add-jit-kernel

name	add-jit-kernel
description	Step-by-step tutorial for adding a new lightweight JIT CUDA kernel to sglang's jit_kernel module

Tutorial: Adding a New JIT Kernel to SGLang

This tutorial walks through adding a simple element-wise scale operation as a JIT 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, passed as C++ template argument)
• Output: x * factor (element-wise), allocated internally
• Supported dtypes: FP16 (torch.float16), BF16 (torch.bfloat16), FP32 (torch.float32)

When to use JIT vs AOT (sgl-kernel)

• JIT (jit_kernel): lightweight, few dependencies, rapid iteration, compiled on first use
• AOT (sgl-kernel): depends on CUTLASS / FlashInfer / DeepGEMM, needs pre-built wheel

---

Common Abstractions in python/sglang/jit_kernel/include/sgl_kernel/

Always prefer these abstractions over raw CUDA primitives. They provide safety, readability, and consistency with the rest of the codebase.

utils.h — Host-side utilities

#include <sgl_kernel/utils.h>

• host::RuntimeCheck(cond, args...) — Assert a condition at runtime; throws PanicError with file/line info on failure. Prefer this over bare assert.
• host::Panic(args...) — Unconditionally throw a PanicError with a descriptive message.
• host::div_ceil(a, b) — Integer ceiling division (a + b - 1) / b.
• host::irange(n) / host::irange(start, end) — Range views for cleaner loops.
• host::pointer::offset(ptr, offsets...) — Byte-safe pointer arithmetic on void*. Use this instead of raw casts.

utils.cuh — Device-side utilities + LaunchKernel

#include <sgl_kernel/utils.cuh>

• Type aliases: fp16_t, bf16_t, fp32_t, fp8_e4m3_t, fp8_e5m2_t and their packed variants fp16x2_t, bf16x2_t, fp32x2_t, etc.
• SGL_DEVICE — Expands to __forceinline__ __device__. Use on all device functions.
• device::kWarpThreads — Constant 32.
• device::load_as<T>(ptr, offset) / device::store_as<T>(ptr, val, offset) — Type-safe loads/stores from void*.
• device::pointer::offset(ptr, offsets...) — Pointer arithmetic on device.
• host::LaunchKernel(grid, block, device_or_stream [, smem]) — RAII kernel launcher that:
  • Resolves the CUDA stream from a DLDevice via TVM-FFI automatically.
  • Checks the CUDA error with file/line info after launch via operator()(kernel, args...).
  • Supports .enable_pdl(bool) for PDL (Programmatic Dependent Launch, SM90+).
• host::RuntimeDeviceCheck(cudaError_t) — Check a CUDA error; throw on failure.

tensor.h — Tensor validation (TensorMatcher, Symbolic types)

#include <sgl_kernel/tensor.h>

This is the primary validation API for all kernel launchers. Use it to validate every tvm::ffi::TensorView argument.

• host::SymbolicSize{"name"} — A named symbolic dimension. Call .set_value(n) to pin it, .unwrap() to extract after verification.
• host::SymbolicDType — Symbolic dtype. Use .set_options<Ts...>() to restrict allowed types.
• host::SymbolicDevice — Symbolic device. Use .set_options<kDLCUDA>() to restrict to CUDA.
• host::TensorMatcher({dims...}) — Fluent builder for tensor validation:
  • .with_dtype<T>() — require a specific C++ type (e.g. fp16_t)
  • .with_dtype<T1, T2, ...>() — allow a set of types
  • .with_device<kDLCUDA>(device_sym) — require CUDA, bind device to symbol
  • .with_strides({strides...}) — validate strides (omit to require contiguous)
  • .verify(tensor_view) — execute the check; throws PanicError with full context on failure; chainable (verify(a).verify(b) to check multiple tensors with the same shape)

