[webgpu] Enable indirect dispatch for flash attention (#26207)

This pull request introduces support for indirect dispatch in the WebGPU
FlashAttention implementation, enabling more dynamic and efficient
kernel launches based on runtime sequence lengths. The changes add new
logic and parameters to propagate sequence length information and
indirect dispatch buffers through the attention pipeline, with
conditional code paths to maintain compatibility with the existing
direct dispatch approach.

It's part of the work to enable graph capture in phi4
#25868

Author: qjia7

onnxruntime/contrib_ops/webgpu/bert/flash_attention.cc

@@ -17,19 +17,24 @@ using namespace onnxruntime::webgpu;
 
 class CopyKVCacheProgram final : public Program<CopyKVCacheProgram> {
  public:
-  CopyKVCacheProgram(const std::string& kernel_name, bool has_past, bool kv_BNSH, bool past_present_share_buffer)
-      : Program{kernel_name}, has_past_(has_past), kv_BNSH_(kv_BNSH), past_present_share_buffer_(past_present_share_buffer) {
+  CopyKVCacheProgram(const std::string& kernel_name, bool has_past, bool kv_BNSH, bool past_present_share_buffer,
+                     bool prepare_indirect_dispatch = false)
+      : Program{kernel_name}, has_past_(has_past), kv_BNSH_(kv_BNSH), past_present_share_buffer_(past_present_share_buffer), prepare_indirect_dispatch_(prepare_indirect_dispatch) {
   }
 
   Status GenerateShaderCode(ShaderHelper& sh) const override;
 
   WEBGPU_PROGRAM_DEFINE_UNIFORM_VARIABLES({"copy_size", ProgramUniformVariableDataType::Uint32},
-                                          {"past_sequence_length", ProgramUniformVariableDataType::Uint32});
+                                          {"total_sequence_length", ProgramUniformVariableDataType::Uint32},
+                                          {"kv_sequence_length", ProgramUniformVariableDataType::Uint32},
+                                          {"tile_size", ProgramUniformVariableDataType::Uint32},
+                                          {"num_heads", ProgramUniformVariableDataType::Uint32});
 
  private:
   bool has_past_;
   bool kv_BNSH_;
   bool past_present_share_buffer_;
+  bool prepare_indirect_dispatch_;
 };
 
 class FlashAttentionProgram final : public Program<FlashAttentionProgram> {
@@ -75,8 +80,8 @@ class FlashAttentionProgram final : public Program<FlashAttentionProgram> {
 class FlashAttentionDecodeQKTProgram final : public Program<FlashAttentionDecodeQKTProgram> {
  public:
   FlashAttentionDecodeQKTProgram(const std::string& kernel_name,
-                                 bool has_attention_bias, uint32_t tile_size)
-      : Program{kernel_name}, has_attention_bias_(has_attention_bias), tile_size_(tile_size) {
+                                 bool has_attention_bias, uint32_t tile_size, bool use_indirect_dispatch)
+      : Program{kernel_name}, has_attention_bias_(has_attention_bias), tile_size_(tile_size), use_indirect_dispatch_(use_indirect_dispatch) {
   }
 
   Status GenerateShaderCode(ShaderHelper& sh) const override;
@@ -86,19 +91,19 @@ class FlashAttentionDecodeQKTProgram final : public Program<FlashAttentionDecode
                                           {"alpha", ProgramUniformVariableDataType::Float32},
                                           {"present_sequence_length", ProgramUniformVariableDataType::Uint32},
                                           {"n_reps", ProgramUniformVariableDataType::Uint32},
-                                          {"num_total_seq_length_tile", ProgramUniformVariableDataType::Uint32},
                                           {"num_present_sequence_length_tile", ProgramUniformVariableDataType::Uint32},
                                           {"num_heads", ProgramUniformVariableDataType::Uint32});
 
  private:
   bool has_attention_bias_;
   uint32_t tile_size_;
+  bool use_indirect_dispatch_;
 };
 
 class FlashAttentionDecodeSplitVxProgram final : public Program<FlashAttentionDecodeSplitVxProgram> {
  public:
-  FlashAttentionDecodeSplitVxProgram(const std::string& kernel_name, uint32_t tile_size, int head_size_vec)
-      : Program{kernel_name}, tile_size_(tile_size), head_size_vec_(head_size_vec) {
+  FlashAttentionDecodeSplitVxProgram(const std::string& kernel_name, uint32_t tile_size, int head_size_vec, bool use_indirect_dispatch)
+      : Program{kernel_name}, tile_size_(tile_size), head_size_vec_(head_size_vec), use_indirect_dispatch_(use_indirect_dispatch) {
   }
 
