Softmax backward optimize (#30249)

* softmax backward optimize

paddle/fluid/operators/softmax_cudnn_op.cu

@@ -17,6 +17,7 @@ limitations under the License. */
 #include "paddle/fluid/operators/softmax_op.h"
 #include "paddle/fluid/platform/cuda_device_function.h"
 #include "paddle/fluid/platform/cudnn_helper.h"
+#include "paddle/fluid/platform/gpu_launch_config.h"
 
 namespace paddle {
 namespace platform {
@@ -39,6 +40,13 @@ using Tensor = framework::Tensor;
         out_data, x->data<T>(), N, dim, dim);                      \
     break;
 
+#define LAUNCH_SOFTMAX_WARP_BACKWARD(Log2Elements)                 \
+  case Log2Elements:                                               \
+    softmax_warp_backward<T, float, Log2Elements><<<               \
+        blocks, threads, 0, ctx.cuda_device_context().stream()>>>( \
+        dx_data, mul_grad.data<T>(), out->data<T>(), N, dim, dim); \
+    break;
+
 static inline int SizeOutAxis(const int axis, DDim dims) {
   int size = 1;
   for (int i = axis + 1; i < dims.size(); i++) {
@@ -199,6 +207,83 @@ __global__ void WarpSoftmaxForward(T* dst, const T* src, const int batch_size,
   }
 }
 
