[Kernel] Layernorm performance optimization (#3662)

cmake/utils.cmake

@@ -100,6 +100,11 @@ function (get_torch_gpu_compiler_flags OUT_GPU_FLAGS GPU_LANG)
 
     if (CUDA_VERSION VERSION_GREATER_EQUAL 11.8)
       list(APPEND GPU_FLAGS "-DENABLE_FP8_E5M2")
+      list(REMOVE_ITEM GPU_FLAGS
+        "-D__CUDA_NO_HALF_OPERATORS__"
+        "-D__CUDA_NO_HALF_CONVERSIONS__"
+        "-D__CUDA_NO_BFLOAT16_CONVERSIONS__"
+        "-D__CUDA_NO_HALF2_OPERATORS__")
     endif()
 
   elseif(${GPU_LANG} STREQUAL "HIP")

csrc/layernorm_kernels.cu

@@ -4,6 +4,16 @@
 
 #include "dispatch_utils.h"
 #include "reduction_utils.cuh"
+#ifndef USE_ROCM
+  #include <cuda_bf16.h>
+  #include <cuda_fp16.h>
+#else
+  #include <hip/hip_bf16.h>
+  #include <hip/hip_fp16.h>
+
+  using __nv_bfloat16 = __hip_bfloat16;
+  using __nv_bfloat162 = __hip_bfloat162;
+#endif
 