Typical pattern:

auto N = SymbolicSize{"num_elements"};
auto device = SymbolicDevice{};
device.set_options<kDLCUDA>();
TensorMatcher({N})  //
    .with_dtype<fp16_t>()
    .with_device(device)
    .verify(dst)
    .verify(src);  // same shape, dtype, device as dst
const size_t n = N.unwrap();
const DLDevice dev = device.unwrap();

type.cuh — dtype_trait<T> and packed_t<T>

#include <sgl_kernel/type.cuh>

• dtype_trait<T> — Static trait struct for each scalar type. Provides:
  • dtype_trait<T>::from(value) — convert from another type (e.g. fp32_t → fp16_t)
  • dtype_trait<T>::abs/sqrt/rsqrt/max/min(x) — type-dispatched math (for fp32_t)
• packed_t<T> — Two-element packed alias: packed_t<fp16_t> = fp16x2_t, packed_t<bf16_t> = bf16x2_t, packed_t<fp32_t> = fp32x2_t. Use for vectorized loads/stores.
• device::cast<To, From>(value) — Type-safe cast using dtype_trait, e.g. cast<fp32x2_t, fp16x2_t>(v).

vec.cuh — Vectorized memory access (AlignedVector)

#include <sgl_kernel/vec.cuh>

• device::AlignedVector<T, N> — Aligned storage for N elements of type T. N must be a power of two, sizeof(T)*N <= 32. Enables 128-bit vector loads/stores for bandwidth efficiency.
  • .load(ptr, offset) — vectorized load from ptr[offset]
  • .store(ptr, offset) — vectorized store to ptr[offset]
  • .fill(value) — fill all lanes
  • operator[](i) — element access

tile.cuh — tile::Memory (strided memory access pattern)

#include <sgl_kernel/tile.cuh>

• device::tile::Memory<T>::cta(blockDim.x) — Creates a tile accessor where each thread handles tid = threadIdx.x with stride blockDim.x. Common for loops over a 1D array.
• .load(ptr, offset) — loads ptr[tid + offset * blockDim.x]
• .store(ptr, val, offset) — stores to ptr[tid + offset * blockDim.x]
• .in_bound(n, offset) — boundary check

math.cuh — Device math (device::math::)

#include <sgl_kernel/math.cuh>

• device::math::max/min/abs/sqrt/rsqrt<T>(a, b) — type-dispatched math via dtype_trait
• device::math::exp/sin/cos(float) — fast float math wrappers

warp.cuh — Warp-level primitives

#include <sgl_kernel/warp.cuh>

• device::warp::reduce_sum<T>(value) — warp-level sum reduction via __shfl_xor_sync
• device::warp::reduce_max<T>(value) — warp-level max reduction

cta.cuh — CTA-level primitives

#include <sgl_kernel/cta.cuh>

• device::cta::reduce_max<T>(value, smem, min_value) — CTA-wide max using shared memory + warp reduction. Caller is responsible for a __syncthreads() after if the result in smem[0] is needed.

atomic.cuh — Atomic operations

#include <sgl_kernel/atomic.cuh>

• device::atomic::max(float* addr, float value) — float atomic max (handles negative values correctly via bit tricks).

runtime.cuh — Occupancy and device info

#include <sgl_kernel/runtime.cuh>

• host::runtime::get_blocks_per_sm(kernel, block_dim) — max active blocks per SM (occupancy)
• host::runtime::get_sm_count(device_id) — number of SMs on the device
• host::runtime::get_cc_major(device_id) — compute capability major version

Persistent kernel pattern (cap blocks to SM count × occupancy):

static const uint32_t max_occ = runtime::get_blocks_per_sm(kernel, kBlockSize);
static const uint32_t num_sm  = runtime::get_sm_count(device.unwrap().device_id);
const auto num_blocks = std::min(num_sm * max_occ, div_ceil(n, kBlockSize));
LaunchKernel(num_blocks, kBlockSize, device.unwrap())(kernel, params);

---

Step 0 (optional): Generate a .clangd config for better IDE support

python -m sglang.jit_kernel

---

Step 1: Implement the CUDA kernel in jit_kernel/csrc/

Create python/sglang/jit_kernel/csrc/elementwise/scale.cuh.

The implementation fully uses the project abstractions described above:

#include <sgl_kernel/tensor.h>   // TensorMatcher, SymbolicSize, SymbolicDevice
#include <sgl_kernel/type.cuh>   // dtype_trait, fp16_t, bf16_t, fp32_t
#include <sgl_kernel/utils.h>    // RuntimeCheck, div_ceil
#include <sgl_kernel/utils.cuh>  // LaunchKernel, SGL_DEVICE
#include <sgl_kernel/vec.cuh>    // AlignedVector

#include <dlpack/dlpack.h>
#include <tvm/ffi/container/tensor.h>

namespace {

// ----------------------------------------------------------------
// Kernel: element-wise scale using vectorized 128-bit loads/stores
// T       = fp16_t | bf16_t | fp32_t
// kVecN   = number of elements per vector load (e.g. 8 for fp16)
// kFactor = scale factor encoded as kFactorNumer / kFactorDenom
// ----------------------------------------------------------------
template <typename T, int kVecN, int32_t kFactorNumer, int32_t kFactorDenom>
__global__ void scale_kernel(T* __restrict__ dst,
                              const T* __restrict__ src,
                              uint32_t n_vecs,
                              uint32_t n_remainder,
                              uint32_t n_total) {
  constexpr float kFactor = static_cast<float>(kFactorNumer)
                          / static_cast<float>(kFactorDenom);

  using vec_t = device::AlignedVector<T, kVecN>;

  // --- vectorised body ---
  const uint32_t vec_stride = blockDim.x * gridDim.x;
  for (uint32_t vi = blockIdx.x * blockDim.x + threadIdx.x;
       vi < n_vecs;
       vi += vec_stride) {
    vec_t v;
    v.load(src, vi);
#pragma unroll
    for (int i = 0; i < kVecN; ++i) {
      v[i] = static_cast<T>(static_cast<float>(v[i]) * kFactor);
    }
    v.store(dst, vi);
  }

  // --- scalar tail ---
  const uint32_t base = n_vecs * kVecN;
  const uint32_t scalar_stride = blockDim.x * gridDim.x;
  for (uint32_t i = blockIdx.x * blockDim.x + threadIdx.x;
       i < n_remainder;
       i += scalar_stride) {
    dst[base + i] = static_cast<T>(static_cast<float>(src[base + i]) * kFactor);
  }
}

// ----------------------------------------------------------------
// Launcher: validates tensors, selects vector width, launches kernel
// ----------------------------------------------------------------
template <typename T, int32_t kFactorNumer, int32_t kFactorDenom>
void scale(tvm::ffi::TensorView dst, tvm::ffi::TensorView src) {
  using namespace host;

  // 1. Validate input tensors with TensorMatcher
  SymbolicSize N = {"num_elements"};
  SymbolicDevice device_;
  device_.set_options<kDLCUDA>();

  TensorMatcher({N})  //
      .with_dtype<T>()
      .with_device(device_)
      .verify(dst)
      .verify(src);  // same shape / dtype / device as dst

  const uint32_t n         = static_cast<uint32_t>(N.unwrap());
  const DLDevice device    = device_.unwrap();

  RuntimeCheck(n > 0, "scale: num_elements must be > 0, got ", n);

  // 2. Choose vector width for 128-bit loads (16 bytes)
  //    fp16/bf16: 8 elements × 2 bytes = 16 bytes
  //    fp32:      4 elements × 4 bytes = 16 bytes
  constexpr int kVecN    = 16 / sizeof(T);
  const uint32_t n_vecs      = n / kVecN;
  const uint32_t n_remainder = n % kVecN;

  // 3. Launch
  constexpr uint32_t kBlockSize = 256;
  const uint32_t grid           = div_ceil(std::max(n_vecs, n_remainder), kBlockSize);

  LaunchKernel(grid, kBlockSize, device)(
      scale_kernel<T, kVecN, kFactorNumer, kFactorDenom>,
      static_cast<T*>(dst.data_ptr()),
      static_cast<const T*>(src.data_ptr()),
      n_vecs,
      n_remainder,
      n);
}

}  // namespace

Key points:

• Include headers from sgl_kernel/ — not raw CUDA headers for anything already covered
• Use TensorMatcher for all tensor validation; never manually check shape/dtype/device
• Use AlignedVector for vectorised 128-bit loads/stores — significant bandwidth win
• Use LaunchKernel — it resolves the stream and checks errors automatically
• Use RuntimeCheck for runtime assertions with useful error messages
• fp16_t / bf16_t / fp32_t are the project's type aliases (from utils.cuh)
• device::cast<To, From> or dtype_trait<T>::from(val) for cross-type conversions
• device::math:: functions for device math instead of bare __ intrinsics

