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.claude/skills/add-sgl-kernel/SKILL.md

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+---
+name: add-sgl-kernel
+description: Step-by-step tutorial for adding a heavyweight AOT CUDA/C++ kernel to sgl-kernel (including tests & benchmarks)
+---
+
+# Tutorial: Adding a New Kernel to `sgl-kernel` (AOT / Heavyweight)
+
+This tutorial walks through adding a simple element-wise scale operation as an AOT kernel. We'll implement `scale(x, factor) = x * factor` to demonstrate the complete workflow.
+
+## Goal
+
+Add a new operation that scales each element of a tensor by a scalar factor:
+
+- Input: tensor `x` (CUDA) and scalar `factor` (float)
+- Output: `x * factor` (element-wise, in-place or into pre-allocated `out`)
+- Supported dtypes: **FP16 (`torch.float16`), BF16 (`torch.bfloat16`), FP32 (`torch.float32`)**
+  - Dispatched via `DISPATCH_PYTORCH_DTYPE_TO_CTYPE_FLOAT_FP16` macro (defined in `sgl-kernel/include/utils.h`)
+
+## Two rules of thumb (must follow)
+
+1. **Prefer `python/sglang/jit_kernel` first** when the kernel does **not** depend on CUTLASS or another large C++ project. This is the default path for lightweight kernels that benefit from rapid iteration.
+2. **Prefer `sgl-kernel`** when the kernel **does** depend on CUTLASS or another large C++ project, or when it should be part of the AOT wheel / torch op registration flow.
+3. **Exception**: if the dependency is `flashinfer`, or CUTLASS that is already provided through `flashinfer`, the kernel can still be implemented as `jit_kernel`.
+
+In addition, every new kernel must ship with:
+
+- **Tests** (pytest)
+- **A benchmark script** (triton.testing)
+
+---
+
+## Repository integration map
+
+You will typically touch these files/areas:
+
+- Implementation: `sgl-kernel/csrc/elementwise/scale.cu` (pick the right subdirectory)
+- Public declarations: `sgl-kernel/include/sgl_kernel_ops.h`
+- Torch extension registration: `sgl-kernel/csrc/common_extension.cc`
+- Build: `sgl-kernel/CMakeLists.txt` (`set(SOURCES ...)`)
+- Python API: `sgl-kernel/python/sgl_kernel/` and `sgl-kernel/python/sgl_kernel/__init__.py`
+- Tests: `sgl-kernel/tests/test_scale.py`
+- Benchmarks: `sgl-kernel/benchmark/bench_scale.py`
+
+---
+
+## Step 1: Implement the kernel in `csrc/`
+
+Pick the right subdirectory:
+
+- `csrc/elementwise/` — for element-wise ops (our example)
+- `csrc/gemm/`, `csrc/attention/`, `csrc/moe/` — for other categories
+
+Create `sgl-kernel/csrc/elementwise/scale.cu`:
+
+```cpp
+#include <ATen/cuda/CUDAContext.h>
+#include <c10/cuda/CUDAGuard.h>
+#include <torch/all.h>
+
+#include "utils.h"  // DISPATCH_PYTORCH_DTYPE_TO_CTYPE_FLOAT_FP16
+
+// scale_kernel: out[i] = input[i] * factor
+// Supports float, half (__half), __nv_bfloat16 via template T
+template <typename T>
+__global__ void scale_kernel(T* __restrict__ out,
+                              const T* __restrict__ input,
+                              float factor,
+                              int64_t n) {
+  int64_t idx = static_cast<int64_t>(blockIdx.x) * blockDim.x + threadIdx.x;
+  if (idx < n) {
+    out[idx] = static_cast<T>(static_cast<float>(input[idx]) * factor);
+  }
+}
+
+void scale(at::Tensor& out, const at::Tensor& input, double factor) {
+  TORCH_CHECK(input.is_cuda(),       "input must be a CUDA tensor");
+  TORCH_CHECK(input.is_contiguous(), "input must be contiguous");
+  TORCH_CHECK(out.is_cuda(),         "out must be a CUDA tensor");
+  TORCH_CHECK(out.is_contiguous(),   "out must be contiguous");
+  TORCH_CHECK(out.sizes() == input.sizes(),  "out and input must have the same shape");
