Writing a non-zero value to a kdamond's 'refresh_ms' sysfs file should
make DAMON periodically update the read-only sysfs files on its own,
without the user writing update keywords such as 'update_schemes_stats'
to the 'state' file.  This behavior has no test coverage.

Add a test that starts a kdamond with refresh_ms set and a 'stat' scheme
whose default access pattern matches every monitored region, then polls
the scheme's 'nr_tried' stats file directly, without requesting an
update.  The value can become non-zero only via the periodic refresh, so
the test confirms refresh_ms works; with refresh_ms disabled the stat
stays zero and the test fails.

Signed-off-by: Ruslan Valiyev <linuxoid@gmail.com>
---
 tools/testing/selftests/damon/Makefile        |  1 +
 .../testing/selftests/damon/sysfs_refresh.py  | 75 +++++++++++++++++++
 2 files changed, 76 insertions(+)
 create mode 100755 tools/testing/selftests/damon/sysfs_refresh.py

diff --git a/tools/testing/selftests/damon/Makefile b/tools/testing/selftests/damon/Makefile
index 2180c328a8252..ece244e5c5b95 100644
--- a/tools/testing/selftests/damon/Makefile
+++ b/tools/testing/selftests/damon/Makefile
@@ -13,6 +13,7 @@ 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
+TEST_PROGS += sysfs_refresh.py
 TEST_PROGS += reclaim.sh lru_sort.sh
 
 # regression tests (reproducers of previously found bugs)
diff --git a/tools/testing/selftests/damon/sysfs_refresh.py b/tools/testing/selftests/damon/sysfs_refresh.py
new file mode 100755
index 0000000000000..012b7e8f509f5
--- /dev/null
+++ b/tools/testing/selftests/damon/sysfs_refresh.py
@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+import os
+import subprocess
+import time
+
+import _damon_sysfs
+
+def main():
+    # Continuously access a memory region for far longer than the test needs,
+    # so the kdamond always has a live target to monitor while we poll.
+    sz_region = 10 * 1024 * 1024
+    proc = subprocess.Popen(
+            ['./access_memory', '1', '%d' % sz_region, '60000', 'repeat'])
+
+    # A 'stat' scheme with the default (maximally wide) access pattern matches
+    # every monitored region, so its 'nr_tried' stat increases as the kdamond
+    # runs.  refresh_ms should make DAMON update the schemes' stats files under
+    # sysfs on its own, without a manual 'update_schemes_stats' request.
+    kdamond = _damon_sysfs.Kdamond(
+            refresh_ms=100,
+            contexts=[_damon_sysfs.DamonCtx(
+                ops='vaddr',
+                targets=[_damon_sysfs.DamonTarget(pid=proc.pid)],
+                schemes=[_damon_sysfs.Damos(action='stat')],
+                )])
+    kdamonds = _damon_sysfs.Kdamonds([kdamond])
+
+    err = kdamonds.start()
+    if err is not None:
+        # Kernels older than the refresh_ms feature have no such file; treat
+        # that as unsupported rather than a failure.
+        if not os.path.exists(os.path.join(kdamond.sysfs_dir(), 'refresh_ms')):
+            proc.terminate()
+            proc.wait()
+            print('kdamond has no refresh_ms file; skipping')
+            exit(_damon_sysfs.ksft_skip)
+        proc.terminate()
+        proc.wait()
+        print('kdamond start failed: %s' % err)
+        exit(1)
+
+    scheme = kdamond.contexts[0].schemes[0]
+    nr_tried_path = os.path.join(scheme.sysfs_dir(), 'stats', 'nr_tried')
+
+    try:
+        # Poll the stat file directly.  We never request an update (e.g.
+        # 'update_schemes_stats'), so 'nr_tried' can become non-zero only
+        # through the periodic refresh that refresh_ms enables.
+        nr_tried = 0
+        deadline = time.monotonic() + 10
+        while time.monotonic() < deadline:
+            if proc.poll() is not None:
+                print('the access_memory target exited unexpectedly')
+                exit(1)
+            content, err = _damon_sysfs.read_file(nr_tried_path)
+            if err is not None:
+                print('reading %s failed: %s' % (nr_tried_path, err))
+                exit(1)
+            nr_tried = int(content)
+            if nr_tried > 0:
+                break
+            time.sleep(0.1)
+    finally:
+        kdamonds.stop()
+        proc.terminate()
+        proc.wait()
+
+    if nr_tried == 0:
+        print('refresh_ms did not auto-update the schemes stats')
+        exit(1)
+
+if __name__ == '__main__':
+    main()
-- 
2.43.0