 namespace vllm {
 
@@ -35,9 +45,199 @@ __global__ void rms_norm_kernel(
   }
 }
 
-// TODO: Further optimize this kernel.
-template<typename scalar_t>
-__global__ void fused_add_rms_norm_kernel(
+
+/* Converter structs for the conversion from torch types to HIP/CUDA types,
+   and the associated type conversions within HIP/CUDA. These helpers need
+   to be implemented for now because the relevant type conversion
+   operators/constructors are not consistently implemented by HIP/CUDA, so
+   a generic conversion via type casts cannot be implemented.
+
+   Each struct should have the member static constexpr bool `exists`:
+   If false, the optimized kernel is not used for the corresponding torch type.
+   If true, the struct should be fully defined as shown in the examples below. 
+ */
+template<typename torch_type>
+struct _typeConvert { static constexpr bool exists = false; };
+
+template<>
+struct _typeConvert<c10::Half> {
+  static constexpr bool exists = true;
+  using hip_type = __half;
+  using packed_hip_type = __half2;
+
+  __device__ static inline float convert(hip_type x) { return __half2float(x); }
+  __device__ static inline float2 convert(packed_hip_type x) { return __half22float2(x); }
+  __device__ static inline hip_type convert(float x) { return __float2half_rn(x); }
+  __device__ static inline packed_hip_type convert(float2 x) { return __float22half2_rn(x); }
+};
+
+#if defined(__CUDA_ARCH__) && __CUDA_ARCH__ >= 800
+// CUDA_ARCH < 800 does not have BF16 support
+// TODO: Add in ROCm support once public headers handle bf16 maturely
+template<>
+struct _typeConvert<c10::BFloat16> {
+  static constexpr bool exists = true;
+  using hip_type = __nv_bfloat16;
+  using packed_hip_type = __nv_bfloat162;
+
+  __device__ static inline float convert(hip_type x) { return __bfloat162float(x); }
+  __device__ static inline float2 convert(packed_hip_type x) { return __bfloat1622float2(x); }
+  __device__ static inline hip_type convert(float x) { return __float2bfloat16(x); }
+  __device__ static inline packed_hip_type convert(float2 x) { return __float22bfloat162_rn(x); }
+};
+#endif
+
+
+/* Vector POD struct to generate vectorized and packed FP16/BF16 ops
+   for appropriate specializations of fused_add_rms_norm_kernel.
+   Only functions that are necessary in that kernel are implemented.
+   Alignment to 16 bytes is required to use 128-bit global memory ops.
+ */
+template<typename scalar_t, int width>
+struct alignas(16) _f16Vec {
+  /* Not theoretically necessary that width is a power of 2 but should 
+     almost always be the case for optimization purposes */ 
+  static_assert(width > 0 && (width & (width - 1)) == 0,
+                "Width is not a positive power of 2!");
+  using Converter = _typeConvert<scalar_t>;
+  using T1 = typename Converter::hip_type;
+  using T2 = typename Converter::packed_hip_type;
+  T1 data[width];
+
+  __device__ _f16Vec& operator+=(const _f16Vec<scalar_t, width>& other) {
+    if constexpr (width % 2 == 0) {
+      #pragma unroll
+      for (int i = 0; i < width; i += 2) {
+        T2 temp{data[i], data[i+1]};
+        temp += T2{other.data[i], other.data[i+1]};
+        data[i] = temp.x;
+        data[i+1] = temp.y;
+      }
+    } else {
+      #pragma unroll
+      for (int i = 0; i < width; ++i)
+        data[i] += other.data[i];
+    }
+    return *this;
+  }
+
+  __device__ _f16Vec& operator*=(const _f16Vec<scalar_t, width>& other) {
+    if constexpr (width % 2 == 0) {
+      #pragma unroll
+      for (int i = 0; i < width; i += 2) {
+        T2 temp{data[i], data[i+1]};
+        temp *= T2{other.data[i], other.data[i+1]};
+        data[i] = temp.x;
+        data[i+1] = temp.y;
+      }
+    } else {
+      #pragma unroll
+      for (int i = 0; i < width; ++i)
+        data[i] *= other.data[i];
+    }
+    return *this;
+  }
+
+  __device__ _f16Vec& operator*=(const float scale) {
+    if constexpr (width % 2 == 0) {
+      #pragma unroll
+      for (int i = 0; i < width; i += 2) {
+        float2 temp_f = Converter::convert(T2{data[i], data[i+1]});
+        temp_f.x *= scale;
+        temp_f.y *= scale;
+        T2 temp = Converter::convert(temp_f);
+        data[i] = temp.x;
+        data[i+1] = temp.y;
+      }
+    } else {
+      #pragma unroll
+      for (int i = 0; i < width; ++i) {
+        float temp = Converter::convert(data[i]) * scale;
+        data[i] = Converter::convert(temp);
+      }
+    }
+    return *this;
+  }
+
+  __device__ float sum_squares() const {
+    float result = 0.0f;
+    if constexpr (width % 2 == 0) {
+      #pragma unroll
+      for (int i = 0; i < width; i += 2) {
+        float2 z = Converter::convert(T2{data[i], data[i+1]});
+        result += z.x * z.x + z.y * z.y;
+      }
+    } else {
+      #pragma unroll
+      for (int i = 0; i < width; ++i) {
+        float x = Converter::convert(data[i]);
+        result += x * x;
+      }
+    }
+    return result;
+  }
+};
+
+/* Function specialization in the case of FP16/BF16 tensors.
+   Additional optimizations we can make in this case are
+   packed and vectorized operations, which help with the
+   memory latency bottleneck. */
+template<typename scalar_t, int width>
+__global__ std::enable_if_t<
+  (width > 0) && _typeConvert<scalar_t>::exists> fused_add_rms_norm_kernel(
+  scalar_t* __restrict__ input,           // [..., hidden_size]
+  scalar_t* __restrict__ residual,        // [..., hidden_size]
+  const scalar_t* __restrict__ weight,    // [hidden_size]
+  const float epsilon,
+  const int num_tokens,
+  const int hidden_size) {
+  // Sanity checks on our vector struct and type-punned pointer arithmetic
+  static_assert(std::is_pod_v<_f16Vec<scalar_t, width>>);
+  static_assert(sizeof(_f16Vec<scalar_t, width>) == sizeof(scalar_t) * width);
+
+  const int vec_hidden_size = hidden_size / width;
+  __shared__ float s_variance;
+  float variance = 0.0f;
+  /* These and the argument pointers are all declared `restrict` as they are
+     not aliased in practice. Argument pointers should not be dereferenced
+     in this kernel as that would be undefined behavior */
+  auto* __restrict__ input_v = reinterpret_cast<_f16Vec<scalar_t, width>*>(input);
+  auto* __restrict__ residual_v = reinterpret_cast<_f16Vec<scalar_t, width>*>(residual);
+  auto* __restrict__ weight_v = reinterpret_cast<const _f16Vec<scalar_t, width>*>(weight);
+
+  for (int idx = threadIdx.x; idx < vec_hidden_size; idx += blockDim.x) {
+    int id = blockIdx.x * vec_hidden_size + idx;
+    _f16Vec<scalar_t, width> temp = input_v[id];
+    temp += residual_v[id];
+    variance += temp.sum_squares();
+    residual_v[id] = temp;
+  }
+  /* Keep the following if-else block in sync with the
+     calculation of max_block_size in fused_add_rms_norm */ 
+  if (num_tokens < 256) {
+    variance = blockReduceSum<float, 1024>(variance);
+  } else variance = blockReduceSum<float, 256>(variance);
+  if (threadIdx.x == 0) {
+    s_variance = rsqrtf(variance / hidden_size + epsilon);
+  }
+  __syncthreads();
+
+  for (int idx = threadIdx.x; idx < vec_hidden_size; idx += blockDim.x) {
+    int id = blockIdx.x * vec_hidden_size + idx;
+    _f16Vec<scalar_t, width> temp = residual_v[id];
+    temp *= s_variance;
+    temp *= weight_v[idx];
+    input_v[id] = temp;
+  }
+}
+
+
+/* Generic fused_add_rms_norm_kernel
+   The width field is not used here but necessary for other specializations.
+ */
+template<typename scalar_t, int width>
+__global__ std::enable_if_t<
+  (width == 0) || !_typeConvert<scalar_t>::exists> fused_add_rms_norm_kernel(
   scalar_t* __restrict__ input,           // [..., hidden_size]
   scalar_t* __restrict__ residual,        // [..., hidden_size]
   const scalar_t* __restrict__ weight,    // [hidden_size]
@@ -48,12 +248,17 @@ __global__ void fused_add_rms_norm_kernel(
   float variance = 0.0f;
 
