VTimeLLM/flash-attention/hopper/flash_prepare_scheduler.cu

/******************************************************************************
 * Copyright (c) 2024, Jay Shah, Ganesh Bikshandi, Ying Zhang, Vijay Thakkar, Pradeep Ramani, Tri Dao.
 ******************************************************************************/

#include <cub/cub.cuh>
#include "cutlass/fast_math.h"
#include "cutlass/barrier.h"
#include "cutlass/arch/barrier.h"

#include "cutlass/arch/grid_dependency_control.h"

#include "flash.h"

#include "static_switch.h"

namespace flash {

// Sort in descending order
template <typename T>
struct PrepareSortOp
{
    __device__ __forceinline__ bool operator()(T const & lhs, T const & rhs)
    {
        return lhs > rhs;
    }
};

template <>
struct PrepareSortOp<int2> {
    __device__ __forceinline__ bool operator()(int2 const & lhs, int2 const & rhs) const {
        return lhs.x > rhs.x;
    }
};

template <>
struct PrepareSortOp<int4> {
    __device__ __forceinline__ bool operator()(int4 const & lhs, int4 const & rhs) const {
        return lhs.x > rhs.x;
    }
};

template <int NumWarps, bool Sort>
__global__ void prepare_varlen_num_blocks_kernel(
        int seqlen_q_static, int seqlen_k_static, int seqlen_k_new_static,
        int const* const cu_seqlens_q, int const* const cu_seqlens_k, int const* const cu_seqlens_k_new,
        int const* const seqused_q, int const* const seqused_k, int const* const leftpad_k_ptr,
        int num_batch, int num_head, int qhead_per_khead, int num_sm, int num_splits_static,
        cutlass::FastDivmod blockm_divmod, cutlass::FastDivmod blockn_divmod,
        int* const tile_count_semaphore,
        int* const num_m_blocks_ptr,
        int* const num_splits_dynamic_ptr,
        int* const varlen_batch_idx_ptr,
        // int* const num_n_blocks_ptr,
        int* const num_nheads_in_l2_ptr,
        bool enable_pdl,
        bool is_causal,
        bool packgqa,
        int max_kvblocks_in_l2) {

    static constexpr int kNumBatchPerWarp = cutlass::NumThreadsPerWarp - 1;
    static constexpr int kSmemSize = 1;
    static constexpr int BLOCK_DIM_X = NumWarps * 32;
    static constexpr int ITEMS_PER_THREAD = 1;
    static_assert(BLOCK_DIM_X * ITEMS_PER_THREAD == NumWarps * 32);
    using BlockMergeSort = cub::BlockMergeSort<int4, BLOCK_DIM_X, ITEMS_PER_THREAD>;

    __shared__ int total_blocks_smem[kSmemSize];

    // Allocate shared memory for BlockMergeSort operations
    __shared__ typename BlockMergeSort::TempStorage temp_storage;

    if (enable_pdl) { cutlass::arch::launch_dependent_grids(); }

    if (threadIdx.x < kSmemSize) { total_blocks_smem[threadIdx.x] = 0; }
    __syncthreads();

    if (threadIdx.x == 0 && tile_count_semaphore) { *tile_count_semaphore = 0; }

    int lane = threadIdx.x % cutlass::NumThreadsPerWarp;

    auto get_num_m_blocks = [&](int batch_idx) {
        int seqlen;
        if (seqused_q) {
            seqlen = batch_idx < num_batch ? seqused_q[batch_idx] : 0;
        } else if (cu_seqlens_q) {
            int cur_cu_seqlen = batch_idx <= num_batch ? cu_seqlens_q[batch_idx] : 0;
            int next_cu_seqlen = __shfl_down_sync(0xffffffff, cur_cu_seqlen, 1);
            seqlen = next_cu_seqlen - cur_cu_seqlen;
        } else {
            seqlen = seqlen_q_static;
        }
        if(packgqa) { seqlen *= qhead_per_khead; }
        return batch_idx < num_batch && lane < kNumBatchPerWarp
            ? blockm_divmod.div(seqlen + blockm_divmod.divisor - 1) : 0;
    };

