kunit: add fallback for os.sched_getaffinity

Python 3.13 added os.process_cpu_count as a cross-platform alternative
for the Linux-only os.sched_getaffinity. Use it when it's available and
provide a fallback when it's not.

This allows kunit to run on macOS.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: tamird

tools/testing/kunit/kunit.py

@@ -312,7 +312,16 @@ def massage_arg(arg: str) -> str:
 	return list(map(massage_arg, argv))
 
 def get_default_jobs() -> int:
-	return len(os.sched_getaffinity(0))
+	if sys.version_info >= (3, 13):
+		if (ncpu := os.process_cpu_count()) is not None:
+			return ncpu
+		raise RuntimeError("os.process_cpu_count() returned None")
+	 # See https://github.com/python/cpython/blob/b61fece/Lib/os.py#L1175-L1186.
+	if sys.platform != "darwin":
+		return len(os.sched_getaffinity(0))
+	if (ncpu := os.cpu_count()) is not None:
+		return ncpu
+	raise RuntimeError("os.cpu_count() returned None")
 
 def add_common_opts(parser: argparse.ArgumentParser) -> None:
 	parser.add_argument('--build_dir',