def test_flash_attn_kvcache(
    seqlen_q,
    seqlen_k,
    d,
    varlen_q,
    has_batch_idx,
    has_leftpad,
    page_size,
    rotary_fraction,
    rotary_interleaved,
    seqlen_new_eq_seqlen_q,
    causal,
    local,
    new_kv,
    mha_type,
    num_splits,
    dtype,
):
    if page_size is not None and seqlen_k % page_size != 0:
        pytest.skip()
    if seqlen_q > seqlen_k and new_kv:
        pytest.skip()
    if not new_kv and rotary_fraction > 0.0:
        pytest.skip()
    device = "cuda"
    # set seed
    torch.random.manual_seed(0)
    batch_size = 5
    # batch_size = 1
    batch_size_cache = batch_size if not has_batch_idx else batch_size * 2
    nheads = 6
    # nheads = 1
    # rotary_dim must be a multiple of 16, and must be <= d
    rotary_dim = math.floor(int(rotary_fraction * d) / 16) * 16
    nheads_k = nheads if mha_type == "mha" else (1 if mha_type == "mqa" else 3)
    assert nheads % nheads_k == 0
    dtype_ref = torch.bfloat16 if dtype == torch.float8_e4m3fn else dtype
    q = torch.randn(batch_size, seqlen_q, nheads, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref)
    if varlen_q:
        query_padding_mask = generate_random_padding_mask(seqlen_q, batch_size, device, mode="random")
        q_unpad, indices_q, cu_seqlens_q, max_seqlen_q, *rest = unpad_input(q, query_padding_mask)
        output_pad_fn = lambda output_unpad: pad_input(
            output_unpad, indices_q, batch_size, seqlen_q
        )
    else:
        query_padding_mask = None
        q_unpad = q
        cu_seqlens_q, max_seqlen_q = None, None
    # Put window_size after QKV randn so that window_size changes from test to test
    window_size = (-1, -1) if not local else torch.randint(0, seqlen_k, (2,))

