_damon_sysfs: implement commit() for online parameters update

Users can update DAMON parameters while it is running, using 'commit'
DAMON sysfs interface command.  For testing the feature in future tests,
implement a function for doing that on the test-purpose DAMON sysfs
interface wrapper Python module.

Link: https://lkml.kernel.org/r/20240625180538.73134-8-sj@kernel.orgSigned-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/_damon_sysfs.py

@@ -481,6 +481,25 @@ class Kdamond:
                     goal.effective_bytes = int(content)
         return None
 
+    def commit(self):
+        nr_contexts_file = os.path.join(self.sysfs_dir(),
+                'contexts', 'nr_contexts')
+        content, err = read_file(nr_contexts_file)
+        if err is not None:
+            return err
+        if int(content) != len(self.contexts):
+            err = write_file(nr_contexts_file, '%d' % len(self.contexts))
+            if err is not None:
+                return err
+
+        for context in self.contexts:
+            err = context.stage()
+            if err is not None:
+                return err
+        err = write_file(os.path.join(self.sysfs_dir(), 'state'), 'commit')
+        return err
+
+
     def commit_schemes_quota_goals(self):
         for context in self.contexts:
             for scheme in context.schemes: