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ggml-webgpu: Fix bug in FlashAttention support check #22492

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Merged
ggerganov merged 2 commits into from
Apr 29, 2026
Merged

ggml-webgpu: Fix bug in FlashAttention support check #22492

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70a4f4f
Fix flashattention support check for devices that don't support subgr…
reeselevine Apr 28, 2026
49db7ee
set path to none if kv_tile doesn't fit
reeselevine Apr 29, 2026
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44 changes: 27 additions & 17 deletions ggml/src/ggml-webgpu/ggml-webgpu-shader-lib.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -494,9 +494,10 @@ struct ggml_webgpu_unary_pipeline_key_hash {
/** FlashAttention */

enum ggml_webgpu_flash_attn_path : uint32_t {
GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX = 0u,
GGML_WEBGPU_FLASH_ATTN_PATH_TILE = 1u,
GGML_WEBGPU_FLASH_ATTN_PATH_VEC = 2u,
GGML_WEBGPU_FLASH_ATTN_PATH_NONE = 0u,
GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX = 1u,
GGML_WEBGPU_FLASH_ATTN_PATH_TILE = 2u,
GGML_WEBGPU_FLASH_ATTN_PATH_VEC = 3u,
};

struct ggml_webgpu_flash_attn_pipeline_key {
Expand Down Expand Up @@ -534,7 +535,7 @@ struct ggml_webgpu_flash_attn_pipeline_key_hash {
};

struct ggml_webgpu_flash_attn_decisions {
uint32_t path = GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX;
uint32_t path = GGML_WEBGPU_FLASH_ATTN_PATH_NONE;
uint32_t q_tile = 0;
uint32_t kv_tile = 0;
uint32_t wg_size = 0;
Expand Down Expand Up @@ -709,19 +710,29 @@ inline ggml_webgpu_flash_attn_decisions ggml_webgpu_flash_attn_get_decisions(
(context.src0->ne[0] % GGML_WEBGPU_FLASH_ATTN_TILE_KV_VEC_WIDTH == 0) &&
(context.src2->ne[0] % GGML_WEBGPU_FLASH_ATTN_TILE_KV_VEC_WIDTH == 0) && !use_vec;

decisions.path = use_vec ? GGML_WEBGPU_FLASH_ATTN_PATH_VEC :
use_tile ? GGML_WEBGPU_FLASH_ATTN_PATH_TILE :
GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX;
decisions.path = use_vec ? GGML_WEBGPU_FLASH_ATTN_PATH_VEC :
use_tile ? GGML_WEBGPU_FLASH_ATTN_PATH_TILE :
context.supports_subgroup_matrix ? GGML_WEBGPU_FLASH_ATTN_PATH_SUBGROUP_MATRIX :
GGML_WEBGPU_FLASH_ATTN_PATH_NONE;

if (decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_NONE) {
return decisions;
}

const ggml_webgpu_flash_attn_pipeline_key key = ggml_webgpu_flash_attn_make_pipeline_key(context, decisions.path);
decisions.kv_direct = key.kv_direct;
const uint32_t max_kv_tile = ggml_webgpu_flash_attn_max_kv_tile(context, key);
// invalidate if even the smallest kv_tile doesn't fit in shared memory
if (max_kv_tile == 0) {
decisions.path = GGML_WEBGPU_FLASH_ATTN_PATH_NONE;
return decisions;
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}