    seqlen_new = seqlen_q if seqlen_new_eq_seqlen_q else torch.randint(1, seqlen_q + 1, (1,)).item()
    cu_seqlens_k_new = None
    key_new_padding_mask = None
    if new_kv:
        k = torch.randn(batch_size, seqlen_new, nheads_k, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref)
        v = torch.randn(batch_size, seqlen_new, nheads_k, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref)
        if varlen_q:  # k & v are also varlen
            key_new_padding_mask = generate_random_padding_mask(seqlen_new, batch_size, device, mode="random")
            k_unpad, indices_k, cu_seqlens_k_new, *rest = unpad_input(k, key_new_padding_mask)
            v_unpad, *rest = unpad_input(v, key_new_padding_mask)
        else:
            k_unpad, v_unpad = k, v
    else:
        k, v, k_unpad, v_unpad = None, None, None, None
    if page_size is None:
        k_cache = torch.randn(batch_size_cache, seqlen_k, nheads_k, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref)
        v_cache = torch.randn(batch_size_cache, seqlen_k, nheads_k, d, device=device, dtype=dtype_ref).to(dtype).to(dtype_ref)
        page_table = None
    else:
        (
            k_cache,
            v_cache,
            page_table,
            k_cache_paged,
            v_cache_paged,
            num_blocks,
        ) = _generate_block_kvcache(
            seqlen_k, page_size, batch_size_cache, nheads_k, d, device, dtype_ref
        )
    cache_seqlens = torch.randint(
        0 if new_kv else 1,
        # If we don't use seqlen_q in the case of causal and rotary, cos/sin won't be long enough
        (
            (seqlen_k - (seqlen_q if (causal or local) and rotary_dim > 1 else seqlen_new) + 1)
            if new_kv
            else (seqlen_k + 1)
        ),
        (batch_size,),
        dtype=torch.int32,
        device=device,
    )
    if has_leftpad:
        cache_leftpad = torch.cat([torch.randint(0, cache_seqlens[i].item(), (1,), dtype=torch.int32, device=device)
                                   if cache_seqlens[i].item() > 0 else torch.zeros(1, dtype=torch.int32, device=device)
                                   for i in range(batch_size)])
    else:
        cache_leftpad = None
    if has_batch_idx:
        cache_batch_idx = torch.randperm(batch_size_cache, dtype=torch.int32, device=device)[
            :batch_size
        ]
    else:
        cache_batch_idx = None
    arange = rearrange(torch.arange(seqlen_k, device=device), "s -> 1 s")
    cache_seqlens_expanded = rearrange(cache_seqlens, "b -> b 1")
    if not new_kv:
        key_padding_mask = arange < cache_seqlens_expanded
    else:
        k_new_seqlens = key_new_padding_mask.sum(-1, keepdims=True) if varlen_q else seqlen_new
        key_padding_mask = arange < cache_seqlens_expanded + k_new_seqlens
    if has_leftpad:
        key_padding_mask = torch.logical_and(
            key_padding_mask, arange >= cache_leftpad.unsqueeze(-1).expand(-1, seqlen_k)
        )
    # cache_seqlens = torch.tensor([64], dtype=torch.int32, device=device)
    if rotary_dim > 0:
        angle = (
            torch.rand(
                seqlen_k if page_size is None else num_blocks * page_size,
                rotary_dim // 2,
                device=device,
            )
            * 2
            * math.pi
        )
        cos = torch.cos(angle).to(dtype=dtype_ref).to(dtype).to(dtype_ref)
        sin = torch.sin(angle).to(dtype=dtype_ref).to(dtype).to(dtype_ref)
        if causal or local:
            q_ro = apply_rotary_emb(
                q, cos, sin, seqlen_offsets=cache_seqlens, interleaved=rotary_interleaved
            )
        else:
            q_ro = rearrange(
                apply_rotary_emb(
                    rearrange(q, "b s h d -> b 1 (s h) d"),
                    cos,
                    sin,
                    seqlen_offsets=cache_seqlens,
                    interleaved=rotary_interleaved,
                ),
                "b 1 (s h) d -> b s h d",
                s=seqlen_q,
            )
        # q_ro = q
        k_ro = apply_rotary_emb(
            k, cos, sin, seqlen_offsets=cache_seqlens, interleaved=rotary_interleaved
        )
    else:
        cos, sin = None, None
        q_ro, k_ro = q, k
    # k_cache[:, 64:] = -1
    k_cache_ref = (k_cache if not has_batch_idx else k_cache[cache_batch_idx]).clone()
    v_cache_ref = (v_cache if not has_batch_idx else v_cache[cache_batch_idx]).clone()
    if new_kv:
        update_mask = torch.logical_and(
            cache_seqlens_expanded <= arange, arange < cache_seqlens_expanded + k_new_seqlens
        )
        k_to_update = rearrange(k_ro, "b s ... -> (b s) ...")
        v_to_update = rearrange(v, "b s ... -> (b s) ...")
        if varlen_q:
            k_to_update = k_to_update[indices_k]
            v_to_update = v_to_update[indices_k]
        k_cache_ref[update_mask] = k_to_update
        v_cache_ref[update_mask] = v_to_update
    k_cache_rep = repeat(k_cache_ref, "b s h d -> b s (h g) d", g=nheads // nheads_k)
    v_cache_rep = repeat(v_cache_ref, "b s h d -> b s (h g) d", g=nheads // nheads_k)
    out_ref, _ = attention_ref(
        q_ro,
        k_cache_rep,
        v_cache_rep,
        query_padding_mask,
        key_padding_mask,
        causal=causal,
        window_size=window_size,
        key_leftpad=cache_leftpad,
    )
    out_pt, _ = attention_ref(
        q_ro,
        k_cache_rep,
        v_cache_rep,
        query_padding_mask,
        key_padding_mask,
        causal=causal,
        window_size=window_size,
        upcast=False,
        reorder_ops=True,
        key_leftpad=cache_leftpad,
        intermediate_dtype=dtype if dtype == torch.float8_e4m3fn else None
    )
    q = q.to(dtype)
    q_unpad = q_unpad.to(dtype) if varlen_q else None
    k_cache = k_cache.to(dtype)
    v_cache = v_cache.to(dtype)
    k_cache_paged = k_cache_paged.to(dtype) if page_size is not None else None
    v_cache_paged = v_cache_paged.to(dtype) if page_size is not None else None
    k = k.to(dtype) if k is not None else None
    v = v.to(dtype) if v is not None else None
    k_unpad = k_unpad.to(dtype) if k_unpad is not None else None
    v_unpad = v_unpad.to(dtype) if v_unpad is not None else None
    cos = cos.to(dtype) if cos is not None else None
    sin = sin.to(dtype) if sin is not None else None
    out, lse, *rest = flash_attn_with_kvcache(
        q if not varlen_q else q_unpad,
        k_cache if page_size is None else k_cache_paged,
        v_cache if page_size is None else v_cache_paged,
        k if not new_kv or not varlen_q else k_unpad,
        v if not new_kv or not varlen_q else v_unpad,
        rotary_cos=cos,
        rotary_sin=sin,
        cache_seqlens=cache_seqlens,
        cache_batch_idx=cache_batch_idx,
        cache_leftpad=cache_leftpad,
        page_table=page_table,
        cu_seqlens_q=cu_seqlens_q,
        cu_seqlens_k_new=cu_seqlens_k_new,
        max_seqlen_q=max_seqlen_q,
        causal=causal,
        window_size=window_size,
        rotary_interleaved=rotary_interleaved,
        num_splits=num_splits,
        return_softmax_lse=True
    )
    if varlen_q:
        out = output_pad_fn(out)
    # out = flash_attn_with_kvcache(
    #     q, k_cache, v_cache, cache_seqlens=cache_seqlens, causal=causal, window_size=window_size
    # )
    # out = flash_attn_with_kvcache(q, k_cache, v_cache, causal=causal, window_size=window_size)
    # qk = torch.einsum("bqhd,bkhd->bhqk", q, k_cache_ref)
    # m = qk.amax(-1, keepdim=True)
    # s_tmp = torch.exp((qk - m) / math.sqrt(d))
    # o1 = torch.einsum('bhst,bthd->bshd', s_tmp, v_cache_ref)
    # lse_ref = torch.logsumexp(qk / math.sqrt(d), -1)
    # probs = torch.softmax(qk, dim=-1)
    print(f"Output max diff: {(out - out_ref).abs().max().item()}")
    print(f"Output mean diff: {(out - out_ref).abs().mean().item()}")
    print(f"Pytorch max diff: {(out_pt - out_ref).abs().max().item()}")
    print(f"Pytorch mean diff: {(out_pt - out_ref).abs().mean().item()}")
    # breakpoint()

