vllm/csrc/quantization/cutlass_w4a8/w4a8_mm_entry.cu
czhu-cohere 3c068c637b
[Kernel] Faster pre-processing time for W4A8 (#23972)
Signed-off-by: czhu-cohere <conway.zhu@cohere.com>
2025-09-17 14:35:32 -07:00

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//
// Based off of:
// https://github.com/NVIDIA/cutlass/blob/main/examples/55_hopper_mixed_dtype_gemm/55_hopper_int4_fp8_gemm.cu
//
#include <ATen/cuda/CUDAContext.h>
#include <c10/cuda/CUDAGuard.h>
#include <torch/all.h>
#include "cutlass_extensions/torch_utils.hpp"
#include "core/registration.h"
#include "cutlass/cutlass.h"
#include <limits>
#include "cute/tensor.hpp"
#include "cutlass/gemm/collective/collective_builder.hpp"
#include "cutlass/epilogue/collective/collective_builder.hpp"
#include "cutlass/gemm/device/gemm_universal_adapter.h"
#include "cutlass/gemm/kernel/gemm_universal.hpp"
#include "cutlass/util/packed_stride.hpp"
#include "cutlass/util/mixed_dtype_utils.hpp"
#include "cutlass_extensions/common.hpp"
#include "cutlass_extensions/epilogue/scaled_mm_epilogues_c3x.hpp"
#include <cuda_runtime.h>
namespace vllm::cutlass_w4a8 {
using namespace cute;
// -------------------------------------------------------------------------------------
// Static configuration shared across all instantiations
// -------------------------------------------------------------------------------------
using MmaType = cutlass::float_e4m3_t; // A/scale element type
using QuantType = cutlass::int4b_t; // B element type (packed int4)
static int constexpr TileShapeK = 128 * 8 / sizeof_bits<MmaType>::value;
static int constexpr ScalePackSize = 8; // pack 8 scale elements together
static int constexpr PackFactor = 8; // 8 4-bit packed into int32
// A matrix configuration
using ElementA = MmaType; // Element type for A matrix operand
using LayoutA = cutlass::layout::RowMajor; // Layout type for A matrix operand
using LayoutA_Transpose =
typename cutlass::layout::LayoutTranspose<LayoutA>::type;
constexpr int AlignmentA =
128 / cutlass::sizeof_bits<
ElementA>::value; // Memory access granularity/alignment of A
// matrix in units of elements (up to 16 bytes)
using StrideA = cutlass::detail::TagToStrideA_t<LayoutA>;
// B matrix configuration
using ElementB = QuantType; // Element type for B matrix operand
using LayoutB =
cutlass::layout::ColumnMajor; // Layout type for B matrix operand
using LayoutB_Transpose =
typename cutlass::layout::LayoutTranspose<LayoutB>::type;
constexpr int AlignmentB =
128 / cutlass::sizeof_bits<
ElementB>::value; // Memory access granularity/alignment of B
// matrix in units of elements (up to 16 bytes)
using StrideB = cutlass::detail::TagToStrideB_t<LayoutB>;
// Define the CuTe layout for reordered quantized tensor B
// LayoutAtomQuant places values that will be read by the same thread in
// contiguous locations in global memory. It specifies the reordering within a
// single warp's fragment
using LayoutAtomQuant =
decltype(cutlass::compute_memory_reordering_atom<MmaType>());
using LayoutB_Reordered = decltype(cute::tile_to_shape(
LayoutAtomQuant{}, Layout<Shape<int, int, int>, StrideB>{}));
// Group-wise scales
using ElementScale = MmaType;
using LayoutScale = cutlass::layout::RowMajor;
// Per-tok, per-chan scales
using ElementSChannel = float;
// C/D matrix configuration
using ElementC =
cutlass::bfloat16_t; // Element type for C and D matrix operands
using LayoutC =
cutlass::layout::RowMajor; // Layout type for C and D matrix operands
constexpr int AlignmentC =
128 / cutlass::sizeof_bits<
ElementC>::value; // Memory access granularity/alignment of C
// matrix in units of elements (up to 16 bytes)
using ElementD = ElementC;
using LayoutD = LayoutC;
constexpr int AlignmentD = 128 / cutlass::sizeof_bits<ElementD>::value;
// Core kernel configurations
using ElementAccumulator = float; // Element type for internal accumulation
using ElementCompute = float; // Element type for epilogue computation
using ArchTag = cutlass::arch::Sm90; // Tag indicating the minimum SM that
// supports the intended feature
using OperatorClass = cutlass::arch::OpClassTensorOp; // Operator class tag
using KernelSchedule =
cutlass::gemm::KernelTmaWarpSpecializedCooperative; // Kernel to launch
// based on the default
// setting in the
// Collective Builder
using EpilogueSchedule = cutlass::epilogue::TmaWarpSpecializedCooperative;
using EpilogueTileType = cutlass::epilogue::collective::EpilogueTileAuto;
// ----------------------------------------------------------------------------
// Kernel template — Tile/Cluster shapes
// ----------------------------------------------------------------------------
template <class TileShape_MN, class ClusterShape_MNK>
struct W4A8GemmKernel {
using TileShape =
decltype(cute::append(TileShape_MN{}, cute::Int<TileShapeK>{}));
using ClusterShape = ClusterShape_MNK;
// Epilogue per-tok, per-chan scales
using ChTokScalesEpilogue =
typename vllm::c3x::ScaledEpilogue<ElementAccumulator, ElementD,
TileShape>;
using EVTCompute = typename ChTokScalesEpilogue::EVTCompute;
using CollectiveEpilogue =
typename cutlass::epilogue::collective::CollectiveBuilder<
ArchTag, OperatorClass, TileShape, ClusterShape, EpilogueTileType,
ElementAccumulator, ElementSChannel,
// Transpose layout of D here since we use explicit swap + transpose
// the void type for C tells the builder to allocate 0 smem for the C
// matrix. We can enable this if beta == 0 by changing ElementC to
// void below.
ElementC, typename cutlass::layout::LayoutTranspose<LayoutC>::type,
AlignmentC, ElementD,
typename cutlass::layout::LayoutTranspose<LayoutD>::type, AlignmentD,
EpilogueSchedule, // This is the only epi supporting the required
// swap + transpose.
EVTCompute>::CollectiveOp;
// The Scale information must get paired with the operand that will be scaled.
// In this example, B is scaled so we make a tuple of B's information and the
// scale information.
using CollectiveMainloopShuffled =
typename cutlass::gemm::collective::CollectiveBuilder<
ArchTag, OperatorClass,
cute::tuple<ElementB, cutlass::Array<ElementScale, ScalePackSize>>,
LayoutB_Reordered, AlignmentB, ElementA, LayoutA_Transpose,
AlignmentA, ElementAccumulator, TileShape, ClusterShape,
cutlass::gemm::collective::StageCountAutoCarveout<static_cast<int>(
sizeof(typename CollectiveEpilogue::SharedStorage))>,
KernelSchedule>::CollectiveOp;
using GemmKernelShuffled = cutlass::gemm::kernel::GemmUniversal<
Shape<int, int, int, int>, // Indicates ProblemShape
CollectiveMainloopShuffled, CollectiveEpilogue>;
using GemmShuffled =
cutlass::gemm::device::GemmUniversalAdapter<GemmKernelShuffled>;
using StrideC = typename GemmKernelShuffled::StrideC;
using StrideD = typename GemmKernelShuffled::StrideD;
using StrideS = typename CollectiveMainloopShuffled::StrideScale;
static torch::Tensor mm(torch::Tensor const& A,
torch::Tensor const& B, // already packed
torch::Tensor const& group_scales, // already packed
int64_t group_size,
torch::Tensor const& channel_scales,
torch::Tensor const& token_scales,
std::optional<at::ScalarType> const& maybe_out_type) {
// TODO: param validation
int m = A.size(0);
int k = A.size(1);
int n = B.size(1);
// safely cast group_size to int
TORCH_CHECK(group_size > 0 && group_size <= std::numeric_limits<int>::max(),
"group_size out of supported range for int: ", group_size);
int const group_size_int = static_cast<int>(group_size);
// Allocate output
const at::cuda::OptionalCUDAGuard device_guard(device_of(A));
auto device = A.device();
auto stream = at::cuda::getCurrentCUDAStream(device.index());
torch::Tensor D =
torch::empty({m, n}, torch::TensorOptions()
.dtype(equivalent_scalar_type_v<ElementD>)
.device(device));
// prepare arg pointers
auto A_ptr = static_cast<MmaType const*>(A.const_data_ptr());
auto B_ptr = static_cast<QuantType const*>(B.const_data_ptr());
auto D_ptr = static_cast<ElementD*>(D.data_ptr());
// can we avoid hardcode the 8 here
auto S_ptr =
static_cast<cutlass::Array<ElementScale, ScalePackSize> const*>(
group_scales.const_data_ptr());
// runtime layout for B
auto shape_B = cute::make_shape(n, k, 1);
LayoutB_Reordered layout_B_reordered =
cute::tile_to_shape(LayoutAtomQuant{}, shape_B);
// strides
int const scale_k = cutlass::ceil_div(k, group_size_int);
StrideA stride_A =
cutlass::make_cute_packed_stride(StrideA{}, cute::make_shape(m, k, 1));
// Reverse stride here due to swap and transpose
StrideD stride_D =
cutlass::make_cute_packed_stride(StrideD{}, cute::make_shape(n, m, 1));
StrideS stride_S = cutlass::make_cute_packed_stride(
StrideS{}, cute::make_shape(n, scale_k, 1));
// Create a structure of gemm kernel arguments suitable for invoking an
// instance of Gemm auto arguments =
// args_from_options<GemmShuffled>(options);
/// Populates a Gemm::Arguments structure from the given arguments
/// Swap the A and B tensors, as well as problem shapes here.
using Args = typename GemmShuffled::Arguments;
using MainloopArguments = typename GemmKernelShuffled::MainloopArguments;
using EpilogueArguments = typename GemmKernelShuffled::EpilogueArguments;
MainloopArguments mainloop_arguments{
B_ptr, layout_B_reordered, A_ptr, stride_A,
S_ptr, stride_S, group_size_int};
EpilogueArguments epilogue_arguments{
ChTokScalesEpilogue::prepare_args(channel_scales, token_scales),
nullptr,
{}, // no C
D_ptr,
stride_D};
Args arguments{cutlass::gemm::GemmUniversalMode::kGemm,
{n, m, k, 1}, // shape
mainloop_arguments,
epilogue_arguments};
// Workspace
size_t workspace_size = GemmShuffled::get_workspace_size(arguments);
torch::Tensor workspace =
torch::empty(workspace_size,
torch::TensorOptions().dtype(torch::kU8).device(device));
// Run GEMM
GemmShuffled gemm;
CUTLASS_CHECK(gemm.can_implement(arguments));
CUTLASS_CHECK(gemm.initialize(arguments, workspace.data_ptr(), stream));
CUTLASS_CHECK(gemm.run(stream));
return D;
}
};
// ----------------------------------------------------------------------------
// Kernel instantiations and dispatch logic
// ----------------------------------------------------------------------------
using Kernel_256x128_1x1x1 =
W4A8GemmKernel<Shape<_256, _128>, Shape<_1, _1, _1>>;
using Kernel_256x64_1x1x1 = W4A8GemmKernel<Shape<_256, _64>, Shape<_1, _1, _1>>;
using Kernel_256x32_1x1x1 = W4A8GemmKernel<Shape<_256, _32>, Shape<_1, _1, _1>>;
using Kernel_256x16_1x1x1 = W4A8GemmKernel<Shape<_256, _16>, Shape<_1, _1, _1>>;
using Kernel_128x256_2x1x1 =
W4A8GemmKernel<Shape<_128, _256>, Shape<_2, _1, _1>>;
using Kernel_128x256_1x1x1 =
W4A8GemmKernel<Shape<_128, _256>, Shape<_1, _1, _1>>;
using Kernel_128x128_1x1x1 =
W4A8GemmKernel<Shape<_128, _128>, Shape<_1, _1, _1>>;
using Kernel_128x64_1x1x1 = W4A8GemmKernel<Shape<_128, _64>, Shape<_1, _1, _1>>;
using Kernel_128x32_1x1x1 = W4A8GemmKernel<Shape<_128, _32>, Shape<_1, _1, _1>>;
using Kernel_128x16_1x1x1 = W4A8GemmKernel<Shape<_128, _16>, Shape<_1, _1, _1>>;
torch::Tensor mm_dispatch(torch::Tensor const& A,
torch::Tensor const& B, // already packed
torch::Tensor const& group_scales, // already packed
int64_t group_size,
torch::Tensor const& channel_scales,
torch::Tensor const& token_scales,
std::optional<at::ScalarType> const& maybe_out_type,
const std::string& schedule) {
if (schedule == "256x128_1x1x1") {
return Kernel_256x128_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "256x64_1x1x1") {
return Kernel_256x64_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "256x32_1x1x1") {
return Kernel_256x32_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "256x16_1x1x1") {
return Kernel_256x16_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x256_2x1x1") {
return Kernel_128x256_2x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x256_1x1x1") {
return Kernel_128x256_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x128_1x1x1") {
return Kernel_128x128_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x64_1x1x1") {
return Kernel_128x64_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x32_1x1x1") {
return Kernel_128x32_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
} else if (schedule == "128x16_1x1x1") {
return Kernel_128x16_1x1x1::mm(A, B, group_scales, group_size,
channel_scales, token_scales,
maybe_out_type);
}
TORCH_CHECK(false, "Unknown W4A8 schedule: ", schedule);
return {};
}
torch::Tensor mm(torch::Tensor const& A,
torch::Tensor const& B, // already packed
torch::Tensor const& group_scales, // already packed
int64_t group_size, torch::Tensor const& channel_scales,
torch::Tensor const& token_scales,
std::optional<at::ScalarType> const& maybe_out_type,
std::optional<std::string> maybe_schedule) {
// requested a specific schedule
if (maybe_schedule) {
return mm_dispatch(A, B, group_scales, group_size, channel_scales,
token_scales, maybe_out_type, *maybe_schedule);
}
std::string schedule;
int M = A.size(0);
int K = A.size(1);
int N = B.size(1);
// heuristic
if (M <= 16) {
schedule = (K == 16384 && N == 18432) ? "256x16_1x1x1" : "128x16_1x1x1";
} else if (M <= 32) {
schedule = (K == 16384 && N == 18432) ? "256x32_1x1x1" : "128x32_1x1x1";
} else if (M <= 64) {
if (K == 16384 && N == 18432)
schedule = "256x64_1x1x1";
else if (N <= 8192 && K <= 8192)
schedule = "128x32_1x1x1";
else
schedule = "128x64_1x1x1";
} else if (M <= 128) {
if (K == 16384 && N == 18432)
schedule = "256x128_1x1x1";
else if (N <= 8192)
schedule = "128x64_1x1x1";
else
schedule = "128x128_1x1x1";
} else if (M <= 256) {
if (N <= 4096)
schedule = "128x64_1x1x1";
else if (N <= 8192)
schedule = "128x128_1x1x1";
else
schedule = "128x256_1x1x1";
} else if (M <= 512 && N <= 4096) {
schedule = "128x128_1x1x1";
} else if (M <= 1024) {
schedule = "128x256_1x1x1";
} else {
schedule = "128x256_2x1x1";
}
return mm_dispatch(A, B, group_scales, group_size, channel_scales,
token_scales, maybe_out_type, schedule);
}
// ----------------------------------------------------------------------------
// Pre-processing utils
// ----------------------------------------------------------------------------
torch::Tensor pack_scale_fp8(torch::Tensor const& scales) {
TORCH_CHECK(scales.dtype() == torch::kFloat8_e4m3fn);
TORCH_CHECK(scales.is_contiguous());
TORCH_CHECK(scales.is_cuda());
auto packed_scales = torch::empty(
{scales.numel() * ScalePackSize},
torch::TensorOptions().dtype(scales.dtype()).device(scales.device()));
auto scales_ptr = static_cast<MmaType const*>(scales.const_data_ptr());
auto packed_scales_ptr =
static_cast<cutlass::Array<ElementScale, ScalePackSize>*>(
packed_scales.data_ptr());
cutlass::pack_scale_fp8(scales_ptr, packed_scales_ptr, scales.numel());
return packed_scales;
}
/*
GPU-accelerated implementation of cutlass::unified_encode_int4b.
Constructs a lookup table in constant memory to map 8 bits
(two 4-bit values) at a time. Assumes memory is contiguous
and pointers are 16-byte aligned.
