From 818370118982554b9495584a678a17b7f8f71bf8 Mon Sep 17 00:00:00 2001
From: Thomas Wouters <thomas@python.org>
Date: Tue, 10 Mar 2026 17:05:23 -0700
Subject: [PATCH 1/3] Cache one datachunk per tstate to prevent alloc/dealloc
 thrashing when repeatedly hitting the same call depth at exactly the wrong
 boundary.

---
 Include/cpython/pystate.h |  1 +
 Python/pystate.c          | 30 ++++++++++++++++++++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index 22df26bd37a5c5..1c56ad5af8072f 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -198,6 +198,7 @@ struct _ts {
     _PyStackChunk *datastack_chunk;
     PyObject **datastack_top;
     PyObject **datastack_limit;
+    _PyStackChunk *datastack_cached_chunk;
     /* XXX signal handlers should also be here */
 
     /* The following fields are here to avoid allocation during init.
diff --git a/Python/pystate.c b/Python/pystate.c
index a8f37bedc81247..560f940c9a3006 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1564,6 +1564,7 @@ init_threadstate(_PyThreadStateImpl *_tstate,
     tstate->datastack_chunk = NULL;
     tstate->datastack_top = NULL;
     tstate->datastack_limit = NULL;
+    tstate->datastack_cached_chunk = NULL;
     tstate->what_event = -1;
     tstate->current_executor = NULL;
     tstate->jit_exit = NULL;
@@ -1714,6 +1715,11 @@ clear_datastack(PyThreadState *tstate)
         _PyObject_VirtualFree(chunk, chunk->size);
         chunk = prev;
     }
+    if (tstate->datastack_cached_chunk != NULL) {
+        _PyObject_VirtualFree(tstate->datastack_cached_chunk,
+                              tstate->datastack_cached_chunk->size);
+        tstate->datastack_cached_chunk = NULL;
+    }
 }
 
 void
@@ -3045,9 +3051,20 @@ push_chunk(PyThreadState *tstate, int size)
     while (allocate_size < (int)sizeof(PyObject*)*(size + MINIMUM_OVERHEAD)) {
         allocate_size *= 2;
     }
-    _PyStackChunk *new = allocate_chunk(allocate_size, tstate->datastack_chunk);
-    if (new == NULL) {
-        return NULL;
+    _PyStackChunk *new;
+    if (tstate->datastack_cached_chunk != NULL
+        && tstate->datastack_cached_chunk->size >= allocate_size)
+    {
+        new = tstate->datastack_cached_chunk;
+        tstate->datastack_cached_chunk = NULL;
+        new->previous = tstate->datastack_chunk;
+        new->top = 0;
+    }
+    else {
+        new = allocate_chunk(allocate_size, tstate->datastack_chunk);
+        if (new == NULL) {
+            return NULL;
+        }
     }
     if (tstate->datastack_chunk) {
         tstate->datastack_chunk->top = tstate->datastack_top -
@@ -3083,12 +3100,17 @@ _PyThreadState_PopFrame(PyThreadState *tstate, _PyInterpreterFrame * frame)
     if (base == &tstate->datastack_chunk->data[0]) {
         _PyStackChunk *chunk = tstate->datastack_chunk;
         _PyStackChunk *previous = chunk->previous;
+        _PyStackChunk *cached = tstate->datastack_cached_chunk;
         // push_chunk ensures that the root chunk is never popped:
         assert(previous);
         tstate->datastack_top = &previous->data[previous->top];
         tstate->datastack_chunk = previous;
-        _PyObject_VirtualFree(chunk, chunk->size);
         tstate->datastack_limit = (PyObject **)(((char *)previous) + previous->size);
+        chunk->previous = NULL;
+        if (cached != NULL) {
+            _PyObject_VirtualFree(cached, cached->size);
+        }
+        tstate->datastack_cached_chunk = chunk;
     }
     else {
         assert(tstate->datastack_top);

From 56a09cba64e2f28da1304be1e21b9563092361fe Mon Sep 17 00:00:00 2001
From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com>
Date: Wed, 11 Mar 2026 00:14:05 +0000
Subject: [PATCH 2/3] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?=
 =?UTF-8?q?rb=5Fit.?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 .../2026-03-11-00-13-59.gh-issue-142183.2iVhJH.rst               | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-00-13-59.gh-issue-142183.2iVhJH.rst

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-00-13-59.gh-issue-142183.2iVhJH.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-00-13-59.gh-issue-142183.2iVhJH.rst
new file mode 100644
index 00000000000000..827224dc71e827
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-11-00-13-59.gh-issue-142183.2iVhJH.rst
@@ -0,0 +1 @@
+Avoid a pathological case where repeated calls at a specific stack depth could be significantly slower.

From 88cef81f5f8850807e401386fe7012e445dbdb3b Mon Sep 17 00:00:00 2001
From: Thomas Wouters <thomas@python.org>
Date: Tue, 10 Mar 2026 17:39:45 -0700
Subject: [PATCH 3/3] Avoid sign comparison issues.

---
 Python/pystate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Python/pystate.c b/Python/pystate.c
index 560f940c9a3006..17b8430b19c188 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -3053,7 +3053,7 @@ push_chunk(PyThreadState *tstate, int size)
     }
     _PyStackChunk *new;
     if (tstate->datastack_cached_chunk != NULL
-        && tstate->datastack_cached_chunk->size >= allocate_size)
+        && (size_t)allocate_size <= tstate->datastack_cached_chunk->size)
     {
         new = tstate->datastack_cached_chunk;
         tstate->datastack_cached_chunk = NULL;