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[Enhancement] Deprecate split&sum in attn bwd examples on Hopper #1065

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Oct 19, 2025
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[Enhancement] Deprecate split&sum in attn bwd examples on Hopper #1065

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270 changes: 43 additions & 227 deletions examples/flash_attention/example_gqa_bwd_wgmma_pipelined.py
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Original file line number Diff line number Diff line change
Expand Up @@ -113,51 +113,20 @@ def flash_bwd_prep(
return flash_bwd_prep


def make_dq_layout(dQ):
# atomicAdd can not be vectorized, so we need to reorder dq to match the 8x8 gemm fragment
return T.Layout(dQ.shape,
lambda b, l, h, d: [b, l // 8, h, d // 8, (d % 2), 4 * (l % 8) + (d % 8) // 2])


@tilelang.jit(
out_idx=[1], pass_configs={
tilelang.PassConfigKey.TL_ENABLE_FAST_MATH: True,
})
def flashattn_bwd_postprocess(batch, heads, seq_len, dim_qk):
dtype = "float16"
accum_dtype = "float"
shape = [batch, seq_len, heads, dim_qk]
blk = 64

@T.prim_func
def flash_bwd_post(
dQ: T.Tensor(shape, accum_dtype), # type: ignore
dQ_out: T.Tensor(shape, dtype), # type: ignore
):
with T.Kernel(T.ceildiv(seq_len, blk), heads, batch, threads=128) as (bx, by, bz):
T.annotate_layout({dQ: make_dq_layout(dQ)})
T.copy(
dQ[bz, bx * blk:(bx + 1) * blk, by, :],
dQ_out[bz, bx * blk:(bx + 1) * blk, by, :],
)

return flash_bwd_post


@tilelang.jit(pass_configs={
tilelang.PassConfigKey.TL_ENABLE_FAST_MATH: True,
})
def flashattn_bwd_atomic_add(batch,
heads,
seq_len,
dim_qk,
dim_v,
is_causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=1):
def flashattn_bwd(batch,
heads,
seq_len,
dim_qk,
dim_v,
is_causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=1):
sm_scale = (1.0 / dim_qk)**0.5
scale = (1.0 / dim_qk)**0.5 * 1.44269504 # log2(e)
head_kv = heads // groups
Expand Down Expand Up @@ -196,10 +165,13 @@ def flash_bwd(
dq = T.alloc_fragment([block_N, dim_qk], accum_dtype)
dk_shared = T.alloc_shared([block_M, dim_qk], accum_dtype)
dv_shared = T.alloc_shared([block_M, dim_v], accum_dtype)
dq_shared = T.alloc_shared([block_N, dim_qk], accum_dtype)

T.annotate_layout({
dQ: make_dq_layout(dQ),
K_shared: tilelang.layout.make_swizzled_layout(K_shared),
dq_shared: tilelang.layout.make_swizzled_layout(dq_shared),
dk_shared: tilelang.layout.make_swizzled_layout(dk_shared),
dv_shared: tilelang.layout.make_swizzled_layout(dv_shared),
})

T.copy(K[bz, by * block_M:(by + 1) * block_M, bx // groups, :], K_shared)
Expand Down Expand Up @@ -244,129 +216,12 @@ def flash_bwd(
T.clear(dq)
T.gemm(dsT_shared, K_shared, dq, transpose_A=True, wg_wait=1)
T.wait_wgmma(0)
for i, j in T.Parallel(block_N, dim_qk):
T.atomic_add(dQ[bz, k * block_N + i, bx, j], dq[i, j])
T.copy(dq, dq_shared)
T.atomic_add(dQ[bz, k * block_N:(k + 1) * block_N, bx, :], dq_shared)
T.copy(dv, dv_shared)
T.atomic_add(dV[bz, by * block_M:(by + 1) * block_M, bx // groups, :], dv_shared)
T.copy(dk, dk_shared)
for i, j in T.Parallel(block_M, dim_qk):
T.atomic_add(dK[bz, by * block_M + i, bx // groups, j], dk_shared[i, j])

return flash_bwd


@tilelang.jit(pass_configs={
tilelang.PassConfigKey.TL_ENABLE_FAST_MATH: True,
})
def flashattn_bwd_split(batch,
heads,
seq_len,
dim_qk,
dim_v,
is_causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=1):
sm_scale = (1.0 / dim_qk)**0.5
scale = (1.0 / dim_qk)**0.5 * 1.44269504 # log2(e)
head_kv = heads // groups
q_shape = [batch, seq_len, heads, dim_qk]
k_shape = [batch, seq_len, head_kv, dim_qk]
v_shape = [batch, seq_len, head_kv, dim_v]
dk_shape = [groups, batch, seq_len, head_kv, dim_qk] # sum after kernel
dv_shape = [groups, batch, seq_len, head_kv, dim_v] # sum after kernel
dtype = "float16"
accum_dtype = "float"