    # Check that FlashAttention's numerical error is at most twice the numerical error
    # of a Pytorch implementation.
    if new_kv:
        if page_size is None:
            k_cache_select = (
                k_cache.to(dtype_ref) if not has_batch_idx else k_cache.to(dtype_ref)[cache_batch_idx]
            )
            v_cache_select = (
                v_cache.to(dtype_ref) if not has_batch_idx else v_cache.to(dtype_ref)[cache_batch_idx]
            )
        else:
            k_cache_select = rearrange(
                k_cache_paged.to(dtype_ref)[(page_table if not has_batch_idx else page_table[cache_batch_idx]).flatten()],
                "(b nblocks) block_size ... -> b (nblocks block_size) ...",
                b=batch_size,
            )[:, :seqlen_k].to(dtype_ref)
            v_cache_select = rearrange(
                v_cache_paged.to(dtype_ref)[(page_table if not has_batch_idx else page_table[cache_batch_idx]).flatten()],
                "(b nblocks) block_size ... -> b (nblocks block_size) ...",
                b=batch_size,
            )[:, :seqlen_k].to(dtype_ref)
        k_cache_ref = k_cache_ref.to(dtype).to(dtype_ref)
        v_cache_ref = v_cache_ref.to(dtype).to(dtype_ref)
        if dtype is not torch.float8_e4m3fn:
            assert torch.equal(v_cache_select, v_cache_ref)
        else:
            assert torch.allclose(v_cache_select, v_cache_ref, rtol=1e-3, atol=1e-3)
        # breakpoint()
        # if rotary_dim == 0 and dtype is not torch.float8_e4m3fn:
        if rotary_dim == 0:
            assert torch.equal(k_cache_select, k_cache_ref)
        else:
            # if not torch.allclose(k_cache_select, k_cache_ref, rtol=1e-3, atol=1e-3):
            #     breakpoint()
            if dtype is not torch.float8_e4m3fn:
                assert torch.allclose(k_cache_select, k_cache_ref, rtol=1e-3, atol=1e-3)
            else:
                assert torch.allclose(k_cache_select, k_cache_ref, rtol=1e-1, atol=1e-1)
    mult = 4 if dtype == torch.float8_e4m3fn else 2
    assert (out - out_ref).abs().max().item() <= mult * (out_pt - out_ref).abs().max().item() + 1e-5
    mult_mean = 3 if dtype == torch.float8_e4m3fn else 1.5
    assert (out - out_ref).abs().mean().item() <= mult_mean * (out_pt - out_ref).abs().mean().item()


