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[CK_TILE] Use Unified Workspace for FMHA BWD
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## Motivation
`dq_acc` is the intermediate accumulation buffer used in FMHA backward
pass for deterministic mode. The current implementation allocates it as
a **single rectangular tensor**:
```
shape = [shape_batch, nhead, nsplits, shape_seqlen_q, hdim_q]
```
where `nsplits = launcher.dq_acc_splits` (a single scalar), computed
from `max_seqlen_k` and shared across all batches.
### Problems
1. **Memory waste**: In group mode, each batch may have a different
`seqlen_k`, but `nsplits` is computed from `max_seqlen_k`, causing
batches with shorter `seqlen_k` to over-allocate in the split dimension.
2. **Interface coupling**: `fmha_bwd_args` exposes internal layout
details such as `stride_dq_acc`, `nhead_stride_dq_acc`,
`batch_stride_dq_acc`, and `split_stride_dq_acc`. The caller is
responsible for computing these strides, but this logic belongs inside
the kernel.
### Goals
1. Switch `dq_acc` buffer to a **compact layout**: batches are
concatenated contiguously, with each batch occupying `nhead * nsplits_i
* seqq_i * hdim_q` elements (nhead outermost).
2. **Remove all `*_stride_dq_acc` fields** from `fmha_bwd_args`,
replacing them with a single `workspace_ptr`; the kernel splits this
internally using a fixed layout.
4. `fmha_bwd_launcher` provides a **workspace management interface**:
the caller only needs to allocate GPU memory and call
`prepare_workspace()` — no layout computation required.
5. **Isolate kernel internals from the caller API**: the `dq_acc` layout
(nsplits, strides, buffer size) is determined entirely inside the
launcher/kernel. Future changes to block shape, pipeline type, or
persistent kernel strategy require no modifications to the caller's
`fmha_bwd_args` or workspace allocation logic.
## Technical Details
### Interface Design
#### New fields in `fmha_bwd_traits`
```cpp
struct fmha_bwd_traits
{
int seqlen_q;
int seqlen_k;
int batch;
int max_seqlen_q;
int max_seqlen_k;
int hdim_q;
int hdim_v;
int nhead_q;
int nhead_k;
std::string data_type;
bool is_group_mode;
mask_enum mask_type;
bias_enum bias_type;
bool has_dbias;
bool has_dropout;
bool is_store_randval;
bool is_deterministic;
// New: cumulative physical seqlen pointers for group mode (pass nullptr for batch mode).
// seqstart_qs[i+1] - seqstart_qs[i] = physical seqlen_q of batch i (including padding); length = batch+1
// seqstart_ks[i+1] - seqstart_ks[i] = physical seqlen_k of batch i (including padding); length = batch+1
const int* seqstart_qs = nullptr;
const int* seqstart_ks = nullptr;
};
```
#### `fmha_bwd_launcher` actual structure
```cpp
struct fmha_bwd_launcher
{
std::function<float(fmha_bwd_args, const ck_tile::stream_config&)> run{};
// Total workspace size in bytes (host_ws_size + device_ws_size), computed by init().
// Zero for kUseQrQtrDorPipeline (writes dq directly, no acc buffer needed).
size_t workspace_size = 0;
fmha_bwd_launcher(const fmha_bwd_traits&);
// Copies auxiliary data (nsplits[], offsets[]) via hipMemcpy to the head of the GPU workspace,
// and zeros the dq_acc buffer portion (tail of workspace) if required.
// The memory pointed to by device_ws must be >= workspace_size bytes.
