[FEA]: Add support for CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY in LaunchConfig

[lijinf2]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this request and that I agree to the Code of Conduct

Area

cuda.core

Is your feature request related to a problem? Please describe.

LaunchConfig does not expose CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY, which overrides the CPU wait policy (SPIN / YIELD / BLOCKING_SYNC) for a launch.

Parent tracking: #496.

Describe the solution you'd like

• Users can set CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY via LaunchConfig.
• Mapping to the native launch attribute is tested.

Describe alternatives you've considered

1. Use cuda.bindings.driver (CUlaunchAttribute / CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY) and cuLaunchKernelEx directly.
2. Set the stream attribute via cuStreamSetAttribute(..., CU_STREAM_ATTRIBUTE_SYNCHRONIZATION_POLICY, ...) instead of a per-launch attribute.

Downside of (1)/(2): leaves cuda.core users without a first-class LaunchConfig API; they must drop to low-level bindings and lose consistency with other LaunchConfig launch attributes (is_cooperative, programmatic_stream_serialization, etc.).

Additional context

Incremental LaunchConfig attribute coverage under #496 (same approach as #1334 for programmatic_stream_serialization).

Example (C API) — agent-generated; reviewed but not executed:

// Per-launch SPIN policy (lowest-latency host sync)
CUlaunchAttribute syncAttr = {
    .id    = CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY,
    .value = { .syncPolicy = CU_SYNC_POLICY_SPIN }
};

CUlaunchConfig config = {
    .gridDimX = 128, .gridDimY = 1, .gridDimZ = 1,
    .blockDimX = 256, .blockDimY = 1, .blockDimZ = 1,
    .sharedMemBytes = 0,
    .hStream = latencyCriticalStream,
    .attrs = &syncAttr,
    .numAttrs = 1
};

cuLaunchKernelEx(&config, kernel, NULL, NULL);

// Subsequent sync on this stream busy-waits on CPU
cuStreamSynchronize(latencyCriticalStream);

[atiaomar1978-hub]

Implementation available in PR #2637

I implemented the requested LaunchConfig support for CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY.

What changed

• New LaunchConfig.synchronization_policy parameter (optional, default None)
• New public enum cuda.core.typing.SynchronizationPolicyType with AUTO, SPIN, YIELD, and BLOCKING_SYNC, aligned with cuda.bindings.driver.CUsynchronizationPolicy
• Native mapping to CU_LAUNCH_ATTRIBUTE_SYNCHRONIZATION_POLICY in _to_native_launch_config() (both the cdef method used by launch() and the cpdef helper used in tests)

Tests added (tests/test_launcher.py)

• Attribute mapping for all four policies (including acceptance of driver enum values)
• Invalid input rejection
• Default None produces no launch attribute
• Combination with is_cooperative
• GPU launch smoke test (skipped when CUDA 13 bindings run on a CUDA 12 driver)

GPU validation (RunPod RTX A5000)

pytest cuda_core/tests/test_launcher.py -k "synchronization_policy" -v
# 10 passed, 4 skipped, 0 failed

Full details: #2637 (comment)

This satisfies both checklist items from this issue: users can set the policy via LaunchConfig, and mapping to the native launch attribute is tested.