fix arithmetic error in backward kernel

sneaxiy · sneaxiy · commit c50c537732bd · 2018-08-13T15:10:48.000Z
diff --git a/paddle/fluid/operators/CMakeLists.txt b/paddle/fluid/operators/CMakeLists.txt
@@ -273,9 +273,9 @@ op_library(squeeze_op DEPS reshape_op)
 op_library(extract_rows_op DEPS memory)
 op_library(flatten_op DEPS reshape_op)
 
-
 if (WITH_GPU)
     op_library(conv_op DEPS vol2col depthwise_conv im2col)
+    op_library(layer_norm_op DEPS cub)
 else()
     op_library(conv_op DEPS vol2col im2col)
 endif()
diff --git a/paddle/fluid/operators/layer_norm_op.cu b/paddle/fluid/operators/layer_norm_op.cu
@@ -45,38 +45,55 @@ inline static int GetDesiredBlockDim(int block_dim) {
 static __device__ __forceinline__ float real_sqrt(float x) { return sqrtf(x); }
 static __device__ __forceinline__ double real_sqrt(double x) { return sqrt(x); }
 
+template <typename T>
+struct PairForLayerNorm {
+  __device__ __forceinline__ PairForLayerNorm() {}
+  __device__ __forceinline__ PairForLayerNorm(const T &first, const T &second)
+      : first_(first), second_(second) {}
+
+  T first_;
+  T second_;
+};
+
+template <typename T>
+struct PairForLayerNormAddFunctor {
+  __device__ __forceinline__ PairForLayerNorm<T> operator()(
+      const PairForLayerNorm<T> &p1, const PairForLayerNorm<T> &p2) {
+    return PairForLayerNorm<T>(p1.first_ + p2.first_, p1.second_ + p2.second_);
+  }
+};
+
 template <typename T, int BlockDim>
 __global__ void LayerNormForward(const T *x, const T *scale, const T *bias,
                                  T *y, T *mean, T *var, float epsilon,
                                  int feature_size) {
-  using BlockReduce = cub::BlockReduce<T, BlockDim>;
+  using BlockReduce = cub::BlockReduce<PairForLayerNorm<T>, BlockDim>;
   __shared__ typename BlockReduce::TempStorage temp_storage;
 
   int beg_idx = blockIdx.x * feature_size + threadIdx.x;
   int end_idx = (blockIdx.x + 1) * feature_size;
 
-  // Step 1: Reduce to calculate mean
+  // Step 1: Reduce to calculate mean and var
   T mean_val = static_cast<T>(0);
-  for (int i = beg_idx; i < end_idx; i += BlockDim) {
-    mean_val += x[i];
-  }
-  mean_val = BlockReduce(temp_storage).Reduce(mean_val, cub::Sum());
-  if (threadIdx.x == 0) mean[blockIdx.x] = mean_val / feature_size;
-  __syncthreads();
-  mean_val = mean[blockIdx.x];
-
-  // Step 2: Reduce to calculate var
   T var_val = static_cast<T>(0);
   for (int i = beg_idx; i < end_idx; i += BlockDim) {
-    T tmp = x[i] - mean_val;
+    T tmp = x[i];
+    mean_val += tmp;
     var_val += (tmp * tmp);
   }
-  var_val = BlockReduce(temp_storage).Reduce(var_val, cub::Sum());
-  if (threadIdx.x == 0) var[blockIdx.x] = var_val / feature_size;
+  auto pair = BlockReduce(temp_storage)
+                  .Reduce(PairForLayerNorm<T>(mean_val, var_val),
+                          PairForLayerNormAddFunctor<T>());
+  if (threadIdx.x == 0) {
+    auto tmp = pair.first_ / feature_size;
+    mean[blockIdx.x] = tmp;
+    var[blockIdx.x] = pair.second_ / feature_size - tmp * tmp;
+  }
   __syncthreads();
+  mean_val = mean[blockIdx.x];
   var_val = static_cast<T>(real_sqrt(var[blockIdx.x] + epsilon));
 