*/
__constant__ uint8_t kNibbleLUT[256];
__global__ void unified_encode_int4b_device(const uint8_t* in, uint8_t* out,
size_t nbytes) {
constexpr size_t V = sizeof(uint4); // 16 bytes
const size_t tid = blockIdx.x * blockDim.x + threadIdx.x;
const size_t nthreads = size_t(gridDim.x) * blockDim.x;
const size_t nvec = nbytes / V;
// 1-D grid-stride loop over 16-byte chunks
for (size_t vec = tid; vec < nvec; vec += nthreads) {
uint4 v = reinterpret_cast<const uint4*>(in)[vec];
uint8_t* b = reinterpret_cast<uint8_t*>(&v);
#pragma unroll
for (int i = 0; i < int(V); ++i) b[i] = kNibbleLUT[b[i]];
reinterpret_cast<uint4*>(out)[vec] = v;
}
}
static bool upload_lut() {
std::array<uint8_t, 256> lut{};
auto map_nib = [](uint8_t v) -> uint8_t {
// 1..7 -> (8 - v); keep 0 and 8..15
return (v == 0 || (v & 0x8)) ? v : uint8_t(8 - v);
};
for (int b = 0; b < 256; ++b) {
uint8_t lo = b & 0xF;
uint8_t hi = (b >> 4) & 0xF;
lut[b] = uint8_t((map_nib(hi) << 4) | map_nib(lo));
}
cudaError_t e = cudaMemcpyToSymbol(kNibbleLUT, lut.data(), lut.size(),
/*offset=*/0, cudaMemcpyHostToDevice);
return (e == cudaSuccess);
}
static bool unified_encode_int4b(cutlass::int4b_t const* in,
cutlass::int4b_t* out, size_t num_int4_elems) {
// Build/upload LUT
if (!upload_lut()) return false;
static_assert(sizeof(typename cutlass::int4b_t::Storage) == 1,
"int4 storage must be 1 byte");
const size_t nbytes = num_int4_elems >> 1;
auto* in_bytes = reinterpret_cast<uint8_t const*>(in);
auto* out_bytes = reinterpret_cast<uint8_t*>(out);
// kernel launch params
constexpr int block = 256;
const size_t nvec = nbytes / sizeof(uint4); // # of 16B vectors
int grid = int((nvec + block - 1) / block);
if (grid == 0) grid = 1; // ensure we still cover the tail in the kernel
unified_encode_int4b_device<<<grid, block>>>(in_bytes, out_bytes, nbytes);
cudaError_t err = cudaGetLastError();
return (err == cudaSuccess);
}
torch::Tensor encode_and_reorder_int4b(torch::Tensor const& B) {
TORCH_CHECK(B.dtype() == torch::kInt32);
TORCH_CHECK(B.dim() == 2);
torch::Tensor B_packed = torch::empty_like(B);
int k = B.size(0) * PackFactor; // logical k
int n = B.size(1);
TORCH_CHECK((n * k) % 32 == 0, "need multiples of 32 int4s for 16B chunks");
auto B_ptr = static_cast<QuantType const*>(B.const_data_ptr());
auto B_packed_ptr = static_cast<QuantType*>(B_packed.data_ptr());
auto shape_B = cute::make_shape(n, k, 1);
auto layout_B = make_layout(shape_B, LayoutRight{}); // row major
LayoutB_Reordered layout_B_reordered =
cute::tile_to_shape(LayoutAtomQuant{}, shape_B);
bool ok =
vllm::cutlass_w4a8::unified_encode_int4b(B_ptr, B_packed_ptr, n * k);
TORCH_CHECK(ok, "unified_encode_int4b failed");
cutlass::reorder_tensor(B_packed_ptr, layout_B, layout_B_reordered);
return B_packed;
}
TORCH_LIBRARY_IMPL_EXPAND(TORCH_EXTENSION_NAME, CUDA, m) {
m.impl("cutlass_w4a8_mm", &mm);
m.impl("cutlass_pack_scale_fp8", &pack_scale_fp8);
m.impl("cutlass_encode_and_reorder_int4b", &encode_and_reorder_int4b);
}
} // namespace vllm::cutlass_w4a8