flash-attention/csrc/flash_attn/src/fmha_kernel.h

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/******************************************************************************
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#pragma once
#include <philox.cuh>
#include <fmha.h>
#include <fmha/utils.h>
#include <fmha/smem_tile.h>
#include <fmha/gmem_tile.h>
#include <fmha/mask.h>
#include <fmha/softmax.h>
namespace fmha {
////////////////////////////////////////////////////////////////////////////////////////////////////
template<int THREADS_PER_CTA>
struct BlockInfoPadded {
template<typename Params>
__device__ BlockInfoPadded(const Params &params,
const int bidb,
const int bidh,
const int tidx)
: bidb(bidb), bidh(bidh), h(params.h) {
// The block index.
sum_s = params.cu_seqlens[bidb];
actual_seqlen = params.cu_seqlens[bidb + 1] - sum_s;
bidx = sum_s * params.h + bidh;
tidx_global = (bidb * params.h + bidh) * THREADS_PER_CTA + tidx;
}
__device__ bool stop_early(const int start_col = 0) const {
return actual_seqlen <= start_col;
}
int actual_seqlen;
int bidx;
int sum_s;
int bidh;
int bidb;
int tidx_global;
int h;
};
////////////////////////////////////////////////////////////////////////////////////////////////////
template<int CHUNKS, typename Cta_tile>
struct Noloop_traits{
// Interpretation of Cta_tile dims, i.e. Cta_tile_p:
enum{ STEP = Cta_tile::M };
enum{ SEQLEN = Cta_tile::N };
template<typename Block_info>
inline __device__ Noloop_traits(const int bidc, const Block_info& binfo)
: bidc_(bidc) {
const int seqlen = binfo.actual_seqlen;
const int steps = (seqlen + STEP - 1) / STEP;
const int steps_per_chunk = (steps + CHUNKS - 1) / CHUNKS;
const int step_begin = bidc_ * steps_per_chunk;
const int step_end = min(steps, (bidc_ + 1) * steps_per_chunk);
const int actual_steps = max(0, step_end - step_begin);
loop_offset_ = step_begin;
num_steps_ = actual_steps;
}
template<typename ... Tiles>
inline __device__ void move_all(Tiles & ... tiles) const {
using expand_type = int[];
for( int s = 0; s < loop_offset_; s++ ) {
expand_type{ (tiles.move(), 0)... };
}
}
inline __device__ int get_idx_dk() const {
//return bidc_;
return bidc_ * 2 + 0;
}
inline __device__ int get_idx_dv() const {
//return CHUNKS + bidc_;
return bidc_ * 2 + 1;
}
inline __device__ int offset_loop_count(const int l) {
// convert loop counter to position in the outer sequence
return (loop_offset_ + l) * STEP;
}
const uint32_t bidc_;
int loop_offset_;
int num_steps_;
};
////////////////////////////////////////////////////////////////////////////////////////////////////
template<typename Kernel_traits>
std::tuple<int , int, int, int, int, int> work_dist(const int total_ctas, const int heads_total) {
constexpr int STEPS_PER_HEAD = Kernel_traits::Cta_tile_p::N / Kernel_traits::Cta_tile_p::M;
const int num_full_heads = heads_total / total_ctas;
const int heads_last_wave = heads_total % total_ctas;
int num_main_groups = 0;
int main_steps = 0;
int rest_steps = 0;
if( heads_last_wave > 0 ) {
// Number of CTA groups that process within heads.
num_main_groups = total_ctas / heads_last_wave;
// Remaining CTAs that process between heads.
const int rest_ctas = total_ctas - (heads_last_wave * num_main_groups);
if(rest_ctas == 0) {
// We have exactly "num_main_groups" CTAs to process each of the remaining heads.
main_steps = (STEPS_PER_HEAD + num_main_groups - 1) / num_main_groups;
num_main_groups = STEPS_PER_HEAD / main_steps; // Here: main_step > 0
rest_steps = STEPS_PER_HEAD % main_steps;
} else {
// Ideal number of steps if we could load-balance as evenly as possible.
const int steps_ideal = (heads_last_wave * STEPS_PER_HEAD + total_ctas - 1) / total_ctas;
// Iterations that a "rest" CTA has to do at most.
const int max_rest_iters = (heads_last_wave + rest_ctas - 1) / rest_ctas;
// Find the first step distribution, s.t. the maximum work of the "rest" CTAs is less than the work of the main CTAs.
main_steps = steps_ideal;
rest_steps = STEPS_PER_HEAD - main_steps * num_main_groups;
for( ; main_steps * num_main_groups < STEPS_PER_HEAD; main_steps++ ) {
rest_steps = STEPS_PER_HEAD - main_steps * num_main_groups;
const int max_rest_total_steps = rest_steps * max_rest_iters;
if( max_rest_total_steps < main_steps )
break;
}
rest_steps = STEPS_PER_HEAD - main_steps * num_main_groups;
}
}
using Cta_tile_p = typename Kernel_traits::Cta_tile_p;
using Mma_tile_p = fmha::Hmma_tile<Cta_tile_p>;
const int max_steps = STEPS_PER_HEAD * num_full_heads + std::max(main_steps, rest_steps);
const int elts_per_thread_per_step = Mma_tile_p::MMAS_M * Mma_tile_p::MMAS_N * 8;
const int elts_per_thread = max_steps * elts_per_thread_per_step;
return {num_full_heads, num_main_groups, heads_last_wave, main_steps, rest_steps, elts_per_thread};
}
////////////////////////////////////////////////////////////////////////////////////////////////////
} // namespace fmha