-  // Step 3: Calculate y
+  // Step 2: Calculate y
   if (scale != nullptr) {
     if (bias != nullptr) {
       for (int i = beg_idx, j = threadIdx.x; i < end_idx;
@@ -104,27 +121,6 @@ __global__ void LayerNormForward(const T *x, const T *scale, const T *bias,
   }
 }
 
-template <typename T>
-struct PairForLayerNormBackward {
-  __device__ __forceinline__ PairForLayerNormBackward() {}
-  __device__ __forceinline__ PairForLayerNormBackward(const T &first,
-                                                      const T &second)
-      : first_(first), second_(second) {}
-
-  T first_;
-  T second_;
-};
-
-template <typename T>
-struct PairForLayerNormBackwardAddFunctor {
-  __device__ __forceinline__ PairForLayerNormBackward<T> operator()(
-      const PairForLayerNormBackward<T> &p1,
-      const PairForLayerNormBackward<T> &p2) {
-    return PairForLayerNormBackward<T>(p1.first_ + p2.first_,
-                                       p1.second_ + p2.second_);
-  }
-};
-
 // Make sure that d_scale != nullptr && d_bias != nullptr
 // Since d_scale != nullptr, scale would not be nullptr
 template <typename T, int BlockDim, bool HasDx>
@@ -133,26 +129,28 @@ __global__ void LayerNormBackwardGradientAll(const T *x, const T *d_y,
                                              const T *mean, const T *var,
                                              const T *scale, float epsilon,
                                              int batch_size, int feature_size) {
-  using BlockReduce = cub::BlockReduce<PairForLayerNormBackward<T>, BlockDim>;
+  using BlockReduce = cub::BlockReduce<PairForLayerNorm<T>, BlockDim>;
   __shared__ typename BlockReduce::TempStorage temp_storage;
 
   int beg_idx = threadIdx.x * feature_size + blockIdx.x;
   int end_idx = batch_size * feature_size + blockIdx.x;
   int stride = BlockDim * feature_size;
+
   T d_scale_partial = 0, d_bias_partial = 0;
 
   for (int i = beg_idx; i < end_idx; i += stride) {
     int row_idx = i / feature_size;
     auto var_val = static_cast<T>(real_sqrt(var[row_idx] + epsilon));
     d_scale_partial += d_y[i] * (x[i] - mean[row_idx]) / var_val;
     d_bias_partial += d_y[i];
-    if (HasDx) d_x[i] = d_y[i] * scale[blockIdx.x] / var_val;
+    if (HasDx) {
+      d_x[i] = d_y[i] * scale[blockIdx.x] / var_val;
+    }
   }
 
-  auto pair =
-      BlockReduce(temp_storage)
-          .Reduce(PairForLayerNormBackward<T>(d_scale_partial, d_bias_partial),
-                  PairForLayerNormBackwardAddFunctor<T>());
+  auto pair = BlockReduce(temp_storage)
+                  .Reduce(PairForLayerNorm<T>(d_scale_partial, d_bias_partial),
+                          PairForLayerNormAddFunctor<T>());
 
   if (threadIdx.x == 0) {
     d_scale[blockIdx.x] = pair.first_;
@@ -205,22 +203,90 @@ __global__ void LayerNormBackwardGradientScaleOrBias(
   }
 }
 