+  TORCH_CHECK(out.scalar_type() == input.scalar_type(),
+              "out and input must have the same dtype");
+
+  const int64_t n = input.numel();
+  const int threads = 256;
+  const int blocks  = (n + threads - 1) / threads;
+
+  const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
+  const at::cuda::OptionalCUDAGuard device_guard(device_of(input));
+
+  // Dispatches over float, float16, bfloat16
+  DISPATCH_PYTORCH_DTYPE_TO_CTYPE_FLOAT_FP16(input.scalar_type(), c_type, [&] {
+    scale_kernel<c_type><<<blocks, threads, 0, stream>>>(
+        static_cast<c_type*>(out.data_ptr()),
+        static_cast<const c_type*>(input.data_ptr()),
+        static_cast<float>(factor),
+        n);
+    cudaError_t status = cudaGetLastError();
+    TORCH_CHECK(status == cudaSuccess,
+                "scale_kernel launch failed: ", cudaGetErrorString(status));
+    return true;
+  });
+}
+```
+
+**Key points:**
+
+- Use `at::Tensor` (PyTorch tensors), `TORCH_CHECK` for validation, `at::cuda::getCurrentCUDAStream()` for stream
+- `DISPATCH_PYTORCH_DTYPE_TO_CTYPE_FLOAT_FP16` covers `float`, `half` (FP16), `__nv_bfloat16` (BF16)
+- Add device error checking after every kernel launch
+- If a kernel only works on certain architectures, enforce that with `TORCH_CHECK` and skip logic in tests
+
+---
+
+## Step 2: Add a C++ declaration in `include/sgl_kernel_ops.h`
+
+Edit `sgl-kernel/include/sgl_kernel_ops.h`, add to the elementwise section:
+
+```cpp
+void scale(at::Tensor& out, const at::Tensor& input, double factor);
+```
+
+---
+
+## Step 3: Register the op in `csrc/common_extension.cc`
+
+Edit `sgl-kernel/csrc/common_extension.cc`, inside `TORCH_LIBRARY_FRAGMENT(sgl_kernel, m)`:
+
+```cpp
+// From csrc/elementwise
+m.def("scale(Tensor! out, Tensor input, float factor) -> ()");
+m.impl("scale", torch::kCUDA, &scale);
+```
+
+**Key points:**
+
+- `Tensor!` means in-place / mutable output argument
+- The schema is important for `torch.compile` and for consistent call signatures
+- If your underlying C++ API uses `float` but PyTorch bindings expect `double`, the implicit cast is fine for scalars; use shims if needed for other types
+
+---
+
+## Step 4: Add the new source file to `CMakeLists.txt`
+
+Edit `sgl-kernel/CMakeLists.txt`, add to `set(SOURCES ...)`:
+
+```cmake
+csrc/elementwise/scale.cu
+```
+
+**Key points:**
+
+- Keep the list **alphabetically sorted** (the file explicitly requires this)
+- If the kernel has arch constraints, reflect that in tests/benchmarks via skip logic
+
+---
+
+## Step 5: Expose a Python API under `sgl-kernel/python/sgl_kernel/`
+
+Prefer following the existing module organization first. For elementwise kernels, the usual pattern is:
+
+- implement the Python wrapper in `sgl-kernel/python/sgl_kernel/elementwise.py`
+- then re-export it from `sgl-kernel/python/sgl_kernel/__init__.py`
+
+For example, in `sgl-kernel/python/sgl_kernel/elementwise.py`, add:
+
+```python
+import torch
+
+def scale(
+    input: torch.Tensor,
+    factor: float,
+    out: torch.Tensor | None = None,
+) -> torch.Tensor:
+    """
+    Element-wise scale: out = input * factor.
+
+    Supported dtypes: torch.float16, torch.bfloat16, torch.float32.
+
+    Parameters
+    ----------
+    input  : CUDA input tensor
+    factor : scale factor (float)
+    out    : optional pre-allocated CUDA output tensor (same shape/dtype as input)
+    """
+    if out is None:
+        out = torch.empty_like(input)
+    torch.ops.sgl_kernel.scale.default(out, input, factor)
+    return out
+```
+
+Then re-export it from `sgl-kernel/python/sgl_kernel/__init__.py` following the existing import style used by other kernels.