def _generate_block_kvcache(seqlen_k, page_size, batch_size, nheads_k, d, device, dtype):
    num_blocks = math.ceil(seqlen_k / page_size) * batch_size * 3
    k_cache_paged = torch.randn(
        num_blocks, page_size, nheads_k, d, device=device, dtype=dtype
    )
    v_cache_paged = torch.randn(
        num_blocks, page_size, nheads_k, d, device=device, dtype=dtype
    )
    page_table = rearrange(
        torch.randperm(num_blocks, dtype=torch.int32, device=device),
        "(b nblocks) -> b nblocks",
        b=batch_size,
    )
    k_cache = rearrange(
        k_cache_paged[page_table.flatten()],
        "(b nblocks) block_size ... -> b (nblocks block_size) ...",
        b=batch_size,
    )[:, :seqlen_k]
    v_cache = rearrange(
        v_cache_paged[page_table.flatten()],
        "(b nblocks) block_size ... -> b (nblocks block_size) ...",
        b=batch_size,
    )[:, :seqlen_k]
    return k_cache, v_cache, page_table, k_cache_paged, v_cache_paged, num_blocks


@pytest.mark.parametrize("dtype", [torch.bfloat16])
@pytest.mark.parametrize("causal", [False, True])
# @pytest.mark.parametrize('causal', [False])
@pytest.mark.parametrize('d', [128])
@pytest.mark.parametrize(
    "seqlen_q,seqlen_k",
    [
        (64, 8192),
    ],
)
def test_flash_attn_cluster(seqlen_q, seqlen_k, d, causal, dtype):
    device = "cuda"
    torch.random.manual_seed(0)
    batch_size = 2
    nheads = 16
    nheads_kv = 4
    # There was a bug where this would cause "unspecified launch failure" due to Cluster
    q = torch.randn(batch_size, seqlen_q, nheads, d, device=device, dtype=dtype)
    k = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype)
    v = torch.randn(batch_size, seqlen_k, nheads_kv, d, device=device, dtype=dtype)
    for _ in range(100):
        flash_attn_func(q, k, v, causal=causal)


# @pytest.mark.parametrize("dtype", ([torch.float16] if is_sm75 else [torch.float16, torch.bfloat16]))
@pytest.mark.parametrize("dtype", [torch.bfloat16])
@pytest.mark.parametrize("causal", [False, True])
# @pytest.mark.parametrize('causal', [False])
@pytest.mark.parametrize("d", [32, 40, 59, 64, 80, 96, 111, 128, 160, 192, 224, 256])
# @pytest.mark.parametrize("d", [32, 40, 59, 64, 80, 96, 111, 128])
# @pytest.mark.parametrize('d', [32, 56, 64, 80, 96, 128])
# @pytest.mark.parametrize("d", [32, 64, 96, 128, 160, 192])
# @pytest.mark.parametrize('d', [80])
@pytest.mark.parametrize(
    "seqlen_q,seqlen_k",
    [
        (1, 239),
        (239, 1),
        (3, 799),
        (799, 3),
        (1024, 128),
        (97, 97),
        (128, 128),
        (200, 200),
        (256, 256),
        (257, 257),
        (384, 384),
        (512, 512),
        (768, 768),
        (1024, 1024),
        (2048, 2048),
    ],
)
def test_flash_attn_race_condition(seqlen_q, seqlen_k, d, causal, dtype):
    device = "cuda"
    # set seed
    torch.random.manual_seed(0)
    # Simulate under memory load
    dummy = torch.empty(70 * 1024 ** 3, dtype=torch.uint8, device=device)
    batch_size = 60  # Sometimes we need large batch size for the race conditions to trigger
    nheads = 4
    q = torch.randn(batch_size, seqlen_q, nheads, d, device=device, dtype=dtype, requires_grad=True)
    k = torch.randn(batch_size, seqlen_k, nheads, d, device=device, dtype=dtype, requires_grad=True)
    v = torch.randn(batch_size, seqlen_k, nheads, d, device=device, dtype=dtype, requires_grad=True)
    torch.random.manual_seed(42)
    out0, lse0 = flash_attn_func(q, k, v, causal=causal)
    g = torch.randn_like(out0)
    dq0, dk0, dv0 = torch.autograd.grad(out0, (q, k, v), g)
    # Numerical error if we just do any arithmetic on dq
    dq_atol = 2 * ((dq0 + 0.3 - 0.3) - dq0).abs().max().item()