   for (int idx = threadIdx.x; idx < hidden_size; idx += blockDim.x) {
-    float x = (float) input[blockIdx.x * hidden_size + idx];
-    x += (float) residual[blockIdx.x * hidden_size + idx];
+    scalar_t z = input[blockIdx.x * hidden_size + idx];
+    z += residual[blockIdx.x * hidden_size + idx];
+    float x = (float) z;
     variance += x * x;
-    residual[blockIdx.x * hidden_size + idx] = (scalar_t) x;
+    residual[blockIdx.x * hidden_size + idx] = z;
   }
-  variance = blockReduceSum<float>(variance);
+  /* Keep the following if-else block in sync with the
+     calculation of max_block_size in fused_add_rms_norm */ 
+  if (num_tokens < 256) {
+    variance = blockReduceSum<float, 1024>(variance);
+  } else variance = blockReduceSum<float, 256>(variance);
   if (threadIdx.x == 0) {
     s_variance = rsqrtf(variance / hidden_size + epsilon);
   }
@@ -93,6 +298,21 @@ void rms_norm(
     });
 }
 
+#define LAUNCH_FUSED_ADD_RMS_NORM(width)              \
+  VLLM_DISPATCH_FLOATING_TYPES(                       \
+    input.scalar_type(),                              \
+    "fused_add_rms_norm_kernel",                      \
+    [&] {                                             \
+      vllm::fused_add_rms_norm_kernel                 \
+      <scalar_t, width><<<grid, block, 0, stream>>>(  \
+        input.data_ptr<scalar_t>(),                   \
+        residual.data_ptr<scalar_t>(),                \
+        weight.data_ptr<scalar_t>(),                  \
+        epsilon,                                      \
+        num_tokens,                                   \
+        hidden_size);                                 \
+    });
+
 void fused_add_rms_norm(
   torch::Tensor& input,    // [..., hidden_size]
   torch::Tensor& residual, // [..., hidden_size]
@@ -102,19 +322,29 @@ void fused_add_rms_norm(
   int num_tokens = input.numel() / hidden_size;
 
