scaled_masked_softmax.cpp

/* coding=utf-8
 * Copyright (c) 2024, NVIDIA CORPORATION.  All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <cuda_fp16.h>
#include <torch/extension.h>
#include <vector>

namespace multihead_attn {
namespace fused_softmax {
namespace scaled_masked_softmax {

torch::Tensor fwd_cuda(torch::Tensor const& input, torch::Tensor const& mask, float scale_factor);

torch::Tensor bwd_cuda(torch::Tensor const& output_grads,
                       torch::Tensor const& softmax_results,
                       float scale_factor);

int get_batch_per_block_cuda(int query_seq_len, int key_seq_len, int batches, int attn_heads);

torch::Tensor fwd(torch::Tensor const& input, torch::Tensor const& mask, float scale_factor)
{
    AT_ASSERTM(input.dim() == 4, "expected 4D tensor");
    AT_ASSERTM((input.scalar_type() == at::ScalarType::Half) ||
                   (input.scalar_type() == at::ScalarType::BFloat16),
               "Only fp16 and bf16 are supported");
    AT_ASSERTM(mask.dim() == 4, "expected 4D tensor");

    return fwd_cuda(input, mask, scale_factor);
}

torch::Tensor bwd(torch::Tensor const& output_grads,
                  torch::Tensor const& softmax_results,
                  float scale_factor)
{
    AT_ASSERTM(output_grads.dim() == 4, "expected 3D tensor");
    AT_ASSERTM(softmax_results.dim() == 4, "expected 3D tensor");

    AT_ASSERTM((output_grads.scalar_type() == at::ScalarType::Half) ||
                   (output_grads.scalar_type() == at::ScalarType::BFloat16),
               "Only fp16 and bf16 are supported");
    AT_ASSERTM((softmax_results.scalar_type() == at::ScalarType::Half) ||
                   (softmax_results.scalar_type() == at::ScalarType::BFloat16),
               "Only fp16 and bf16 are supported");

    return bwd_cuda(output_grads, softmax_results, scale_factor);
}

int get_batch_per_block(int query_seq_len, int key_seq_len, int batches, int attn_heads)
{
    return get_batch_per_block_cuda(query_seq_len, key_seq_len, batches, attn_heads);
}

}  // end namespace scaled_masked_softmax
}  // end namespace fused_softmax
}  // end namespace multihead_attn

PYBIND11_MODULE(TORCH_EXTENSION_NAME, m)
{
    m.def("forward",
          &multihead_attn::fused_softmax::scaled_masked_softmax::fwd,
          "Self Multihead Attention scaled, time masked softmax -- Forward.");

    m.def("backward",
          &multihead_attn::fused_softmax::scaled_masked_softmax::bwd,
          "Self Multihead Attention scaled, time masked softmax -- Backward.");

    m.def("get_batch_per_block",
          &multihead_attn::fused_softmax::scaled_masked_softmax::get_batch_per_block,
          "Return Batch per block size.");
}