    for i in range(1000):
        torch.random.manual_seed(42)
        out, lse = flash_attn_func(q, k, v, causal=causal)
        assert torch.equal(out, out0)
        assert torch.equal(lse, lse0)

        dq, dk, dv = torch.autograd.grad(out, (q, k, v), g)
        dq_equal = torch.allclose(dq, dq0, atol=dq_atol)
        if not dq_equal:
            print(f"Iter {i}, {dq_atol = }, dQ max diff: {(dq - dq0).abs().max().item()}")
            # breakpoint()
        assert torch.equal(dv, dv0)
        assert torch.equal(dk, dk0)
        assert dq_equal


def attention_combine_ref(out_partial, lse_partial):
    """
    out_partial: (num_splits, batch_size, seqlen, nheads, d)
    lse_partial: (num_splits, batch_size, nheads, seqlen)
    """
    lse = torch.logsumexp(lse_partial, dim=0)
    scale = torch.exp(lse_partial - lse)
    scale = torch.where(torch.isinf(scale) | torch.isnan(scale), torch.zeros_like(scale), scale)
    out = (scale.unsqueeze(-1) * out_partial).sum(0)
    return out, lse


@pytest.mark.parametrize("dtype", [torch.float32, torch.float16, torch.bfloat16])
# @pytest.mark.parametrize("dtype", [torch.float32])
# @pytest.mark.parametrize("d", [32, 40, 59, 64, 80, 96, 111, 128, 160, 192, 224, 256])
@pytest.mark.parametrize("d", [64, 96, 128, 192, 256])
# @pytest.mark.parametrize("d", [128])
@pytest.mark.parametrize("seqlen", [1, 2, 3, 32, 64, 256, 113, 108, 640, 1024, 2048])
# @pytest.mark.parametrize("seqlen", [12, 32, 64, 256, 112, 108, 640, 1024, 2048, 8192])
# @pytest.mark.parametrize("seqlen", [15])
@pytest.mark.parametrize("num_splits", [1, 2, 3, 5, 17, 32, 55, 97, 155])
# @pytest.mark.parametrize("num_splits", [1, 2, 3, 5, 11])
# @pytest.mark.parametrize("num_splits", [128])
def test_flash_attn_combine(num_splits, seqlen, d, dtype):
    if DISABLE_SPLIT:
        pytest.skip()
    device = "cuda"
    # set seed
    torch.random.manual_seed(1)
    batch_size = 5
    nheads = 16
    # batch_size = 1
    # nheads = 1
    out_partial = torch.randn(num_splits * 2, batch_size, nheads, seqlen, d, device=device, dtype=torch.float32).transpose(2, 3)[:num_splits]  # To test non-contiguous tensor
    lse_partial = torch.randn(num_splits, batch_size, nheads * 2, seqlen, device=device, dtype=torch.float32).transpose(-1, -2)[:, :, :, :nheads]  # To test non-contiguous tensor
    # To test short-circuiting based on num_splits
    lse_partial[num_splits // 2:, :batch_size // 3] = -float("inf")
    out, lse = flash_attn_combine(out_partial, lse_partial, out_dtype=dtype)
    out_ref, lse_ref = attention_combine_ref(out_partial, lse_partial)
    out_pt = out_ref.to(dtype)

    print(f"LSE max diff: {(lse - lse_ref).abs().max().item()}")
    print(f"LSE mean diff: {(lse - lse_ref).abs().mean().item()}")
    print(f"Output max diff: {(out - out_ref).abs().max().item()}")
    print(f"Output mean diff: {(out - out_ref).abs().mean().item()}")
    print(f"Pytorch max diff: {(out_pt - out_ref).abs().max().item()}")
    print(f"Pytorch mean diff: {(out_pt - out_ref).abs().mean().item()}")
    # breakpoint()

    assert torch.allclose(lse, lse_ref, atol=1e-5, rtol=1e-5)
    multiple = 2
    assert ((out - out_ref).abs().max().item() <= multiple * (out_pt - out_ref).abs().max().item()) or torch.allclose(out, out_pt, atol=1e-5, rtol=1e-5)

    # from flash_attn.utils.benchmark import pytorch_profiler
    # # pytorch_profiler(torch.sum, lse_partial)
    # pytorch_profiler(flash_attn_combine, out_partial, lse_partial)
    # pytorch_profiler(torch.sum, out_partial)