std::function<void(void* device_ws)> prepare_workspace{};
template <typename... Args>
float operator()(Args&&... args) const { return run(std::forward<Args>(args)...); }
private:
size_t host_ws_size = 0; // CPU workspace size (nsplits[] + offsets[] arrays)
size_t device_ws_size = 0; // GPU-only data size (dq_acc buffer)
std::unique_ptr<char[]> ws_host; // host-side workspace buffer
public:
template <typename T0, typename T1, typename T2, typename Arch>
void init(const fmha_bwd_traits& traits);
};
```
The `init<>()` template method (invoked by codegen dispatch branches as
`this->init<...>(t)`) is responsible for:
1. Setting the `run` lambda
2. Calling `FmhaBwdDQDKDVKernel::GetWorkspaceHostSize(batch)` to obtain
`host_ws_size`
3. Allocating `ws_host` (host memory)
4. Calling `FmhaBwdDQDKDVKernel::PrepareWorkspaceHost(ws_host.get(),
...)` to fill nsplits/offsets; return value is `device_ws_size`
5. `workspace_size = host_ws_size + device_ws_size`
6. Setting the `prepare_workspace` lambda (captures `this`, calls
`PrepareWorkspaceDevice`)
When no kernel matches the given traits, both `run` and
`prepare_workspace` are initialized to default lambdas that print a
warning to `std::cerr` and return gracefully (no exception).
#### Workspace overall layout
The workspace is managed by `FmhaBwdWorkspaceManager` and consists of
two segments:
```
Offset 0 (CPU-prepared segment, host_ws_size bytes; also hipMemcpy'd to the head of GPU workspace):
index_t nsplits[batch or 1] — per-batch nsplits array
group mode: batch elements
batch mode / non-deterministic: 1 element
[group mode only] long_index_t dq_acc_offsets[batch+1]
— per-batch element offset (inclusive prefix sum)
offsets[0]=0, offsets[i+1] = offsets[i] + nhead*nsplits_i*seqq_i*hdim_q
Offset host_ws_size (device data segment, device_ws_size bytes):
AccDataType dq_acc[total_elements] — compact dq_acc buffer (zeroed if required)
total_elements = sum_i(nhead * nsplits_i * seqq_i * hdim_q)
layout within each batch: [nhead, nsplits_i, seqq_i, hdim_q]
note: seqq_i uses the physical length (including padding)
```
Alignment constant (`ALIGNMENT = 16`):
```
nsplits_size = align_up(sizeof(index_t) * N, 16) // N = batch (group) or 1 (batch/non-det)
offsets_size = align_up(sizeof(long_index_t) * (batch+1), 16) // group mode only
host_ws_size = nsplits_size + offsets_size
dq_acc_offset = host_ws_size // GetDqAccDataOffset(batch)
```
**Key benefits**:
- The kernel reads nsplits/offsets directly from the workspace head — no
device-side recomputation.
- `FmhaBwdConvertQGradKernel` is completely decoupled from the pipeline
block shape (`kN0`): nsplits is read from `nsplits_ptr`, `kN0` is no
longer a template parameter, and multiple dq_dk_dv tiles with different
`F_bn0` values now share a single convert_dq kernel instance (under
receipt 1/2, deterministic convert_dq kernel count drops from ~300 to
60).
- nsplits/offsets are computed on the host and transferred in one
`hipMemcpy`; the dq_acc buffer follows immediately, at the offset given
by `GetDqAccDataOffset`.
#### Workspace size by scenario
| Scenario | `workspace_size` | Notes |
|----------|-----------------|-------|
| **kUseQrQtrDorPipeline** (any mode) | `0` | Writes dq directly; no acc
buffer; `PrepareWorkspaceHost` returns 0 |
| **Non-deterministic + batch mode** | `> 0` | nsplits[1]=1; dq_acc used
for atomic add; `workspace_size = host_ws_size +
batch*nhead*seqlen_q*hdim_q*ebytes` |
| **Non-deterministic + group mode** | `> 0` | nsplits[1]=1; dq_acc
contiguous layout; `workspace_size = host_ws_size +
nhead*seqstart_qs[batch]*hdim_q*ebytes` |
| **Deterministic + group mode** | `> 0` | nsplits[batch],
offsets[batch+1], compact dq_acc; nsplits_i computed independently per
batch |
| **Deterministic + batch mode persistent** | `> 0` | nsplits[1]
(uniform across batches); dq_acc `batch*nhead*nsplits*seqlen_q*hdim_q` |
**NeedsZeroDqAcc** (determines whether `PrepareWorkspaceDevice` calls
`hipMemset`):
- Persistent kernel (deterministic batch mode) or non-deterministic:
**must zero** (atomic add requires zero initialization)
- Deterministic group mode + no mask: **no zeroing needed** (every tile
writes its full region)
- Deterministic + with mask: **must zero** (some blocks are skipped,
leaving uninitialized tiles that would contribute to the reduction)
#### Caller usage
```cpp
// 1. Create launcher (traits include seqstart_qs/ks pointers; workspace_size is computed during construction)