+template <typename T, typename AccT, int Log2Elements>
+__global__ void softmax_warp_backward(T* gradInput, const T* grad,
+                                      const T* output, int batch_size,
+                                      int stride, int element_count) {
+  constexpr int next_power_of_two = 1 << Log2Elements;
+  constexpr int warp_size_softmax =
+      (next_power_of_two < 32) ? next_power_of_two : 32;
+  constexpr int WARP_ITERATIONS = next_power_of_two / warp_size_softmax;
+  constexpr int WARP_BATCH = (next_power_of_two <= 128) ? 2 : 1;
+
+  int first_batch = (blockDim.y * blockIdx.x + threadIdx.y) * WARP_BATCH;
+
+  int local_batches = batch_size - first_batch;
+  if (local_batches > WARP_BATCH) {
+    local_batches = WARP_BATCH;
+  }
+
+  int local_idx = threadIdx.x % warp_size_softmax;
+
+  int thread_offset = first_batch * stride + local_idx;
+  grad += thread_offset;
+  output += thread_offset;
+  gradInput += thread_offset;
+
+  // load data from global memory
+  AccT grad_reg[WARP_BATCH][WARP_ITERATIONS];
+  AccT output_reg[WARP_BATCH][WARP_ITERATIONS];
+  for (int i = 0; i < WARP_BATCH; ++i) {
+    int batch_element_count = (i >= local_batches) ? 0 : element_count;
+    for (int it = 0; it < WARP_ITERATIONS; ++it) {
+      int element_index = local_idx + it * warp_size_softmax;
+      if (element_index < batch_element_count) {
+        grad_reg[i][it] =
+            static_cast<AccT>(grad[i * element_count + it * warp_size_softmax]);
+        output_reg[i][it] = static_cast<AccT>(
+            output[i * element_count + it * warp_size_softmax]);
+      } else {
+        grad_reg[i][it] = AccT(0);
+        output_reg[i][it] = AccT(0);
+      }
+    }
+  }
+
+  AccT sum[WARP_BATCH];
+#pragma unroll
+  for (int i = 0; i < WARP_BATCH; ++i) {
+    sum[i] = grad_reg[i][0];
+#pragma unroll
+    for (int it = 1; it < WARP_ITERATIONS; ++it) {
+      sum[i] += grad_reg[i][it];
+    }
+  }
+  warp_reduce_sum<AccT, WARP_BATCH, warp_size_softmax>(sum);
+
+// store result
+#pragma unroll
+  for (int i = 0; i < WARP_BATCH; ++i) {
+    if (i >= local_batches) break;
+#pragma unroll
+    for (int it = 0; it < WARP_ITERATIONS; ++it) {
+      int element_index = local_idx + it * warp_size_softmax;
+      if (element_index < element_count) {
+        // compute gradients
+        gradInput[i * element_count + it * warp_size_softmax] =
+            (grad_reg[i][it] - output_reg[i][it] * sum[i]);
+      }
+    }
+  }
+}
+
+template <typename T>
+__global__ void MultiplyCUDAKernel(T* C, const T* A, const T* B, int N) {
+  CUDA_KERNEL_LOOP(i, N) {
+    C[i] = static_cast<T>(static_cast<float>(A[i]) * static_cast<float>(B[i]));
+  }
+}
+
 template <typename T, int VPT, int WARP_PER_BLOCK>
 __global__ void VecSoftmaxBackward(T* dst, const T* grad, const T* src,
                                    const int batch_size,
@@ -340,19 +425,20 @@ class SoftmaxGradCUDNNKernel : public framework::OpKernel<T> {
     constexpr bool warp_softmax_available =
         std::is_same<T, float>::value ||
         std::is_same<T, platform::float16>::value;
-    if (D == 1 && dim == 128 && N % warps_per_block == 0 &&
-        warp_softmax_available) {
+    bool optimize = false;
+    if (D == 1 && warp_softmax_available) {
+      if (dim == 128 && N % warps_per_block == 0) {
+        optimize = true;
         if (std::is_same<T, float>::value) {
-        VecSoftmaxBackward<
-            float, 4,
-            warps_per_block><<<N / warps_per_block, warps_per_block * WARP_SIZE,
-                               0, ctx.cuda_device_context().stream()>>>(
-            dx->data<float>(), dout->data<float>(), out->data<float>(), N, dim);
+          VecSoftmaxBackward<float, 4, warps_per_block><<<
+              N / warps_per_block, warps_per_block * WARP_SIZE, 0,
+              ctx.cuda_device_context().stream()>>>(dx->data<float>(),
+                                                    dout->data<float>(),
+                                                    out->data<float>(), N, dim);
         } else if (std::is_same<T, platform::float16>::value) {
-        VecSoftmaxBackward<
-            platform::float16, 4,
-            warps_per_block><<<N / warps_per_block, warps_per_block * WARP_SIZE,
-                               0, ctx.cuda_device_context().stream()>>>(
+          VecSoftmaxBackward<platform::float16, 4, warps_per_block><<<
+              N / warps_per_block, warps_per_block * WARP_SIZE, 0,
+              ctx.cuda_device_context().stream()>>>(
               dx->data<platform::float16>(), dout->data<platform::float16>(),
               out->data<platform::float16>(), N, dim);
         } else {
@@ -361,7 +447,52 @@ class SoftmaxGradCUDNNKernel : public framework::OpKernel<T> {
               platform::errors::Unimplemented(
                   "Warp softmax backward is only available for fp32 and fp16"));
         }
-    } else {
+      } else if (dim < 40 && dim % 32 != 0) {
+        optimize = true;
+        Tensor mul_grad;
+        int numel = N * dim;
+        mul_grad.mutable_data<T>({numel}, ctx.GetPlace());
+
+        auto stream = ctx.cuda_device_context().stream();
+        auto& dev_ctx =
+            ctx.template device_context<platform::CUDADeviceContext>();
+        auto config = GetGpuLaunchConfig1D(dev_ctx, numel);
+
+        MultiplyCUDAKernel<T><<<config.block_per_grid.x,
+                                config.thread_per_block.x, 0, stream>>>(
+            mul_grad.data<T>(), dout->data<T>(), out->data<T>(), numel);
+
+        int log2_elements = log2_ceil(dim);
+        const int next_power_of_two = 1 << log2_elements;
+
+        int warp_size = (next_power_of_two < 32) ? next_power_of_two : 32;
+
+        int batches_per_warp = (next_power_of_two <= 128) ? 2 : 1;
+
+        constexpr int threads_per_block = 128;
+
+        int warps_per_block = (threads_per_block / warp_size);
+        int batches_per_block = warps_per_block * batches_per_warp;
+        int blocks = (N + batches_per_block - 1) / batches_per_block;
+        dim3 threads(warp_size, warps_per_block, 1);
+
+        switch (log2_elements) {
+          LAUNCH_SOFTMAX_WARP_BACKWARD(0);  // 1
+          LAUNCH_SOFTMAX_WARP_BACKWARD(1);  // 2
+          LAUNCH_SOFTMAX_WARP_BACKWARD(2);  // 4
+          LAUNCH_SOFTMAX_WARP_BACKWARD(3);  // 8
+          LAUNCH_SOFTMAX_WARP_BACKWARD(4);  // 16
+          LAUNCH_SOFTMAX_WARP_BACKWARD(5);  // 32
+          LAUNCH_SOFTMAX_WARP_BACKWARD(6);  // 64
+          LAUNCH_SOFTMAX_WARP_BACKWARD(7);  // 128
+          LAUNCH_SOFTMAX_WARP_BACKWARD(8);  // 256
+          LAUNCH_SOFTMAX_WARP_BACKWARD(9);  // 512
+          default:
+            break;
+        }
+      }
+    }
+    if (!optimize) {
       ScopedTensorDescriptor desc;
       std::vector<int> tensor_dims = {N, dim, D, 1};
       DataLayout layout = DataLayout::kNCHW;