+template <typename T, int BlockDim>
+__global__ void LayerNormBackwardPostProcessToCalculateDX(const T *x, T *d_x,
+                                                          const T *mean,
+                                                          const T *var,
+                                                          float epsilon,
+                                                          int feature_size) {
+  using BlockReduce = cub::BlockReduce<PairForLayerNorm<T>, BlockDim>;
+  __shared__ typename BlockReduce::TempStorage temp_storage;
+  __shared__ T d_x_reduce_tmp[2];
+
+  int beg_idx = blockIdx.x * feature_size + threadIdx.x;
+  int end_idx = (blockIdx.x + 1) * feature_size;
+
+  T block_mean = mean[blockIdx.x];
+  T block_var = var[blockIdx.x];
+  T d_x_mean_partial = 0, d_x_var_partial = 0;
+  for (int i = beg_idx; i < end_idx; i += BlockDim) {
+    d_x_mean_partial += d_x[i];
+    d_x_var_partial += d_x[i] * (x[i] - block_mean);
+  }
+
+  auto pair =
+      BlockReduce(temp_storage)
+          .Reduce(PairForLayerNorm<T>(d_x_mean_partial, d_x_var_partial),
+                  PairForLayerNormAddFunctor<T>());
+
+  if (threadIdx.x == 0) {
+    d_x_reduce_tmp[0] = pair.first_ / feature_size;
+    d_x_reduce_tmp[1] = pair.second_ / (feature_size * (block_var + epsilon));
+  }
+  __syncthreads();
+
+  d_x_mean_partial = d_x_reduce_tmp[0];
+  d_x_var_partial = d_x_reduce_tmp[1];
+  for (int i = beg_idx; i < end_idx; i += BlockDim) {
+    d_x[i] -= d_x_mean_partial;
+    d_x[i] -= (x[i] - block_mean) * d_x_var_partial;
+  }
+}
+
 // Here, we only calculate d_x
-template <typename T>
-__global__ void LayerNormBackwardGradientOnlyX(const T *d_y, T *d_x,
-                                               const T *var, const T *scale,
-                                               float epsilon, int batch_size,
-                                               int feature_size) {
-  int idx = threadIdx.x + blockIdx.x * blockDim.x;
-  if (idx < batch_size * feature_size) {
-    int row_idx = idx / feature_size;
-    auto var_val = static_cast<T>(real_sqrt(var[row_idx] + epsilon));
+template <typename T, int BlockDim>
+__global__ void LayerNormBackwardGradientOnlyDX(const T *x, const T *d_y,
+                                                T *d_x, const T *mean,
+                                                const T *var, const T *scale,
+                                                float epsilon,
+                                                int feature_size) {
+  using BlockReduce = cub::BlockReduce<PairForLayerNorm<T>, BlockDim>;
+  __shared__ typename BlockReduce::TempStorage temp_storage;
+  __shared__ T d_x_reduce_tmp[2];
+
+  int beg_idx = blockIdx.x * feature_size + threadIdx.x;
+  int end_idx = (blockIdx.x + 1) * feature_size;
+
+  T block_mean = mean[blockIdx.x], block_var = var[blockIdx.x];
+  T d_x_mean_partial = 0, d_x_var_partial = 0;
+  for (int i = beg_idx; i < end_idx; i += BlockDim) {
+    auto var_val = static_cast<T>(real_sqrt(block_var + epsilon));
     if (scale != nullptr) {
-      int col_idx = idx % feature_size;
-      d_x[idx] = d_y[idx] * scale[col_idx] / var_val;
+      int col_idx = i % feature_size;
+      d_x[i] = d_y[i] * scale[col_idx] / var_val;
     } else {
-      d_x[idx] = d_y[idx] / var_val;
+      d_x[i] = d_y[i] / var_val;
     }
+    d_x_mean_partial += d_x[i];
+    d_x_var_partial += d_x[i] * (x[i] - block_mean);
+  }
+
+  auto pair =
+      BlockReduce(temp_storage)
+          .Reduce(PairForLayerNorm<T>(d_x_mean_partial, d_x_var_partial),
+                  PairForLayerNormAddFunctor<T>());
+
+  if (threadIdx.x == 0) {
+    d_x_reduce_tmp[0] = pair.first_ / feature_size;
+    d_x_reduce_tmp[1] = pair.second_ / (feature_size * (block_var + epsilon));
+  }
+  __syncthreads();
+
+  d_x_mean_partial = d_x_reduce_tmp[0];
+  d_x_var_partial = d_x_reduce_tmp[1];
+  for (int i = beg_idx; i < end_idx; i += BlockDim) {
+    d_x[i] -= d_x_mean_partial;
+    d_x[i] -= (x[i] - block_mean) * d_x_var_partial;
   }
 }
 