    auto get_num_n_blocks = [&](int batch_idx) {
        int leftpad_k = batch_idx < num_batch && leftpad_k_ptr != nullptr ? leftpad_k_ptr[batch_idx] : 0;
        int seqlen;
        if (seqused_k) {
            seqlen = batch_idx < num_batch ? seqused_k[batch_idx] : 0;
        } else if (cu_seqlens_k) {
            int cur_cu_seqlen = batch_idx <= num_batch ? cu_seqlens_k[batch_idx] : 0;
            int next_cu_seqlen = __shfl_down_sync(0xffffffff, cur_cu_seqlen, 1);
            seqlen = next_cu_seqlen - cur_cu_seqlen;
        } else {
            seqlen = seqlen_k_static;
        }
        int seqlen_new;
        if (cu_seqlens_k_new) {
            int cur_cu_seqlen_new = batch_idx <= num_batch ? cu_seqlens_k_new[batch_idx] : 0;
            int next_cu_seqlen_new = __shfl_down_sync(0xffffffff, cur_cu_seqlen_new, 1);
            seqlen_new = next_cu_seqlen_new - cur_cu_seqlen_new;
        } else {
            seqlen_new = seqlen_k_new_static;
        }
        // if (threadIdx.x == 0) { printf("seqlen = %d, seqlen_new = %d, leftpad_k = %d\n", seqlen, seqlen_new, leftpad_k); }
        seqlen = seqlen - leftpad_k + seqlen_new;
        return batch_idx < num_batch && lane < kNumBatchPerWarp
            ? blockn_divmod.div(seqlen + blockn_divmod.divisor - 1) : 0;
    };

    int warp_idx = threadIdx.x / cutlass::NumThreadsPerWarp;
    int batch_cta_idx_offset = int(blockIdx.x) * 992;
    int bidb_start = batch_cta_idx_offset + kNumBatchPerWarp * warp_idx;
    int batch_idx = lane + bidb_start;
    int num_m_blocks = get_num_m_blocks(batch_idx);
    int num_n_blocks = get_num_n_blocks(batch_idx);

    auto get_nheads_in_l2 = [&](int n_blocks) {
        int nheads_in_l2 = n_blocks * 16 <= max_kvblocks_in_l2 ? 16
            : n_blocks * 8 <= max_kvblocks_in_l2 ? 8
            : n_blocks * 4 <= max_kvblocks_in_l2 ? 4
            : n_blocks * 2 <= max_kvblocks_in_l2 ? 2
            : 1;
        if(!packgqa) { nheads_in_l2 *= qhead_per_khead; }
        return min(nheads_in_l2, num_head);
    };
    
    int num_splits_dynamic;
    if (int(gridDim.x) > 1 || num_splits_static == 1) {
        // set num splits for all batches to 1 (note that user expects num_splits_static to mean upper bound on splits)
        // for batch size > 992, we expect GPU occupancy to not be an issue except in degenerate cases (e.g., most are zero-length)
        num_splits_dynamic = 1;
    } else {
        int total_blocks = num_m_blocks * num_n_blocks;
        // Warp sum
        #pragma unroll
        for (int i = cutlass::NumThreadsPerWarp / 2; i >= 1; i /= 2) {
            total_blocks += __shfl_down_sync(0xffffffff, total_blocks, i);
        }
        if (lane == 0) { atomicAdd(total_blocks_smem, total_blocks); }
        __syncthreads();
        total_blocks = total_blocks_smem[0];
        // 10% margin
        int blocks_per_sm = static_cast<int>(ceilf(float(total_blocks) * 1.1f * float(num_head) / float(num_sm)));
        // blocks_per_sm = std::max(1, blocks_per_sm);  // 1 is the minimum number of blocks per SM
        num_splits_dynamic = std::max(std::min((num_n_blocks + blocks_per_sm - 1) / blocks_per_sm, num_splits_static), 1);
        // num_n_blocks per work tile for the batch
        num_n_blocks = cutlass::ceil_div(num_n_blocks, num_splits_dynamic); 
    }

    if constexpr (Sort) {
        if(lane == kNumBatchPerWarp || batch_idx >= num_batch) {
            num_n_blocks = INT_MIN; // sort last
        } else if (is_causal) {
            // sort by shortest member to process
            num_n_blocks = num_n_blocks * blockn_divmod.divisor - num_m_blocks * blockm_divmod.divisor;
        }
        int4 batch_coords[ITEMS_PER_THREAD]; // 1 item per thread
        batch_coords[0] = make_int4(num_n_blocks, num_m_blocks, num_splits_dynamic, batch_idx);

        // if (threadIdx.x == 0) {
        //     printf("Unsorted: num_n_blocks - num_m_blocks = %d, num_m_blocks = %d, num_splits = %d, batch_idx = %d.\n", 
        //         batch_coords[0].x, batch_coords[0].y, batch_coords[0].z, batch_coords[0].w);
        // } __syncthreads();

        // Sort batches by num_n_blocks in descending order
        BlockMergeSort(temp_storage).Sort(batch_coords, PrepareSortOp<int4>());

