selftests/damon: add python and drgn-based DAMON sysfs test

Add a python-written DAMON sysfs functionality selftest.  It sets DAMON
parameters using Python module _damon_sysfs, reads updated kernel internal
DAMON status and parameters using a 'drgn' script, namely
drgn_dump_damon_status.py, and compare if the resulted DAMON internal
status is as expected.  The test is very minimum at the moment.

Link: https://lkml.kernel.org/r/20250628160428.53115-4-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

@@ -7,6 +7,7 @@ TEST_FILES = _damon_sysfs.py
 
 # functionality tests
 TEST_PROGS += sysfs.sh
+TEST_PROGS += sysfs.py
 TEST_PROGS += sysfs_update_schemes_tried_regions_wss_estimation.py
 TEST_PROGS += damos_quota.py damos_quota_goal.py damos_apply_interval.py
 TEST_PROGS += damos_tried_regions.py damon_nr_regions.py

tools/testing/selftests/damon/sysfs.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+import json
+import os
+import subprocess
+
+import _damon_sysfs
+
+def dump_damon_status_dict(pid):
+    file_dir = os.path.dirname(os.path.abspath(__file__))
+    dump_script = os.path.join(file_dir, 'drgn_dump_damon_status.py')
+    rc = subprocess.call(['drgn', dump_script, pid, 'damon_dump_output'],
+                         stderr=subprocess.DEVNULL)
+    if rc != 0:
+        return None, 'drgn fail'
+    try:
+        with open('damon_dump_output', 'r') as f:
+            return json.load(f), None
+    except Exception as e:
+        return None, 'json.load fail (%s)' % e
+
+def main():
+    kdamonds = _damon_sysfs.Kdamonds(
+            [_damon_sysfs.Kdamond(contexts=[_damon_sysfs.DamonCtx()])])
+    err = kdamonds.start()
+    if err is not None:
+        print('kdamond start failed: %s' % err)
+        exit(1)
+
+    status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
+    if err is not None:
+        print(err)
+        exit(1)
+
+    if len(status['contexts']) != 1:
+        print('number of contexts: %d' % len(status['contexts']))
+        exit(1)
+    kdamonds.stop()
+
+if __name__ == '__main__':
+    main()