selftests: cgroup: Add cg_read_key_long_poll() to poll a cgroup key with retries

Introduce a new helper function `cg_read_key_long_poll()` in
cgroup_util.h. This function polls the specified key in a cgroup file
until it matches the expected value or the retry limit is reached,
with configurable wait intervals between retries.

This helper is particularly useful for handling asynchronously updated
cgroup statistics (e.g., memory.stat), where immediate reads may
observe stale values, especially on busy systems. It allows tests and
other utilities to handle such cases more flexibly.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c
index 44c52f620fda17b50762a245ff198946d09bdb24..ce6c2642fd9bbb622123e3ba23b6a14846f85e7f 100644 (file)
--- a/tools/testing/selftests/cgroup/lib/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/lib/cgroup_util.c
@@ -168,6 +168,27 @@ long cg_read_key_long(const char *cgroup, const char *control, const char *key)
        return atol(ptr + strlen(key));
 }
 
+long cg_read_key_long_poll(const char *cgroup, const char *control,
+                          const char *key, long expected, int retries,
+                          useconds_t wait_interval_us)
+{
+       long val = -1;
+       int i;
+
+       for (i = 0; i < retries; i++) {
+               val = cg_read_key_long(cgroup, control, key);
+               if (val < 0)
+                       return val;
+
+               if (val == expected)
+                       break;
+
+               usleep(wait_interval_us);
+       }
+
+       return val;
+}
+
 long cg_read_lc(const char *cgroup, const char *control)
 {
        char buf[PAGE_SIZE];

diff --git a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
index 7ab2824ed7b54d4ba2217cf4bd3d373bceb27c7f..77f386dab5e8fbe16b6859316b2e0d2200ca78c4 100644 (file)
--- a/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
+++ b/tools/testing/selftests/cgroup/lib/include/cgroup_util.h
@@ -17,6 +17,8 @@
 #define CG_NAMED_NAME "selftest"
 #define CG_PATH_FORMAT (!cg_test_v1_named ? "0::%s" : (":name=" CG_NAMED_NAME ":%s"))
 
+#define DEFAULT_WAIT_INTERVAL_US (100 * 1000) /* 100 ms */
+
 /*
  * Checks if two given values differ by less than err% of their sum.
  */
@@ -64,6 +66,9 @@ extern int cg_read_strstr(const char *cgroup, const char *control,
 extern long cg_read_long(const char *cgroup, const char *control);
 extern long cg_read_long_fd(int fd);
 long cg_read_key_long(const char *cgroup, const char *control, const char *key);
+long cg_read_key_long_poll(const char *cgroup, const char *control,
+                          const char *key, long expected, int retries,
+                          useconds_t wait_interval_us);
 extern long cg_read_lc(const char *cgroup, const char *control);
 extern int cg_write(const char *cgroup, const char *control, char *buf);
 extern int cg_open(const char *cgroup, const char *control, int flags);