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gh-115103: Update gc.collect to process delayed objects #116238

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Merged
corona10 merged 5 commits into from
Mar 2, 2024
+18 −0
Merged

gh-115103: Update gc.collect to process delayed objects #116238

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c28c79f
gh-115103: Update refleak checker to trigger _PyMem_ProcessDelayed
corona10 Mar 2, 2024
81f333a
Update Lib/test/support/__init__.py
corona10 Mar 2, 2024
59b3fc4
Address code review
corona10 Mar 2, 2024
85b419c
Address code review
corona10 Mar 2, 2024
a187b03
nit
corona10 Mar 2, 2024
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Address code review
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@corona10
corona10 committed Mar 2, 2024
commit 85b419c2b0107e5be77d87fd314ae3213d805753
3 changes: 0 additions & 3 deletions Include/internal/pycore_gc.h
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Expand Up @@ -288,9 +288,6 @@ extern PyObject *_PyGC_GetReferrers(PyInterpreterState *interp, PyObject *objs);
extern void _PyGC_ClearAllFreeLists(PyInterpreterState *interp);
extern void _Py_ScheduleGC(PyThreadState *tstate);
extern void _Py_RunGC(PyThreadState *tstate);
#ifdef Py_GIL_DISABLED
extern void _PyGC_Clear_DelayedObjects(PyInterpreterState *interp);
#endif

#ifdef __cplusplus
}
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2 changes: 0 additions & 2 deletions Lib/test/support/__init__.py
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Expand Up @@ -785,8 +785,6 @@ def gc_collect():
gc.collect()
gc.collect()
gc.collect()
if Py_GIL_DISABLED:
gc._collect_delayed_objects()


@contextlib.contextmanager
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28 changes: 1 addition & 27 deletions Modules/clinic/gcmodule.c.h
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19 changes: 0 additions & 19 deletions Modules/gcmodule.c
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Expand Up @@ -93,24 +93,6 @@ gc_collect_impl(PyObject *module, int generation)
return _PyGC_Collect(tstate, generation, _Py_GC_REASON_MANUAL);
}

#ifdef Py_GIL_DISABLED
/*[clinic input]
gc._collect_delayed_objects

Process delayed free requests by force

[clinic start generated code]*/

static PyObject *
gc__collect_delayed_objects_impl(PyObject *module)
/*[clinic end generated code: output=a016a10f967d4229 input=1064c31903cd9fac]*/
{
PyInterpreterState *interp = _PyInterpreterState_GET();
_PyGC_Clear_DelayedObjects(interp);
Py_RETURN_NONE;
}
#endif

/*[clinic input]
gc.set_debug

Expand Down Expand Up @@ -526,7 +508,6 @@ static PyMethodDef GcMethods[] = {
GC_FREEZE_METHODDEF
GC_UNFREEZE_METHODDEF
GC_GET_FREEZE_COUNT_METHODDEF
GC__COLLECT_DELAYED_OBJECTS_METHODDEF
{NULL, NULL} /* Sentinel */
};

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30 changes: 18 additions & 12 deletions Python/gc_free_threading.c
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Expand Up @@ -324,6 +324,23 @@ merge_all_queued_objects(PyInterpreterState *interp, struct collection_state *st
HEAD_UNLOCK(&_PyRuntime);
}

static void
process_delayed_frees(PyInterpreterState *interp)
{
// In STW status, we can observe the latest write sequence by
// advancing the write sequence immediately.
_Py_qsbr_advance(&interp->qsbr);
_PyThreadStateImpl *current_tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
_Py_qsbr_quiescent_state(current_tstate->qsbr);
HEAD_LOCK(&_PyRuntime);
PyThreadState *tstate = interp->threads.head;
while (tstate != NULL) {
_PyMem_ProcessDelayed(tstate);
tstate = (PyThreadState *)tstate->next;
}
HEAD_UNLOCK(&_PyRuntime);
}

// Subtract an incoming reference from the computed "gc_refs" refcount.
static int
visit_decref(PyObject *op, void *arg)
Expand Down Expand Up @@ -1006,6 +1023,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state)
_PyEval_StopTheWorld(interp);
// merge refcounts for all queued objects
merge_all_queued_objects(interp, state);
process_delayed_frees(interp);
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@pablogsal pablogsal Mar 2, 2024

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Nit/question: _PyEval_StopTheWorld also locks _PyRuntime. Maybe is worth moving the lock up?

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@colesbury colesbury Mar 2, 2024

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_PyEval_StopTheWorld locks _PyRuntime temporarily to loop over threads, but does not hold the lock

We want process_delayed_frees after the stop-the-world call because it guarantees that we can free all the "delayed free" pointers. If we moved it up before the _PyEval_StopTheWorld there might be some memory blocks we don't free.

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@pablogsal pablogsal Mar 2, 2024

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Makes sense, thanks for clarifying

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@pablogsal pablogsal Mar 2, 2024

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If we moved it up before the _PyEval_StopTheWorld there might be some memory blocks we don't free.

With "moving the lock up" i meant to have a unique locking for both functions, not to move this call before the other. But your comment still applies


// Find unreachable objects
int err = deduce_unreachable_heap(interp, state);
Expand Down Expand Up @@ -1758,16 +1776,4 @@ _PyGC_ClearAllFreeLists(PyInterpreterState *interp)
HEAD_UNLOCK(&_PyRuntime);
}

void
_PyGC_Clear_DelayedObjects(PyInterpreterState *interp)
{
HEAD_LOCK(&_PyRuntime);
PyThreadState *tstate = interp->threads.head;
while (tstate != NULL) {
_PyMem_ProcessDelayed(tstate);
tstate = (PyThreadState *)tstate->next;
}
HEAD_UNLOCK(&_PyRuntime);
}

#endif // Py_GIL_DISABLED