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gh-137110: Untrack immortal objects from expand_region_transitivity_reachable #137111

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gh-137110: Untrack immortal objects from expand_region_transitivity_reachable #137111

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@sergey-miryanov sergey-miryanov commented Jul 25, 2025 •
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There is some inconsistency in handling of immortal objects in default-build's GC.

In update_refs immortal objects just untracked (all immortal objects that we face in GC is an accidentally immortalized).

cpython/Python/gc.c

Lines 495 to 500 in 1e69cd1

if (_Py_IsImmortal(op)) {
assert(!_Py_IsStaticImmortal(op));
_PyObject_GC_UNTRACK(op);
gc = next;
continue;
}

update_refs called for all collections (young, increment and full). But for incremental collection if increment_size < gcstate->work_to_do we steal some objects from neighbor gen and call expand_region_transitively_reachable where immortal objects moved to permanent generation:

cpython/Python/gc.c

Lines 1397 to 1402 in 1e69cd1

if (_Py_IsImmortal(op)) {
PyGC_Head *next = GC_NEXT(gc);
gc_list_move(gc, &gcstate->permanent_generation.head);
gc = next;
continue;
}

So, one part of immortal objects can be untracked, and other part can be moved to perm gen.

As I understand from #127519 it is preferring to untrack immortal objects in all cases.

CPython versions tested on:

CPython main branch

@bedevere-app bedevere-app bot mentioned this pull request Jul 25, 2025
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I'm not sure, but this probably should skip news.

@sobolevn sobolevn requested review from nascheme and markshannon July 26, 2025 10:40
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@nascheme nascheme Awaiting requested review from nascheme

@markshannon markshannon Awaiting requested review from markshannon

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