@T.prim_func
def flash_bwd(
Q: T.Tensor(q_shape, dtype), # type: ignore
K: T.Tensor(k_shape, dtype), # type: ignore
V: T.Tensor(v_shape, dtype), # type: ignore
dO: T.Tensor([batch, seq_len, heads, dim_v], dtype), # type: ignore
lse: T.Tensor([batch, heads, seq_len], accum_dtype), # type: ignore
Delta: T.Tensor([batch, heads, seq_len], accum_dtype), # type: ignore
dQ: T.Tensor(q_shape, accum_dtype), # type: ignore
dK: T.Tensor(dk_shape, dtype), # type: ignore
dV: T.Tensor(dv_shape, dtype), # type: ignore
):
with T.Kernel(heads, T.ceildiv(seq_len, block_M), batch, threads=threads) as (bx, by, bz):
K_shared = T.alloc_shared([block_M, dim_qk], dtype)
dsT_shared = T.alloc_shared([block_M, block_N], dtype)
q = T.alloc_shared([block_N, dim_qk], dtype)
V_shared = T.alloc_shared([block_M, dim_v], dtype)
qkT = T.alloc_fragment([block_M, block_N], accum_dtype)
dsT = T.alloc_fragment([block_M, block_N], accum_dtype)
qkT_cast = T.alloc_fragment([block_M, block_N], dtype)
dsT_cast = T.alloc_fragment([block_M, block_N], dtype)
lse_shared = T.alloc_shared([block_N], accum_dtype)
delta = T.alloc_shared([block_N], accum_dtype)
do = T.alloc_shared([block_N, dim_v], dtype)
dv = T.alloc_fragment([block_M, dim_v], accum_dtype)
dk = T.alloc_fragment([block_M, dim_qk], accum_dtype)
dq = T.alloc_fragment([block_N, dim_qk], accum_dtype)
dv_shared = T.alloc_shared([block_M, dim_v], dtype)
dk_shared = T.alloc_shared([block_M, dim_qk], dtype)

T.annotate_layout({
dQ: make_dq_layout(dQ),
K_shared: tilelang.layout.make_swizzled_layout(K_shared),
dv_shared: tilelang.layout.make_swizzled_layout(dv_shared),
dk_shared: tilelang.layout.make_swizzled_layout(dk_shared),
})

T.copy(K[bz, by * block_M:(by + 1) * block_M, bx // groups, :], K_shared)
T.copy(V[bz, by * block_M:(by + 1) * block_M, bx // groups, :], V_shared)
T.clear(dv)
T.clear(dk)
loop_st = T.floordiv(by * block_M, block_N) if is_causal else 0
loop_ed = T.ceildiv(seq_len, block_N)
for k in T.Pipelined(loop_st, loop_ed, num_stages=num_stages):
T.copy(Q[bz, k * block_N:(k + 1) * block_N, bx, :], q)
T.clear(qkT)
T.gemm(
K_shared, q, qkT, transpose_B=True, policy=T.GemmWarpPolicy.FullRow, wg_wait=-1)
T.copy(dO[bz, k * block_N:(k + 1) * block_N, bx, :], do)
T.clear(dsT)
T.gemm(
V_shared,
do,
dsT,
transpose_B=True,
policy=T.GemmWarpPolicy.FullRow,
wg_wait=-1)
T.wait_wgmma(1)

T.copy(lse[bz, bx, k * block_N:(k + 1) * block_N], lse_shared)
for i, j in T.Parallel(block_M, block_N):
qkT[i, j] = T.exp2(qkT[i, j] * scale - lse_shared[j])
if is_causal:
for i, j in T.Parallel(block_M, block_N):
qkT[i, j] = T.if_then_else(by * block_M + i <= k * block_N + j, qkT[i, j],
0)
T.wait_wgmma(0)
T.copy(qkT, qkT_cast)
T.gemm(qkT_cast, do, dv, policy=T.GemmWarpPolicy.FullRow, wg_wait=-1)

T.copy(Delta[bz, bx, k * block_N:(k + 1) * block_N], delta)

for i, j in T.Parallel(block_M, block_N):
dsT_cast[i, j] = qkT[i, j] * (dsT[i, j] - delta[j]) * sm_scale
T.gemm(dsT_cast, q, dk, policy=T.GemmWarpPolicy.FullRow, wg_wait=1)

T.copy(dsT_cast, dsT_shared)
T.clear(dq)
T.gemm(dsT_shared, K_shared, dq, transpose_A=True, wg_wait=1)
T.wait_wgmma(0)
for i, j in T.Parallel(block_N, dim_qk):
T.atomic_add(dQ[bz, k * block_N + i, bx, j], dq[i, j])