+
+---
+
+## Step 6: Write tests (required)
+
+Create `sgl-kernel/tests/test_scale.py`:
+```python
+import pytest
+
+import torch
+import sgl_kernel
+
+@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float32])
+@pytest.mark.parametrize("size", [128, 1024, 4096, 65536])
+@pytest.mark.parametrize("factor", [0.5, 1.0, 2.0])
+def test_scale_correctness(dtype, size, factor):
+    input = torch.randn(size, dtype=dtype, device="cuda")
+    out   = torch.empty_like(input)
+
+    result = sgl_kernel.scale(input, factor, out=out)
+    assert result is out
+
+    expected = input * factor
+    rtol, atol = (1e-5, 1e-6) if dtype == torch.float32 else (1e-2, 1e-2)
+    torch.testing.assert_close(out, expected, rtol=rtol, atol=atol)
+
+
+def test_scale_shape_mismatch():
+    input = torch.randn(128, dtype=torch.float16, device="cuda")
+    out   = torch.empty(256, dtype=torch.float16, device="cuda")
+    with pytest.raises(RuntimeError, match="same shape"):
+        sgl_kernel.scale(input, 2.0, out=out)
+
+
+def test_scale_cpu_input():
+    input = torch.randn(128, dtype=torch.float16)  # CPU
+    out   = torch.empty_like(input)
+    with pytest.raises(RuntimeError, match="CUDA"):
+        sgl_kernel.scale(input, 2.0, out=out)
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-q"])
+```
+
+---
+
+## Step 7: Add a benchmark (required)
+
+Create `sgl-kernel/benchmark/bench_scale.py`:
+
+```python
+import itertools
+import os
+
+import torch
+import triton
+import triton.testing
+
+import sgl_kernel
+
+IS_CI = (
+    os.getenv("CI", "false").lower() == "true"
+    or os.getenv("GITHUB_ACTIONS", "false").lower() == "true"
+)
+
+dtypes  = [torch.float16] if IS_CI else [torch.float16, torch.bfloat16, torch.float32]
+sizes   = [4096] if IS_CI else [2**n for n in range(10, 20)]  # 1K … 512K
+factors = [2.0]
+
+configs = list(itertools.product(dtypes, sizes))
+
+
+def torch_scale(input: torch.Tensor, factor: float) -> torch.Tensor:
+    return input * factor
+
+
+@triton.testing.perf_report(
+    triton.testing.Benchmark(
+        x_names=["dtype", "size"],
+        x_vals=configs,
+        line_arg="provider",
+        line_vals=["sglang", "torch"],
+        line_names=["SGL Kernel", "PyTorch"],
+        styles=[("green", "-"), ("red", "--")],
+        ylabel="µs (median)",
+        plot_name="scale-performance",
+        args={},
+    )
+)
+def benchmark(dtype, size, provider):
+    input  = torch.randn(size, dtype=dtype, device="cuda")
+    out    = torch.empty_like(input)
+    factor = 2.0
+
+    if provider == "sglang":
+        fn = lambda: sgl_kernel.scale(input, factor, out=out)
+    else:
+        fn = lambda: torch_scale(input, factor)
+
+    ms, min_ms, max_ms = triton.testing.do_bench_cudagraph(
+        fn, quantiles=[0.5, 0.2, 0.8]
+    )
+    return 1000 * ms, 1000 * max_ms, 1000 * min_ms
+
+
+if __name__ == "__main__":
+    benchmark.run(print_data=True)
+```
+
+---
+
+## Step 8: Build
+
+Build:
+
+```bash
+cd sgl-kernel
+make build -j16
+```
+
+If you need to limit host resource usage:
+
+```bash
+cd sgl-kernel
+make build -j1 MAX_JOBS=2 CMAKE_ARGS="-DSGL_KERNEL_COMPILE_THREADS=1"
+```
+
+---
+
+## Step 9: Validate
+
+After building successfully, run the test and benchmark:
+
+```bash
+pytest sgl-kernel/tests/test_scale.py -q
+python sgl-kernel/benchmark/bench_scale.py
+```
+
+---
+
+## Troubleshooting
+
+- **Async CUDA errors**: `CUDA_LAUNCH_BLOCKING=1`
+- **Memory errors**: `compute-sanitizer --tool memcheck python ...`
+- **Build is too slow / OOM**: reduce `MAX_JOBS` and `SGL_KERNEL_COMPILE_THREADS`
+- **Binary bloat**: use `sgl-kernel/analyze_whl_kernel_sizes.py`
+- **CMake sources list**: if your `.cu` file is missing from `SOURCES`, the symbol will be undefined at link time
+
+---
+
+## References
+
+- `sgl-kernel/README.md`
+- `sgl-kernel/include/sgl_kernel_ops.h`
+- `sgl-kernel/csrc/common_extension.cc`
+- `sgl-kernel/CMakeLists.txt`
+- `sgl-kernel/include/utils.h` — `DISPATCH_PYTORCH_DTYPE_TO_CTYPE_FLOAT_FP16` macro and friends
+- `sgl-kernel/csrc/elementwise/activation.cu` — reference for the FP16/BF16/FP32 dispatch pattern
+
+## Summary of Files Created/Modified
+
+```
+sgl-kernel/csrc/elementwise/scale.cu          # NEW: CUDA kernel + launcher
+sgl-kernel/include/sgl_kernel_ops.h           # MODIFIED: C++ declaration
+sgl-kernel/csrc/common_extension.cc           # MODIFIED: schema + dispatch registration
+sgl-kernel/CMakeLists.txt                     # MODIFIED: add source file (alphabetical)
+sgl-kernel/python/sgl_kernel/elementwise.py   # MODIFIED: Python wrapper
+sgl-kernel/python/sgl_kernel/__init__.py      # MODIFIED: re-export Python API
+sgl-kernel/tests/test_scale.py                # NEW: tests
+sgl-kernel/benchmark/bench_scale.py           # NEW: benchmark
+```