   Status GenerateShaderCode(ShaderHelper& sh) const override;
@@ -107,19 +112,19 @@ class FlashAttentionDecodeSplitVxProgram final : public Program<FlashAttentionDe
                                           {"head_size_vec", ProgramUniformVariableDataType::Uint32},
                                           {"present_sequence_length", ProgramUniformVariableDataType::Uint32},
                                           {"n_reps", ProgramUniformVariableDataType::Uint32},
-                                          {"num_total_seq_length_tile", ProgramUniformVariableDataType::Uint32},
                                           {"num_present_sequence_length_tile", ProgramUniformVariableDataType::Uint32},
                                           {"num_heads", ProgramUniformVariableDataType::Uint32});
 
  private:
   uint32_t tile_size_;
   int head_size_vec_;
+  bool use_indirect_dispatch_;
 };
 
 class FlashAttentionDecodeVxReduceProgram final : public Program<FlashAttentionDecodeVxReduceProgram> {
  public:
-  FlashAttentionDecodeVxReduceProgram(const std::string& kernel_name, uint32_t tile_size)
-      : Program{kernel_name}, tile_size_(tile_size) {
+  FlashAttentionDecodeVxReduceProgram(const std::string& kernel_name, uint32_t tile_size, uint32_t seq_tile_size, bool use_indirect_dispatch)
+      : Program{kernel_name}, tile_size_(tile_size), seq_tile_size_(seq_tile_size), use_indirect_dispatch_(use_indirect_dispatch) {
   }
 
   Status GenerateShaderCode(ShaderHelper& sh) const override;
@@ -132,11 +137,13 @@ class FlashAttentionDecodeVxReduceProgram final : public Program<FlashAttentionD
 
  private:
   uint32_t tile_size_;
+  uint32_t seq_tile_size_;
+  bool use_indirect_dispatch_;
 };
 
 Status ApplyFlashAttention(const Tensor* Q, const Tensor* K, const Tensor* V, const Tensor* attention_bias,
                            Tensor* output, const Tensor* past_key, Tensor* present_key, const Tensor* past_value, Tensor* present_value,
-                           const WebgpuAttentionParameters& parameters, onnxruntime::webgpu::ComputeContext& context);
+                           const WebgpuAttentionParameters& parameters, onnxruntime::webgpu::ComputeContext& context, const Tensor* seqlen_k = nullptr);
 
 bool CanApplyFlashAttention(const Tensor* bias, const Tensor* present_key, const Tensor* present_value,
                             const WebgpuAttentionParameters& parameters, onnxruntime::webgpu::ComputeContext& context);

onnxruntime/contrib_ops/webgpu/bert/flash_attention.h

@@ -5,6 +5,7 @@
 #param tile_size
 #param tile_size_k_vec
 #param sub_tile_count
+#param use_indirect_dispatch
 
 // Note that this shader adopts similar algorithm with dp4a generation shader.
 //
@@ -48,10 +49,15 @@ var<workgroup> tile_qk: array<q_element_t, tile_size>;
 $MAIN {
   let local_row = u32(local_idx / tile_size_k_vec);
   let local_col = local_idx % tile_size_k_vec;
-  let total_seq_offset = (workgroup_idx % uniforms.num_total_seq_length_tile) * tile_size;
-  let head_idx = u32(workgroup_idx / uniforms.num_total_seq_length_tile);
+#if use_indirect_dispatch
+  let total_sequence_length = u32(seqlens_k[0]) + 1u;
+#else
+  let total_sequence_length = uniforms.total_sequence_length;
+#endif
+  let num_total_seq_length_tile = (total_sequence_length + tile_size - 1) / tile_size;
+  let total_seq_offset = (workgroup_idx % num_total_seq_length_tile) * tile_size;
+  let head_idx = u32(workgroup_idx / num_total_seq_length_tile);
   let q_offset = head_idx * uniforms.head_size_vec;
-  var total_sequence_length = uniforms.total_sequence_length;
   let present_offset = u32(head_idx / uniforms.n_reps) * uniforms.present_sequence_length * uniforms.head_size_vec;
   for (var k: u32 = 0u; k < uniforms.head_size_vec; k += tile_size_k_vec) {
     if (local_idx < tile_size_k_vec && k + local_idx < uniforms.head_size_vec) {
@@ -95,7 +101,7 @@ $MAIN {
     for (var i = 0u; i < tile_size && (total_seq_offset + i) < total_sequence_length; i++) {
       l_sum += exp(f32(tile_qk[i]) - l_max);
     }
-    let meta_offset = head_idx * uniforms.num_present_sequence_length_tile + workgroup_idx % uniforms.num_total_seq_length_tile;
+    let meta_offset = head_idx * uniforms.num_present_sequence_length_tile + workgroup_idx % num_total_seq_length_tile;
     metadata[meta_offset] = metadata_value_t(l_max, l_sum);
   }
 }

onnxruntime/contrib_ops/webgpu/bert/flash_attention_decode_qkt.wgsl.template

@@ -5,6 +5,7 @@
 #param head_size_vec
 #param tile_size_k_vec
 #param sub_tile_count
+#param use_indirect_dispatch
 
