gh-140358: Bring back elapsed time and unreachable count to gc debug output

Lib/test/test_gc.py

@@ -801,6 +801,32 @@ def __del__(self):
  rc, out, err = assert_python_ok('-c', code)
  self.assertEqual(out.strip(), b'__del__ called')
 
+ @unittest.skipIf(Py_GIL_DISABLED, "requires GC generations or increments")
+ def test_gc_debug_stats(self):
+ # Checks that debug information is printed to stderr
+ # when DEBUG_STATS is set.
+ code = """if 1:
+ import gc
+ gc.set_debug(%s)
+ gc.collect()
+ """
+ _, _, err = assert_python_ok("-c", code % "gc.DEBUG_STATS")
+ self.assertRegex(err, b"gc: collecting generation [0-9]+")
+ self.assertRegex(
+ err,
+ b"gc: objects in each generation: [0-9]+ [0-9]+ [0-9]+",
+ )
+ self.assertRegex(
+ err, b"gc: objects in permanent generation: [0-9]+"
+ )
+ self.assertRegex(
+ err,
+ b"gc: done, .* unreachable, .* uncollectable, .* elapsed",
+ )
+
+ _, _, err = assert_python_ok("-c", code % "0")
+ self.assertNotIn(b"elapsed", err)
+
  def test_global_del_SystemExit(self):
  code = """if 1:
  class ClassWithDel:

Misc/NEWS.d/next/Core_and_Builtins/2025-10-20-11-24-36.gh-issue-140358.UQuKdV.rst

@@ -0,0 +1,4 @@
+Restore elapsed time and unreachable object count in GC debug output. These
+were inadvertently removed during a refactor of ``gc.c``. The debug log now
+again reports elapsed collection time and the number of unreachable objects.
+Contributed by Pål Grønås Drange.

Python/gc.c

@@ -2077,8 +2077,10 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
  if (reason != _Py_GC_REASON_SHUTDOWN) {
  invoke_gc_callback(gcstate, "start", generation, &stats);
  }
+ PyTime_t t1;
  if (gcstate->debug & _PyGC_DEBUG_STATS) {
  PySys_WriteStderr("gc: collecting generation %d...\n", generation);
+ (void)PyTime_PerfCounterRaw(&t1);
  show_stats_each_generations(gcstate);
  }
  if (PyDTrace_GC_START_ENABLED()) {
@@ -2115,6 +2117,17 @@ _PyGC_Collect(PyThreadState *tstate, int generation, _PyGC_Reason reason)
 #endif
  validate_spaces(gcstate);
  _Py_atomic_store_int(&gcstate->collecting, 0);
+
+ if (gcstate->debug & _PyGC_DEBUG_STATS) {
+ PyTime_t t2;
+ (void)PyTime_PerfCounterRaw(&t2);
+ double d = PyTime_AsSecondsDouble(t2 - t1);
+ PySys_WriteStderr(
+ "gc: done, %zd unreachable, %zd uncollectable, %.4fs elapsed\n",
+ stats.collected + stats.uncollectable, stats.uncollectable, d
+ );
+ }
+
  return stats.uncollectable + stats.collected;
 }