   dim3 grid(num_tokens);
-  dim3 block(std::min(hidden_size, 1024));
+  /* This kernel is memory-latency bound in many scenarios.
+     When num_tokens is large, a smaller block size allows
+     for increased block occupancy on CUs and better latency
+     hiding on global mem ops. */
+  const int max_block_size = (num_tokens < 256) ? 1024 : 256;
+  dim3 block(std::min(hidden_size, max_block_size));
   const at::cuda::OptionalCUDAGuard device_guard(device_of(input));
   const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
-  VLLM_DISPATCH_FLOATING_TYPES(
-    input.scalar_type(),
-    "fused_add_rms_norm_kernel",
-    [&] {
-      vllm::fused_add_rms_norm_kernel<scalar_t><<<grid, block, 0, stream>>>(
-        input.data_ptr<scalar_t>(),
-        residual.data_ptr<scalar_t>(),
-        weight.data_ptr<scalar_t>(),
-        epsilon,
-        num_tokens,
-        hidden_size);
-    });
+  /*If the tensor types are FP16/BF16, try to use the optimized kernel
+    with packed + vectorized ops.
+    Max optimization is achieved with a width-8 vector of FP16/BF16s
+    since we can load at most 128 bits at once in a global memory op.
+    However, this requires each tensor's data to be aligned to 16
+    bytes.
+   */
+  auto inp_ptr = reinterpret_cast<std::uintptr_t>(input.data_ptr());
+  auto res_ptr = reinterpret_cast<std::uintptr_t>(residual.data_ptr());
+  auto wt_ptr = reinterpret_cast<std::uintptr_t>(weight.data_ptr());
+  bool ptrs_are_aligned = inp_ptr % 16 == 0 && res_ptr % 16 == 0 \
+                          && wt_ptr % 16 == 0;
+  if (ptrs_are_aligned && hidden_size % 8 == 0) {
+    LAUNCH_FUSED_ADD_RMS_NORM(8);
+  } else {
+    LAUNCH_FUSED_ADD_RMS_NORM(0);
+  }
 }

csrc/reduction_utils.cuh

@@ -20,43 +20,45 @@
 #include "cuda_compat.h"
 
 namespace vllm {
-
-template<typename T>
+template<typename T, int numLanes = WARP_SIZE>
 __inline__ __device__ T warpReduceSum(T val) {
-#pragma unroll
-  for (int mask = WARP_SIZE/2; mask > 0; mask >>= 1)
+  static_assert(numLanes > 0 && (numLanes & (numLanes - 1)) == 0,
+                "numLanes is not a positive power of 2!");
+  static_assert(numLanes <= WARP_SIZE);
+  #pragma unroll
+  for (int mask = numLanes >> 1; mask > 0; mask >>= 1)
     val += VLLM_SHFL_XOR_SYNC(val, mask);
   return val;
 }
 
-__inline__ __device__ constexpr int _calculateLaneMask(int warp_size) {
-  return warp_size - 1;
-}
-
-__inline__ __device__ constexpr int _calculateWidShift(int warp_size) {
-  return 5 + (warp_size >> 6);
+// Helper function to return the next largest power of 2
+static constexpr int _nextPow2(unsigned int num) {
+  if (num <= 1) return num;
+  return 1 << (CHAR_BIT * sizeof(num) - __builtin_clz(num - 1));
 }
 
 /* Calculate the sum of all elements in a block */
-template<typename T>
+template<typename T, int maxBlockSize = 1024>
 __inline__ __device__ T blockReduceSum(T val) {
-  static __shared__ T shared[WARP_SIZE];
-  constexpr auto LANE_MASK = _calculateLaneMask(WARP_SIZE);
-  constexpr auto WID_SHIFT = _calculateWidShift(WARP_SIZE);
-  int lane = threadIdx.x & LANE_MASK;
-  int wid = threadIdx.x >> WID_SHIFT;
-
-  val = warpReduceSum<T>(val);
-
-  if (lane == 0)
-    shared[wid] = val;
+  static_assert(maxBlockSize <= 1024);
+  if constexpr (maxBlockSize > WARP_SIZE) {
+    val = warpReduceSum<T>(val);
+    // Calculates max number of lanes that need to participate in the last warpReduce
+    constexpr int maxActiveLanes = (maxBlockSize + WARP_SIZE - 1) / WARP_SIZE;
+    static __shared__ T shared[maxActiveLanes];
+    int lane = threadIdx.x % WARP_SIZE;
+    int wid = threadIdx.x / WARP_SIZE;
+    if (lane == 0)
+      shared[wid] = val;
 
-  __syncthreads();
+    __syncthreads();
 
-  // Modify from blockDim.x << 5 to blockDim.x / 32. to prevent
-  // blockDim.x is not divided by 32
-  val = (threadIdx.x < (blockDim.x / (WARP_SIZE * 1.0f))) ? shared[lane] : (T)(0.0f);
-  val = warpReduceSum<T>(val);
+    val = (threadIdx.x < blockDim.x / float(WARP_SIZE)) ? shared[lane] : (T)(0.0f);
+    val = warpReduceSum<T, _nextPow2(maxActiveLanes)>(val);
+  } else {
+    // A single warpReduce is equal to blockReduce
+    val = warpReduceSum<T, _nextPow2(maxBlockSize)>(val);
+  }
   return val;
 }