@@ -328,6 +328,27 @@ def test_sys_api_with_existing_perf_jit_trampoline(self):
                 """
         assert_python_ok("-c", code, PYTHON_JIT="0")
 
+    @unittest.skipUnless(
+        "-D_Py_JIT" in (sysconfig.get_config_var("PY_CORE_CFLAGS") or "").split(),
+        "requires a real JIT (_Py_JIT)",
+    )
+    def test_sys_api_perf_jit_backend_in_subinterpreter(self):
+        # gh-157247: a subinterpreter must not bypass the JIT/perf exclusion.
+        code = """if 1:
+                import sys
+                from contextlib import closing
+                from concurrent import interpreters
+
+                assert sys._jit.is_enabled(), "expected the JIT to be enabled"
+
+                with closing(interpreters.create()) as interp:
+                    interp.exec(
+                        "import sys; sys.activate_stack_trampoline('perf_jit')")
+                """
+        rc, out, err = assert_python_failure("-c", code, PYTHON_JIT="1")
+        self.assertIn(
+            b"Cannot activate the perf trampoline if the JIT is active", err)
+
 
 def is_unwinding_reliable_with_frame_pointers():
     cflags = sysconfig.get_config_var("PY_CORE_CFLAGS")
 
@@ -0,0 +1,2 @@
+Fix :func:`sys.activate_stack_trampoline` allowing activation from a
+subinterpreter while the JIT is enabled in the main interpreter.
@@ -2370,7 +2370,9 @@ sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
 #ifdef _Py_JIT
-    if (_PyInterpreterState_GET()->jit) {
+    // Perf state is process-wide, and only the main interpreter can enable
+    // the JIT. Check it even when called from a subinterpreter (gh-157247).
+    if (_PyInterpreterState_Main()->jit) {
         PyErr_SetString(PyExc_ValueError, "Cannot activate the perf trampoline if the JIT is active");
         return NULL;
     }
Original file line number	Diff line number	Diff line change
@@ -0,0 +1,2 @@
	1	+ Fix :func:`sys.activate_stack_trampoline` allowing activation from a
	2	+ subinterpreter while the JIT is enabled in the main interpreter.
Original file line number	Diff line number	Diff line change
@@ -2370,7 +2370,9 @@ sys_activate_stack_trampoline_impl(PyObject module, const char backend)
2370	2370	{
2371	2371	#ifdef PY_HAVE_PERF_TRAMPOLINE
2372	2372	#ifdef _Py_JIT
2373		- if (_PyInterpreterState_GET()->jit) {
	2373	+ // Perf state is process-wide, and only the main interpreter can enable
	2374	+ // the JIT. Check it even when called from a subinterpreter (gh-157247).
	2375	+ if (_PyInterpreterState_Main()->jit) {
2374	2376	PyErr_SetString(PyExc_ValueError, "Cannot activate the perf trampoline if the JIT is active");
2375	2377	return NULL;
2376	2378	}