---

Step 2: Add the Python wrapper in jit_kernel/

Create python/sglang/jit_kernel/scale.py:

from __future__ import annotations

from typing import TYPE_CHECKING

import torch

from sglang.jit_kernel.utils import cache_once, load_jit, make_cpp_args

if TYPE_CHECKING:
    from tvm_ffi.module import Module


@cache_once
def _jit_scale_module(dtype: torch.dtype, factor_numer: int, factor_denom: int) -> Module:
    """Compile and cache the JIT scale module for a given dtype and factor."""
    args = make_cpp_args(dtype, factor_numer, factor_denom)
    return load_jit(
        "scale",
        *args,
        cuda_files=["elementwise/scale.cuh"],
        cuda_wrappers=[("scale", f"scale<{args}>")],
    )


def scale(src: torch.Tensor, factor: float, out: torch.Tensor | None = None) -> torch.Tensor:
    """
    Element-wise scale: dst = src * factor.

    Supported dtypes: torch.float16, torch.bfloat16, torch.float32.

    Parameters
    ----------
    src    : CUDA tensor (FP16 / BF16 / FP32)
    factor : scale factor
    out    : optional pre-allocated output tensor (same shape/dtype as src)

    Returns
    -------
    Scaled tensor (dst = src * factor).
    """
    assert src.is_cuda, "src must be a CUDA tensor"
    assert src.dtype in (torch.float16, torch.bfloat16, torch.float32), (
        f"Unsupported dtype {src.dtype}. Supported: float16, bfloat16, float32"
    )
    if out is None:
        out = torch.empty_like(src)
    else:
        assert out.shape == src.shape, "out shape must match src"
        assert out.dtype == src.dtype,  "out dtype must match src"

    # Encode factor as integer ratio; denom=1000 gives 3 decimal places of precision
    factor_denom = 1000
    factor_numer = round(factor * factor_denom)

    module = _jit_scale_module(src.dtype, factor_numer, factor_denom)
    module.scale(out, src)
    return out

Key points:

• Use cache_once — not functools.lru_cache (incompatible with torch.compile)
• load_jit first arg(s) form the unique build marker; same marker = same cached binary
• cuda_wrappers: (export_name, kernel_symbol) — export_name is called from Python
• make_cpp_args(dtype, ...) converts torch.dtype to C++ type alias:

torch.dtype	C++ type
torch.float16	fp16_t
torch.bfloat16	bf16_t
torch.float32	fp32_t

---

Step 3 (optional): Tune JIT build flags

return load_jit(
    "scale",
    *args,
    cuda_files=["elementwise/scale.cuh"],
    cuda_wrappers=[("scale", f"scale<{args}>")],
    extra_cuda_cflags=["-O3", "--use_fast_math"],
)

If your kernel requires SM90+, raise a clear Python error before calling load_jit:

if torch.cuda.get_device_capability()[0] < 9:
    raise RuntimeError("This kernel requires SM90 (Hopper) or later")

---

Step 4: Write tests (required)

Create python/sglang/jit_kernel/tests/test_scale.py:

import pytest
import torch
from sglang.jit_kernel.scale import scale


@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float32])
@pytest.mark.parametrize("size", [1, 127, 128, 1024, 4097])  # cover tail remainder
@pytest.mark.parametrize("factor", [0.5, 1.0, 2.0, 3.0])
def test_scale_correctness(dtype, size, factor):
    src = torch.randn(size, dtype=dtype, device="cuda")
    out = scale(src, factor)
    expected = src * 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)


@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float32])
def test_scale_out_param(dtype):
    src = torch.randn(1024, dtype=dtype, device="cuda")
    out = torch.empty_like(src)
    result = scale(src, 2.0, out=out)
    assert result is out
    torch.testing.assert_close(out, src * 2.0, rtol=1e-2, atol=1e-2)


def test_scale_cpu_error():
    src = torch.randn(128, dtype=torch.float16)  # CPU tensor
    with pytest.raises(AssertionError, match="CUDA"):
        scale(src, 2.0)


def test_scale_unsupported_dtype():
    src = torch.randint(0, 10, (128,), dtype=torch.int32, device="cuda")
    with pytest.raises(AssertionError, match="Unsupported dtype"):
        scale(src, 2.0)


if __name__ == "__main__":
    pytest.main([__file__, "-q"])