fmha_bwd_launcher launcher(fmha_traits);
// 2. Read launcher.workspace_size directly
const auto ws_size = launcher.workspace_size;
// 3. Allocate a single GPU workspace
ck_tile::DeviceMem ws_buf(ws_size);
// 4. Copy nsplits/offsets to GPU head and zero dq_acc if required
launcher.prepare_workspace(ws_buf.GetDeviceBuffer());
// 5. Build args with a single workspace pointer; the kernel splits it internally
fmha_bwd_args args{
...,
ws_size > 0 ? ws_buf.GetDeviceBuffer() : nullptr, // workspace_ptr
};
launcher(args, stream_config);
```
Composable Kernel Tile
concept
ck_tile provides a programming model with templated abstractions to enable users to implement performance-critical kernels for machine learning workloads. introduces following basic concepts to help users building your own operator
- tensor coordinate transformation, this is the core concept of layout/index transform abstraction in both compiler time and run time.
- tile-based programming model, including tile-level api and the concept of distributed tensor.
ck_tile is independently from the old ck, located under /include/ck_tile. You don't need to include anything from old CK, ck_tile has similiar (indeed almost the same) implementations for users to build operators. We will have a transition period to pull everything from old ck into ck_tile, stay tuned.
component
ck_tile is splitted into several componenets including core, host, ops/gemm, ops/fmha... each component you only need to include a single header (e.g #include "ck_tile/core.hpp", #include "ck_tile/ops/fmha.hpp") then you are able to use the function/structure inside (different from old ck)
[core]
ck_tile/core contains all the basic data structure and function to build the kernel, you can only include this header and build your own operators that utilizing all the basic building blocks introduced in ck.
core/container
- array, store runtime variables with fixed length (tensor index, register buffer, etc...)
- tuple, same as std::tuple, hold different type of data, and one of the solution to achieve multiple buffer.
- sequence, compile time integer sequence used to build various internal structures, or to describe tile size
- other convenient structure build on top of above 3
core/numeric
- gpu data type like
fp16_t,bf16_t,fp8_t... and the conversion between each other - constexpr integer similiar to std::integral_constant to be used as compile time integer.
- math functions and numeric utilities
core/algorithm
- coordinate transformation system, used to build tensor transform and compile time indexing. This is the core idea introduced in old
ckto describe how a tensor is build by several basic transform primitives likemerge/unmerge/embedetc... and how we indexing into a ND tensor that finally mapped to 1D memory offset.
core/tensor
- tensor descriptor, to describe how a ND tensor
- distributed tensor, describe the storage of this tensor, and the distribution of how a collection of threads collaborately work for this tensor.
- tile level API, including
load_tile,store_tile,shuffle_tile,slice_tile, etc...
[host]
ck_tile/host contains all the host side utilities to launch a kernel, create the device buffer, and some reference implementations. This can be used to create examples (like that under ck_tile example folder) and simple executable to invoke this kernel, so if you only need ck_tile to build your own device library then it's OK to not include this. Based on this, it is recommended to include the specific header you needed under this folder to avoid including unwanted headers (e.g, only include ck_tile/host/kernel_launch.hpp), unless you are writing a host executable.
[ops/gemm, ops/fmha, ops/reduce...]
our implementation of different device operators.
- warp, warp tile level operator
- block, block tile level operator
- pipeline, pipeline that can achieve a customized tile level mainloop (or epilogue). By switching different pipeline to the kernel template you can have different kind of pipeline optimizations.
- kernel, template interface for users to instantiate a particular kernel
[ops/epilogue]
epilogue part of our kernel. We may extend this epilogue part to let users to build their own cutomized epilogues.
[ref]
reference implementation of cpu or gpu. This folder is supposed to include a specific header on demand.
examples
currently we put all ck_tile related example under /example/ck_tile folder. Please check each example's subfolder.