xserv/csrc/attention/reshape_and_cache.cu

#include <cuda_bf16.h>
#include "../common.cuh"

// Scatter [num_tokens] new K/V into a paged KV pool for ONE sequence.
//
// Source layouts (BF16, contiguous):
//   k_src, v_src : [num_kv_heads, num_tokens, head_dim]   (head-major)
//
// Pool layouts (BF16, contiguous):
//   k_pool, v_pool : [num_blocks_total, num_kv_heads, BLOCK_SIZE, head_dim]
//
// For token t (0 <= t < num_tokens):
//   p           = start_pos + t
//   logical_blk = p / BLOCK_SIZE
//   slot_in_blk = p % BLOCK_SIZE
//   phys        = block_ids[logical_blk]
//   pool[phys, h, slot_in_blk, :] := src[h, t, :]
//
// Replaces a Rust-side per-token, per-head cudaMemcpy loop. With Qwen3-8B
// (8 KV heads, 36 layers) and a 1024-token prefill, that loop fired
// ~290k device-side memcpys; one kernel launch per layer is dramatically
// less overhead.
//
// Grid : (num_tokens, num_kv_heads)
// Block: head_dim threads (≤128 in practice; head_dim is padded to a
//        multiple of 32 by the model and all our shipping configs are
//        128, so a single warp's worth handles two slots in flight).

__global__ void reshape_and_cache_bf16_kernel(
    const __nv_bfloat16* __restrict__ k_src,
    const __nv_bfloat16* __restrict__ v_src,
    __nv_bfloat16* __restrict__ k_pool,
    __nv_bfloat16* __restrict__ v_pool,
    const int* __restrict__ block_ids,
    int num_tokens, int num_heads,
    int head_dim, int start_pos, int block_size
) {
    int t = blockIdx.x;
    int h = blockIdx.y;
    if (t >= num_tokens || h >= num_heads) return;

    int p           = start_pos + t;
    int logical_blk = p / block_size;
    int slot_in_blk = p - logical_blk * block_size;
    int phys        = block_ids[logical_blk];

    long long src_off = ((long long)h * num_tokens + t) * head_dim;
    long long dst_off = (((long long)phys * num_heads + h) * block_size + slot_in_blk) * head_dim;

    int tid = threadIdx.x;
    int blockSize = blockDim.x;

    // Per-thread strided copy. head_dim is typically 128 and blockSize is
    // 128, so each thread copies exactly one element — but the loop keeps
    // the kernel correct for non-128 head_dim configs (Phi-style 64, etc.).
    for (int d = tid; d < head_dim; d += blockSize) {
        k_pool[dst_off + d] = k_src[src_off + d];
        v_pool[dst_off + d] = v_src[src_off + d];
    }
}

// Batched variant: writes one new K/V token per sequence into a paged
// pool, indexed by a per-batch block table that also drives the paged
// attention kernel. Used in the decode path where every seq advances
// by exactly one position per step.
//
// Source layouts (BF16, contiguous):
//   k_src, v_src : [batch, num_kv_heads, head_dim]
//
// Pool layouts (BF16, contiguous):
//   k_pool, v_pool : [num_blocks_total, num_kv_heads, BLOCK_SIZE, head_dim]
//
// block_tables : int32 [batch, max_blocks_per_seq]
// kv_lens      : int32 [batch]   (current seq_len BEFORE this step + 1
//                                  — i.e. the same buffer paged attention
//                                  reads. The new token's logical index
//                                  is `kv_lens[b] - 1`.)
//
// Grid : (batch, num_kv_heads)
// Block: head_dim threads.

__global__ void reshape_and_cache_batched_bf16_kernel(
    const __nv_bfloat16* __restrict__ k_src,
    const __nv_bfloat16* __restrict__ v_src,
    __nv_bfloat16* __restrict__ k_pool,
    __nv_bfloat16* __restrict__ v_pool,
    const int* __restrict__ block_tables,
    const int* __restrict__ kv_lens,
    int num_heads, int head_dim,
    int block_size, int max_blocks_per_seq
) {
    int b = blockIdx.x;
    int h = blockIdx.y;

    int new_pos     = kv_lens[b] - 1;
    int logical_blk = new_pos / block_size;
    int slot_in_blk = new_pos - logical_blk * block_size;
    int phys        = block_tables[b * max_blocks_per_seq + logical_blk];

    long long src_off = ((long long)b * num_heads + h) * head_dim;
    long long dst_off = (((long long)phys * num_heads + h) * block_size + slot_in_blk) * head_dim;

    int tid = threadIdx.x;
    int blockSize = blockDim.x;
    for (int d = tid; d < head_dim; d += blockSize) {
        k_pool[dst_off + d] = k_src[src_off + d];
        v_pool[dst_off + d] = v_src[src_off + d];
    }
}

extern "C" {

void launch_reshape_and_cache_bf16(
    const void* k_src, const void* v_src,
    void* k_pool, void* v_pool,
    const void* block_ids,
    int num_tokens, int num_heads,
    int head_dim, int start_pos, int block_size,
    void* stream
) {
    if (num_tokens <= 0) return;
    int threads = head_dim < 32 ? 32 : head_dim;
    if (threads > 1024) threads = 1024;
    dim3 grid(num_tokens, num_heads);
    reshape_and_cache_bf16_kernel<<<grid, threads, 0, (cudaStream_t)stream>>>(
        (const __nv_bfloat16*)k_src,
        (const __nv_bfloat16*)v_src,
        (__nv_bfloat16*)k_pool,
        (__nv_bfloat16*)v_pool,
        (const int*)block_ids,
        num_tokens, num_heads,
        head_dim, start_pos, block_size
    );
    CUDA_CHECK_LAST_ERROR();
}

void launch_reshape_and_cache_batched_bf16(
    const void* k_src, const void* v_src,
    void* k_pool, void* v_pool,
    const void* block_tables, const void* kv_lens,
    int batch, int num_heads,
    int head_dim, int block_size, int max_blocks_per_seq,
    void* stream
) {
    if (batch <= 0 || num_heads <= 0) return;
    int threads = head_dim < 32 ? 32 : head_dim;
    if (threads > 1024) threads = 1024;
    dim3 grid(batch, num_heads);
    reshape_and_cache_batched_bf16_kernel<<<grid, threads, 0, (cudaStream_t)stream>>>(
        (const __nv_bfloat16*)k_src,
        (const __nv_bfloat16*)v_src,
        (__nv_bfloat16*)k_pool,
        (__nv_bfloat16*)v_pool,
        (const int*)block_tables,
        (const int*)kv_lens,
        num_heads, head_dim, block_size, max_blocks_per_seq
    );
    CUDA_CHECK_LAST_ERROR();
}

}