Run:

pytest python/sglang/jit_kernel/tests/test_scale.py -q

---

Step 5: Add a benchmark (required)

Create python/sglang/jit_kernel/benchmark/bench_scale.py:

import itertools

import torch
import triton
import triton.testing

from sglang.jit_kernel.benchmark.utils import (
    DEFAULT_DEVICE,
    DEFAULT_DTYPE,
    get_benchmark_range,
    run_benchmark,
)
from sglang.jit_kernel.scale import scale as jit_scale


SIZE_LIST = get_benchmark_range(
    full_range=[2**n for n in range(10, 20)],  # 1K … 512K elements
    ci_range=[4096, 65536],
)

configs = list(itertools.product(SIZE_LIST))


@triton.testing.perf_report(
    triton.testing.Benchmark(
        x_names=["size"],
        x_vals=configs,
        line_arg="provider",
        line_vals=["jit", "torch"],
        line_names=["SGL JIT Kernel", "PyTorch"],
        styles=[("blue", "-"), ("red", "--")],
        ylabel="us",
        plot_name="scale-performance",
        args={},
    )
)
def benchmark(size: int, provider: str):
    src = torch.randn(size, dtype=DEFAULT_DTYPE, device=DEFAULT_DEVICE)
    factor = 2.0

    if provider == "jit":
        fn = lambda: jit_scale(src, factor)
    else:
        fn = lambda: src * factor

    return run_benchmark(fn)


if __name__ == "__main__":
    benchmark.run(print_data=True)

Run:

python python/sglang/jit_kernel/benchmark/bench_scale.py

---

Troubleshooting

• JIT compilation fails: ensure the .cuh file is under python/sglang/jit_kernel/csrc/; reduce template argument combinations
• CUDA crash / illegal memory access: CUDA_LAUNCH_BLOCKING=1; compute-sanitizer --tool memcheck python ...
• Unstable benchmark results: run_benchmark uses CUDA-graph-based timing by default

---

References

• docs/developer_guide/development_jit_kernel_guide.md
• python/sglang/jit_kernel/utils.py — cache_once, load_jit, make_cpp_args
• python/sglang/jit_kernel/include/sgl_kernel/tensor.h — TensorMatcher, SymbolicSize/DType/Device
• python/sglang/jit_kernel/include/sgl_kernel/utils.cuh — type aliases, LaunchKernel, SGL_DEVICE
• python/sglang/jit_kernel/include/sgl_kernel/vec.cuh — AlignedVector
• python/sglang/jit_kernel/include/sgl_kernel/tile.cuh — tile::Memory
• python/sglang/jit_kernel/include/sgl_kernel/type.cuh — dtype_trait, packed_t, device::cast
• python/sglang/jit_kernel/include/sgl_kernel/math.cuh — device::math::
• python/sglang/jit_kernel/include/sgl_kernel/warp.cuh — warp::reduce_sum/max
• python/sglang/jit_kernel/include/sgl_kernel/cta.cuh — cta::reduce_max
• python/sglang/jit_kernel/include/sgl_kernel/atomic.cuh — atomic::max
• python/sglang/jit_kernel/include/sgl_kernel/runtime.cuh — occupancy / SM count helpers
• python/sglang/jit_kernel/csrc/add_constant.cuh — minimal runnable reference
• python/sglang/jit_kernel/csrc/elementwise/rmsnorm.cuh — real example using TensorMatcher + LaunchKernel + tile::Memory
• python/sglang/jit_kernel/csrc/elementwise/qknorm.cuh — real example using runtime::get_blocks_per_sm + persistent kernel pattern
• python/sglang/jit_kernel/benchmark/utils.py — benchmark helpers

Summary of Files Created

python/sglang/jit_kernel/csrc/elementwise/scale.cuh   # NEW: CUDA kernel
python/sglang/jit_kernel/scale.py                     # NEW: Python wrapper
python/sglang/jit_kernel/tests/test_scale.py          # NEW: Tests
python/sglang/jit_kernel/benchmark/bench_scale.py     # NEW: Benchmark