        // if (threadIdx.x == 0) {
        //     printf("Sorted: num_n_blocks - num_m_blocks = %d, num_m_blocks = %d, num_splits = %d, batch_idx = %d.\n", 
        //         batch_coords[0].x, batch_coords[0].y, batch_coords[0].z, batch_coords[0].w);
        // } __syncthreads();

        if (is_causal) {
            // reset value to num_n_blocks
            batch_coords[0].x = blockn_divmod.div(batch_coords[0].x + batch_coords[0].y * blockm_divmod.divisor);
        }

        // When sorting, we re-index some metadata by 'virtual batch index'
        // and also store the vbidx -> bidx mapping.
        // 1. num_nheads_in_l2_ptr: virtual_batch_idx -> num_nheads_in_l2[batch_idx]
        // 2. num_splits_dynamic_ptr: virtual_batch_idx -> num_splits[batch_idx]
        // 3. num_m_blocks_ptr: virtual_batch_idx -> num_m_blocks[batch_idx]
        // 4. varlen_batch_idx_ptr: virtual_batch_idx -> batch_idx      
        batch_idx = batch_cta_idx_offset + threadIdx.x;
        if (batch_idx < num_batch && threadIdx.x < 992) {
            // num_n_blocks_ptr[threadIdx.x] = max(batch_coords[0].x, 1);
            if(num_nheads_in_l2_ptr) { num_nheads_in_l2_ptr[batch_idx] = get_nheads_in_l2(max(batch_coords[0].x, 1)); }
            num_m_blocks_ptr[batch_idx] = batch_coords[0].y;
            num_splits_dynamic_ptr[batch_idx] = batch_coords[0].z;
            varlen_batch_idx_ptr[batch_idx] = batch_coords[0].w;
        }  
    } else {
        if (batch_idx < num_batch && lane < kNumBatchPerWarp) {
            // num_n_blocks_ptr[batch_idx] = max(num_n_blocks, 1);
            if(num_nheads_in_l2_ptr) { num_nheads_in_l2_ptr[batch_idx] = get_nheads_in_l2(max(num_n_blocks, 1)); }
            num_splits_dynamic_ptr[batch_idx] = num_splits_dynamic;
            num_m_blocks_ptr[batch_idx] = num_m_blocks;
            // printf("idx = %d, num_m_blocks = %d, num_n_blocks = %d, num_split_static = %d, num_splits_dynamic = %d\n", bidb_start + lane, num_m_blocks_ptr[bidb_start + lane], num_n_blocks, num_splits_static, num_splits_dynamic);
        }
    }
    
}

} // flash

void prepare_varlen_num_blocks(Flash_fwd_params &params, cudaStream_t stream, bool packgqa,
                               int blockM, int blockN, bool enable_pdl) {
    int qhead_per_khead = cutlass::ceil_div(params.h, params.h_k);
    int num_warps = cutlass::ceil_div(params.b, 31); // warp switch will cap this at 32
    int num_ctas = cutlass::ceil_div(params.b, 31 * 32);
    // int const size_l2 = 50 * 1024 * 1024; // 50 MB
    int const size_l2 = 8 * 1024 * 1024; // underestimate seems better in practice
    int const element_size = params.is_e4m3 ? 1 : 2;
    int const size_one_kvblock = blockN * (params.d + params.dv) * element_size;
    // printf("block size = %d, element size = %d, headdim = %d, headdim_v = %d, size 1 kblock = %d.\n", blockN, element_size, params.d, params.dv, size_one_kvblock);
    int const max_kvblocks_in_l2 = size_l2 / size_one_kvblock;
    BOOL_SWITCH(params.varlen_sort_batches, Sort, [&] {
        NUM_WARP_SWITCH(num_warps, NumWarps, [&] {
            flash::prepare_varlen_num_blocks_kernel<NumWarps, Sort><<<num_ctas /*grid*/, 32 * NumWarps /*block*/, 0, stream>>>(
                params.seqlen_q, params.seqlen_k, params.seqlen_knew,
                params.cu_seqlens_q, params.cu_seqlens_k, params.cu_seqlens_knew,
                params.seqused_q, params.seqused_k, params.leftpad_k,
                params.b, !packgqa ? params.h : params.h_k, qhead_per_khead, params.num_sm, params.num_splits,
                cutlass::FastDivmod(blockM), cutlass::FastDivmod(blockN),
                params.tile_count_semaphore,
                params.num_m_blocks_ptr,
                params.num_splits_dynamic_ptr,
                params.varlen_batch_idx_ptr,
                // params.num_n_blocks_ptr,
                params.num_nheads_in_l2_ptr,
                enable_pdl,
                params.is_causal,
                packgqa,
                max_kvblocks_in_l2);
        });
    });
}