From ce791541769a41beabec0f515cd62e504d46ff1c Mon Sep 17 00:00:00 2001
From: Edward Xu <xuxiangad@gmail.com>
Date: Wed, 19 Nov 2025 08:57:59 +0800
Subject: [PATCH] gh-139103: fix free-threading `dataclass.__init__` perf issue
 (gh-141596)

The dataclasses `__init__` function is generated dynamically by a call to `exec()` and so doesn't have deferred reference counting enabled. Enable deferred reference counting on functions when assigned as an attribute to type objects to avoid reference count contention when creating dataclass instances.
---
 .../2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst   |  1 +
 Objects/typeobject.c                                 | 12 ++++++++++++
 Tools/ftscalingbench/ftscalingbench.py               | 12 ++++++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst
new file mode 100644
index 00000000000..c038dc742cc
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-11-15-23-58-23.gh-issue-139103.9cVYJ0.rst
@@ -0,0 +1 @@
+Improve multithreaded scaling of dataclasses on the free-threaded build.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 61bcc21ce13..c99c6b3f637 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -6546,6 +6546,18 @@ type_setattro(PyObject *self, PyObject *name, PyObject *value)
     assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_INLINE_VALUES));
     assert(!_PyType_HasFeature(metatype, Py_TPFLAGS_MANAGED_DICT));
 
+#ifdef Py_GIL_DISABLED
+    // gh-139103: Enable deferred refcounting for functions assigned
+    // to type objects.  This is important for `dataclass.__init__`,
+    // which is generated dynamically.
+    if (value != NULL &&
+        PyFunction_Check(value) &&
+        !_PyObject_HasDeferredRefcount(value))
+    {
+        PyUnstable_Object_EnableDeferredRefcount(value);
+    }
+#endif
+
     PyObject *old_value = NULL;
     PyObject *descr = _PyType_LookupRef(metatype, name);
     if (descr != NULL) {
diff --git a/Tools/ftscalingbench/ftscalingbench.py b/Tools/ftscalingbench/ftscalingbench.py
index 1a59e25189d..097a065f368 100644
--- a/Tools/ftscalingbench/ftscalingbench.py
+++ b/Tools/ftscalingbench/ftscalingbench.py
@@ -27,6 +27,7 @@
 import sys
 import threading
 import time
+from dataclasses import dataclass
 from operator import methodcaller
 
 # The iterations in individual benchmarks are scaled by this factor.
@@ -202,6 +203,17 @@ def method_caller():
     for i in range(1000 * WORK_SCALE):
         mc(obj)
 
+@dataclass
+class MyDataClass:
+    x: int
+    y: int
+    z: int
+
+@register_benchmark
+def instantiate_dataclass():
+    for _ in range(1000 * WORK_SCALE):
+        obj = MyDataClass(x=1, y=2, z=3)
+
 def bench_one_thread(func):
     t0 = time.perf_counter_ns()
     func()