@@ -263,6 +329,14 @@ static void LayerNormBackward(const T *x, const T *d_y, const T *scale,
         T><<<(feature_size + kMaxBlockDim - 1) / kMaxBlockDim, kMaxBlockDim, 0,
              stream>>>(x, d_y, d_x, d_scale, d_bias, mean, var, scale, epsilon,
                        feature_size);
+
+    if (d_x != nullptr) {
+      switch (GetDesiredBlockDim(feature_size)) {
+        FIXED_BLOCK_DIM_CASE(LayerNormBackwardPostProcessToCalculateDX<
+                             T, kBlockDim><<<1, kBlockDim, 0, stream>>>(
+            x, d_x, mean, var, epsilon, feature_size));
+      }
+    }
     return;
   }
 
@@ -296,10 +370,12 @@ static void LayerNormBackward(const T *x, const T *d_y, const T *scale,
       }
       break;
     case 4:  // d_x != nullptr, d_scale == nullptr, d_bias == nullptr
-      LayerNormBackwardGradientOnlyX<
-          T><<<(batch_size * feature_size + kMaxBlockDim - 1) / kMaxBlockDim,
-               kMaxBlockDim, 0, stream>>>(d_y, d_x, var, scale, epsilon,
-                                          batch_size, feature_size);
+      switch (GetDesiredBlockDim(feature_size)) {
+        FIXED_BLOCK_DIM_CASE(
+            LayerNormBackwardGradientOnlyDX<
+                T, kBlockDim><<<batch_size, kBlockDim, 0, stream>>>(
+                x, d_y, d_x, mean, var, scale, epsilon, feature_size));
+      }
       break;
     case 5:  // d_x != nulptr, d_scale == nullptr, d_bias != nullptr
       switch (block_dim) {
@@ -309,6 +385,12 @@ static void LayerNormBackward(const T *x, const T *d_y, const T *scale,
             x, d_y, d_scale, d_bias, d_x, mean, var, scale, epsilon, batch_size,
             feature_size));
       }
+      switch (GetDesiredBlockDim(feature_size)) {
+        FIXED_BLOCK_DIM_CASE(
+            LayerNormBackwardPostProcessToCalculateDX<
+                T, kBlockDim><<<batch_size, kBlockDim, 0, stream>>>(
+                x, d_x, mean, var, epsilon, feature_size));
+      }
       break;
     case 6:  // d_x != nullptr, d_scale != nullptr, d_bias == nullptr
       switch (block_dim) {
@@ -318,6 +400,12 @@ static void LayerNormBackward(const T *x, const T *d_y, const T *scale,
             x, d_y, d_scale, d_bias, d_x, mean, var, scale, epsilon, batch_size,
             feature_size));
       }
+      switch (GetDesiredBlockDim(feature_size)) {
+        FIXED_BLOCK_DIM_CASE(
+            LayerNormBackwardPostProcessToCalculateDX<
+                T, kBlockDim><<<batch_size, kBlockDim, 0, stream>>>(
+                x, d_x, mean, var, epsilon, feature_size));
+      }
       break;
     case 7:  // d_x != nullptr, d_scale != nullptr, d_bias != nullptr
       switch (block_dim) {
@@ -327,6 +415,12 @@ static void LayerNormBackward(const T *x, const T *d_y, const T *scale,
                 x, d_y, d_scale, d_bias, d_x, mean, var, scale, epsilon,
                 batch_size, feature_size));
       }
+      switch (GetDesiredBlockDim(feature_size)) {
+        FIXED_BLOCK_DIM_CASE(
+            LayerNormBackwardPostProcessToCalculateDX<
+                T, kBlockDim><<<batch_size, kBlockDim, 0, stream>>>(
+                x, d_x, mean, var, epsilon, feature_size));
+      }
       break;
     default:
       break;
