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CUDA: Fix data-races when reusing SMEM in block_reduce (llama/26385)

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* CUDA: Fix data-races when reusing block_reduce

block_reduce currently doesn't resync after reading from SMEM, causing
potential data-races when reusing SMEM for multiple reductions.

One may consider simply always adding this in block_reduce, but this
comes at a potential perf cost

* double-buffering for single-row softmax

* double-buffering for norm as well

* Add comment

* Add explanatory comment to block_reduce

* Specify need for + do memory barrier only in multi-warp scenario

* Implement review-suggestion from @gaugarg-nv
This commit is contained in:
Oliver Simons
2026-08-04 13:37:47 +03:00
committed by Georgi Gerganov
parent e5f6c74112
commit 3fee8a1e05
3 changed files with 18 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ggml/src/ggml-cuda/common.cuh
+2 -1
View File
@@ -627,7 +627,8 @@ template <typename T> struct block_reduce_policy<block_reduce_method::MAX, T> {
};
template <block_reduce_method reduce_method_t, const unsigned int block_size_template = 0, typename T>
static __device__ T block_reduce(T val, T * shared_vals) {
static __device__ T block_reduce(T val, [[maybe_unused]] T * shared_vals) {
// for multi-warp reductions, callers must not reuse shared_vals until all reads from this invocation have completed
val = block_reduce_policy<reduce_method_t, T>::reduce(val);
const unsigned int block_size = block_size_template == 0 ? blockDim.x : block_size_template;
if (block_size > WARP_SIZE) {
ggml/src/ggml-cuda/norm.cu
+2 -2
View File
@@ -64,7 +64,7 @@ static __global__ void group_norm_f32(const float * x, float * dst, const int gr
tmp += xi * xi;
}
tmp = block_reduce<block_reduce_method::SUM, block_size>(tmp, s_sum);
tmp = block_reduce<block_reduce_method::SUM, block_size>(tmp, s_sum + 32);
const float variance = tmp / group_size;
const float scale = rsqrtf(variance + eps);
@@ -297,7 +297,7 @@ static void group_norm_f32_cuda(
group_norm_f32<WARP_SIZE><<<num_groups, block_dims, 0, stream>>>(x, dst, group_size, ne_elements, eps);
} else {
const dim3 block_dims(1024, 1, 1);
group_norm_f32<1024><<<num_groups, block_dims, block_dims.x > WARP_SIZE ? 32 * sizeof(float): 0, stream>>>(x, dst, group_size, ne_elements, eps);
group_norm_f32<1024><<<num_groups, block_dims, block_dims.x > WARP_SIZE ? 2 * 32 * sizeof(float): 0, stream>>>(x, dst, group_size, ne_elements, eps);
}
}
ggml/src/ggml-cuda/softmax.cu
+14 -6
View File
@@ -116,6 +116,11 @@ static __global__ void soft_max_f32(
vals[col] = val;
}
if (block_size > WARP_SIZE) {
// sync is needed as we reuse buf_iw across block_reduce invocations, see #26385
// for block_size <= WARP_SIZE, block_reduce does not access buf_iw
__syncthreads();
}
// find the sum of exps in the block
tmp = block_reduce<block_reduce_method::SUM, block_size_template>(tmp, buf_iw);
@@ -142,6 +147,8 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
float * __restrict__ dst,
float * __restrict__ tmp_maxs,
float * __restrict__ tmp_sums,
float * shared_vals_max,
float * shared_vals_sum,
const soft_max_params p) {
namespace cg = cooperative_groups;
@@ -154,7 +161,6 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
float local_vals[n_elem_per_thread] = { -INFINITY, -INFINITY, -INFINITY, -INFINITY };
float local_max = -INFINITY;
const int step_size = gridDim.x * blockDim.x;
__shared__ float shared_vals[32];
// Compute thread-local max
for (int col = col_start; col < p.ncols;) {
@@ -171,7 +177,7 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
}
// Compute CTA-level max
local_max = block_reduce<block_reduce_method::MAX>(local_max, shared_vals);
local_max = block_reduce<block_reduce_method::MAX>(local_max, shared_vals_max);
// Store CTA-level max to GMEM
if (tid == 0) {
@@ -186,7 +192,7 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
} else {
local_max = -INFINITY;
}
local_max = block_reduce<block_reduce_method::MAX>(local_max, shared_vals);
local_max = block_reduce<block_reduce_method::MAX>(local_max, shared_vals_max);
// Compute softmax dividends, accumulate divisor
float tmp_expf = 0.0f;
@@ -209,7 +215,7 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
}
// Reduce divisor within CTA
tmp_expf = block_reduce<block_reduce_method::SUM>(tmp_expf, shared_vals);
tmp_expf = block_reduce<block_reduce_method::SUM>(tmp_expf, shared_vals_sum);
// Store CTA-level sum to GMEM
if (tid == 0) {
@@ -223,7 +229,7 @@ static __device__ void soft_max_f32_parallelize_cols_single_row(const float * __
} else {
tmp_expf = 0.0f;
}
tmp_expf = block_reduce<block_reduce_method::SUM>(tmp_expf, shared_vals);
tmp_expf = block_reduce<block_reduce_method::SUM>(tmp_expf, shared_vals_sum);
// Divide dividend by global sum + store data
for (int col = col_start; col < p.ncols;) {
@@ -310,9 +316,11 @@ __launch_bounds__(8*WARP_SIZE, 1) static __global__ void soft_max_f32_paralleliz
// https://docs.nvidia.com/cuda/cuda-programming-guide/05-appendices/device-callable-apis.html#grid-synchronization
// https://docs.nvidia.com/cuda/cuda-programming-guide/05-appendices/device-callable-apis.html#class-cluster-group
{
__shared__ float shared_vals[2][32];
for (int rowx = 0; rowx < p.ne01 * p.ne02 * p.ne03; rowx++) {
soft_max_f32_parallelize_cols_single_row(x + int64_t(rowx) * p.ncols, dst + int64_t(rowx) * p.ncols, tmp_maxs,
tmp_sums, p);
tmp_sums, shared_vals[0], shared_vals[1], p);
}
}