selftests/damon: add a test for update_schemes_tried_regions hang bug

Add a test for reproducing the update_schemes_tried_{regions,bytes}
command-causing indefinite hang bug that fixed by commit 7d6fa31a2f
("mm/damon/sysfs-schemes: add timeout for update_schemes_tried_regions"),
to avoid mistakenly re-introducing the bug.  Refer to the fix commit for
more details of the bug.

Link: https://lkml.kernel.org/r/20231212194810.54457-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/Makefile

@@ -10,6 +10,7 @@ TEST_PROGS += debugfs_empty_targets.sh debugfs_huge_count_read_write.sh
 TEST_PROGS += debugfs_duplicate_context_creation.sh
 TEST_PROGS += debugfs_rm_non_contexts.sh
 TEST_PROGS += sysfs.sh sysfs_update_removed_scheme_dir.sh
+TEST_PROGS += sysfs_update_schemes_tried_regions_hang.py
 TEST_PROGS += sysfs_update_schemes_tried_regions_wss_estimation.py
 TEST_PROGS += reclaim.sh lru_sort.sh
 

tools/testing/selftests/damon/sysfs_update_schemes_tried_regions_hang.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+import subprocess
+import time
+
+import _damon_sysfs
+
+def main():
+    proc = subprocess.Popen(['sleep', '2'])
+    kdamonds = _damon_sysfs.Kdamonds([_damon_sysfs.Kdamond(
+            contexts=[_damon_sysfs.DamonCtx(
+                ops='vaddr',
+                targets=[_damon_sysfs.DamonTarget(pid=proc.pid)],
+                schemes=[_damon_sysfs.Damos(
+                    access_pattern=_damon_sysfs.DamosAccessPattern(
+                        nr_accesses=[200, 200]))] # schemes
+                )] # contexts
+            )]) # kdamonds
+
+    err = kdamonds.start()
+    if err != None:
+        print('kdmaond start failed: %s' % err)
+        exit(1)
+
+    while proc.poll() == None:
+        err = kdamonds.kdamonds[0].update_schemes_tried_bytes()
+        if err != None:
+            print('tried bytes update failed: %s' % err)
+            exit(1)
+
+if __name__ == '__main__':
+    main()