T.copy(dv, dv_shared)
T.copy(dv_shared, dV[bx % groups, bz, by * block_M:(by + 1) * block_M, bx // groups, :])
T.copy(dk, dk_shared)
T.copy(dk, dK[bx % groups, bz, by * block_M:(by + 1) * block_M, bx // groups, :])
T.atomic_add(dK[bz, by * block_M:(by + 1) * block_M, bx // groups, :], dk_shared)

return flash_bwd

Expand Down Expand Up @@ -403,54 +258,30 @@ def maybe_contiguous(x):
block_M = 128
block_N = 32
mod_prep = flashattn_bwd_preprocess(BATCH, H, N_CTX, D_HEAD_V)
mod_post = flashattn_bwd_postprocess(BATCH, H, N_CTX, D_HEAD_QK)
delta = mod_prep(o, do)

if ctx.use_atomic:
kernel = flashattn_bwd_atomic_add(
BATCH,
H,
N_CTX,
D_HEAD_QK,
D_HEAD_V,
ctx.causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=groups)
shape_q = [BATCH, N_CTX, H, D_HEAD_QK]
shape_k = [BATCH, N_CTX, HEAD_KV, D_HEAD_QK]
shape_v = [BATCH, N_CTX, HEAD_KV, D_HEAD_V]
dq = torch.zeros(shape_q, dtype=torch.float32, device=q.device)
dk = torch.zeros(shape_k, dtype=torch.float32, device=q.device)
dv = torch.zeros(shape_v, dtype=torch.float32, device=q.device)
kernel(q, k, v, do, lse, delta, dq, dk, dv)
dq = mod_post(dq)
dk = dk.to(torch.float16)
dv = dv.to(torch.float16)
else:
kernel = flashattn_bwd_split(
BATCH,
H,
N_CTX,
D_HEAD_QK,
D_HEAD_V,
ctx.causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=groups)
shape_q = [BATCH, N_CTX, H, D_HEAD_QK]
shape_k = [groups, BATCH, N_CTX, HEAD_KV, D_HEAD_QK] # sum after kernel
shape_v = [groups, BATCH, N_CTX, HEAD_KV, D_HEAD_V] # sum after kernel
dq = torch.zeros(shape_q, dtype=torch.float32, device=q.device)
dk = torch.empty(shape_k, dtype=torch.float16, device=q.device)
dv = torch.empty(shape_v, dtype=torch.float16, device=q.device)
kernel(q, k, v, do, lse, delta, dq, dk, dv)
dq = mod_post(dq)
dk, dv = dk.sum(0), dv.sum(0)
kernel = flashattn_bwd(
BATCH,
H,
N_CTX,
D_HEAD_QK,
D_HEAD_V,
ctx.causal,
block_M,
block_N,
threads=256,
num_stages=2,
groups=groups)
shape_q = [BATCH, N_CTX, H, D_HEAD_QK]
shape_k = [BATCH, N_CTX, HEAD_KV, D_HEAD_QK]
shape_v = [BATCH, N_CTX, HEAD_KV, D_HEAD_V]
dq = torch.zeros(shape_q, dtype=torch.float32, device=q.device)
dk = torch.zeros(shape_k, dtype=torch.float32, device=q.device)
dv = torch.zeros(shape_v, dtype=torch.float32, device=q.device)
kernel(q, k, v, do, lse, delta, dq, dk, dv)
dq = dq.to(torch.float16)
dk = dk.to(torch.float16)
dv = dv.to(torch.float16)

return dq, dk, dv, None, None, None

Expand Down Expand Up @@ -489,8 +320,7 @@ def main(BATCH: int = 1,
D_HEAD_QK: int = 192,
D_HEAD_V: int = 128,
groups: int = 16,
causal: bool = False,
use_atomic: bool = True):
causal: bool = False):
flops_per_qk = 2.0 * BATCH * H * N_CTX * N_CTX * D_HEAD_QK
flops_per_v = 2.0 * BATCH * H * N_CTX * N_CTX * D_HEAD_V
total_flops = 3 * flops_per_qk + 2 * flops_per_v
Expand All @@ -510,7 +340,7 @@ def main(BATCH: int = 1,
dO = (
torch.empty(BATCH, N_CTX, H, D_HEAD_V, dtype=torch.half,
device="cuda").normal_().requires_grad_())
O = attention(Q, K, V, causal, groups, use_atomic)
O = attention(Q, K, V, causal, groups)
O.backward(dO, retain_graph=True)
dQ, Q.grad = Q.grad.clone(), None
dK, K.grad = K.grad.clone(), None
Expand Down Expand Up @@ -553,20 +383,6 @@ def run1():
parser.add_argument('--d_head_v', type=int, default=128, help='Head dimension for V')
parser.add_argument('--causal', action='store_true', help='Causal flag')
parser.add_argument('--groups', type=int, default=16, help='groups')
parser.add_argument(
'--use_atomic', action='store_true', default=False, help='Use atomic add for dK/dV')
parser.add_argument(
'--use_split', action='store_true', default=False, help='Use split for dK/dV')
args = parser.parse_args()

# Handle backward compatibility and logic
if args.use_split:
use_atomic = False
elif args.use_atomic:
use_atomic = True
else:
# Default: use atomic
use_atomic = True

main(args.batch, args.h, args.n_ctx, args.d_head_qk, args.d_head_v, args.groups, args.causal,
use_atomic)
main(args.batch, args.h, args.n_ctx, args.d_head_qk, args.d_head_v, args.groups, args.causal)
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