if (decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_VEC) {
const uint32_t min_kv_tile = ggml_webgpu_flash_attn_max_kv_tile(context, key);
decisions.q_tile = 1u;
decisions.kv_tile = std::max(8u, std::min(32u, min_kv_tile));
decisions.kv_tile = (decisions.kv_tile / 8u) * 8u;
decisions.wg_size = std::max(1u, std::min<uint32_t>(32u, context.max_subgroup_size));
decisions.q_tile = 1u;
decisions.kv_tile = std::max(8u, std::min(32u, max_kv_tile));
decisions.kv_tile = (decisions.kv_tile / 8u) * 8u;
decisions.wg_size = std::max(1u, std::min<uint32_t>(32u, context.max_subgroup_size));
if (decisions.kv_direct) {
decisions.kv_tile = std::min(decisions.kv_tile, GGML_WEBGPU_KV_SEQ_PAD);
while (GGML_WEBGPU_KV_SEQ_PAD % decisions.kv_tile != 0) {
Expand All @@ -734,9 +745,8 @@ inline ggml_webgpu_flash_attn_decisions ggml_webgpu_flash_attn_get_decisions(
decisions.q_tile =
decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_TILE ? GGML_WEBGPU_FLASH_ATTN_TILE_Q_TILE : context.sg_mat_m;
decisions.kv_tile = decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_TILE ?
std::min(64u, ggml_webgpu_flash_attn_max_kv_tile(context, key)) :
std::min(ggml_webgpu_flash_attn_max_kv_tile(context, key),
context.sg_mat_n * GGML_WEBGPU_FLASH_ATTN_PREFERRED_KV_SG_TILES);
std::min(64u, max_kv_tile) :
std::min(max_kv_tile, context.sg_mat_n * GGML_WEBGPU_FLASH_ATTN_PREFERRED_KV_SG_TILES);
decisions.wg_size = decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_TILE ?
GGML_WEBGPU_FLASH_ATTN_PREFERRED_WG_SIZE :
std::max(context.max_subgroup_size, GGML_WEBGPU_FLASH_ATTN_PREFERRED_WG_SIZE);
Expand All @@ -755,7 +765,6 @@ inline ggml_webgpu_flash_attn_decisions ggml_webgpu_flash_attn_get_decisions(
context.sg_mat_n;
}
}

return decisions;
}

Expand Down Expand Up @@ -1364,7 +1373,7 @@ class ggml_webgpu_shader_lib {
if (key.src_type == GGML_TYPE_Q1_0) {
defines.push_back("BLOCK_SIZE=128u");
} else if ((key.src_type >= GGML_TYPE_Q4_0 && key.src_type <= GGML_TYPE_Q8_1) ||
key.src_type == GGML_TYPE_IQ4_NL) {
key.src_type == GGML_TYPE_IQ4_NL) {
defines.push_back("BLOCK_SIZE=32u");
} else if (key.src_type >= GGML_TYPE_Q2_K) {
defines.push_back("BLOCK_SIZE=256u");
Expand Down Expand Up @@ -2325,6 +2334,7 @@ class ggml_webgpu_shader_lib {
size_t storage_offset_alignment) {
const ggml_webgpu_flash_attn_decisions decisions =
ggml_webgpu_flash_attn_get_decisions(context, storage_offset_alignment);
GGML_ASSERT(decisions.path != GGML_WEBGPU_FLASH_ATTN_PATH_NONE);
ggml_webgpu_flash_attn_pipeline_key key = ggml_webgpu_flash_attn_make_pipeline_key(context, decisions.path);
auto it = flash_attn_pipelines.find(key);
if (it != flash_attn_pipelines.end()) {
Expand Down
4 changes: 4 additions & 0 deletions ggml/src/ggml-webgpu/ggml-webgpu.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3918,6 +3918,10 @@ static bool ggml_backend_webgpu_device_supports_op(ggml_backend_dev_t dev, const
shader_lib_ctx, ctx->webgpu_global_ctx->capabilities.limits.minStorageBufferOffsetAlignment);
const size_t limit_bytes = ctx->webgpu_global_ctx->capabilities.limits.maxComputeWorkgroupStorageSize;
const bool has_mask = op->src[3] != nullptr;
if (decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_NONE) {
supports_op = false;
break;
}
if (decisions.path == GGML_WEBGPU_FLASH_ATTN_PATH_VEC) {
const size_t min_bytes =
ggml_webgpu_flash_attn_wg_mem_bytes(decisions.q_tile, decisions.kv_tile, (uint32_t) src0->ne[0],
Expand Down
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