 // Note that this shader adopts similar algorithm with dp4a generation shader.
 //
@@ -40,22 +41,27 @@ var<workgroup> qkv_values: array<array<present_value_value_t, tile_size_k_vec>,
 $MAIN {
   let local_row = u32(local_idx / tile_size_k_vec);
   let local_col = local_idx % tile_size_k_vec;
-  let total_seq_offset = (workgroup_idx % uniforms.num_total_seq_length_tile) * tile_size;
-  let head_idx = u32(workgroup_idx / uniforms.num_total_seq_length_tile);
-  var total_sequence_length = uniforms.total_sequence_length;
+  #if use_indirect_dispatch
+  let total_sequence_length = u32(seqlens_k[0]) + 1u;
+  #else
+  let total_sequence_length = uniforms.total_sequence_length;
+  #endif
+  let num_total_seq_length_tile = (total_sequence_length + tile_size - 1) / tile_size;
+  let total_seq_offset = (workgroup_idx % num_total_seq_length_tile) * tile_size;
+  let head_idx = u32(workgroup_idx / num_total_seq_length_tile);
   let present_offset = u32(head_idx / uniforms.n_reps) * head_size_vec * uniforms.present_sequence_length;
 
   // Calculate the global max and sum in qk.
   if (head_idx < uniforms.num_heads)
   {
     var g_max = f32(-3.402823e+38f);
     var g_sum = f32(0);
-    for (var i = 0u; i < uniforms.num_total_seq_length_tile; i++)
+    for (var i = 0u; i < num_total_seq_length_tile; i++)
     {
       let meta_offset = head_idx * uniforms.num_present_sequence_length_tile + i;
       g_max = max(g_max, metadata[meta_offset].x);
     }
-    for (var i = 0u; i < uniforms.num_total_seq_length_tile; i++)
+    for (var i = 0u; i < num_total_seq_length_tile; i++)
     {
       let meta_offset = head_idx * uniforms.num_present_sequence_length_tile + i;
       let m_value = metadata[meta_offset];
@@ -95,7 +101,7 @@ $MAIN {
   }
 
   for (var i = local_idx; i < head_size_vec; i += workgroup_size_x) {
-    let out_offset = head_idx * uniforms.num_present_sequence_length_tile * head_size_vec + (workgroup_idx % uniforms.num_total_seq_length_tile) * head_size_vec + i;
+    let out_offset = head_idx * uniforms.num_present_sequence_length_tile * head_size_vec + (workgroup_idx % num_total_seq_length_tile) * head_size_vec + i;
     out_split_vx[out_offset] = tile_output[i];
   }
 }

onnxruntime/contrib_ops/webgpu/bert/flash_attention_decode_split_vx.wgsl.template

@@ -1,7 +1,9 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+#param seq_tile_size
 #param tile_size
+#param use_indirect_dispatch
 
 // Inputs are splits of the GQA output, split into num_total_seq_length_tiles
 // rows. This shader needs to add these splits across the row dimension to
@@ -23,10 +25,16 @@ $MAIN {
   var value = output_value_t(0);
   let local_row = u32(local_idx / tile_size);
   let local_col = local_idx % tile_size;
+  #if use_indirect_dispatch
+  let total_sequence_length = u32(seqlens_k[0]) + 1u;
+  let num_total_seq_length_tile = (total_sequence_length + seq_tile_size - 1) / seq_tile_size;
+  #else
+  let num_total_seq_length_tile = uniforms.num_total_seq_length_tile;
+  #endif
 
   if (head_size_offset + local_col < uniforms.head_size_vec) {
-    for (var r = 0u; r < uniforms.num_total_seq_length_tile; r += tile_size) {
-      if (r + local_row < uniforms.num_total_seq_length_tile) {
+    for (var r = 0u; r < num_total_seq_length_tile; r += tile_size) {
+      if (r + local_row < num_total_seq_length_tile) {
         value += input[in_offset + (r + local_row) * uniforms.head_size_vec + head_size_offset + local_col];
       }
     }

onnxruntime/contrib_ops/webgpu/bert/flash_attention_decode_vx_reduce.wgsl.template

@@ -206,7 +206,7 @@ Status GroupQueryAttention::ComputeInternal(onnxruntime::webgpu::ComputeContext&
       !use_sliding_window &&
       CanApplyFlashAttention(attention_bias, present_key, present_value, parameters, context)) {
     return ApplyFlashAttention(query, key, value, attention_bias, output, past_key, present_key, past_value,
-                               present_value, parameters, context);
+                               present_value, parameters, context, seqlen_k);
   }
 
   Tensor qSplit;

onnxruntime/contrib_ops/webgpu/bert/group_query_attention.cc

@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "core/framework/execution_provider.h"
+#include "core/providers/webgpu/webgpu_execution_provider.h"
 
 #include "core/providers/webgpu/program.h"
 #include "core/providers/webgpu/webgpu_context.h"
@@ -16,7 +17,6 @@
 namespace onnxruntime {
 
 class Tensor;
-class WebGpuExecutionProvider;
 
 namespace webgpu {
 
@@ -42,6 +42,9 @@ class ComputeContext {
   inline bool HasFeature(wgpu::FeatureName feature) const {
     return webgpu_context_.DeviceHasFeature(feature);
   }
+  inline bool IsGraphCaptureEnabled() const {
+    return ep_.IsGraphCaptureEnabled();
+  }
 #if !defined(__wasm__)
   inline const wgpu::AdapterPropertiesSubgroupMatrixConfigs& SubgroupMatrixConfigs() const {
     return webgpu_context_.SubgroupMatrixConfigs();