selftests: cgroup: Simplify task self migration

Simplify task migration by being oblivious about its PID during
migration. This allows to easily migrate individual threads as well.
This change brings no functional change and prepares grounds for thread
granularity migrating tests.
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/cgroup_util.c

@@ -282,10 +282,12 @@ int cg_enter(const char *cgroup, int pid)
 
 int cg_enter_current(const char *cgroup)
 {
-	char pidbuf[64];
+	return cg_write(cgroup, "cgroup.procs", "0");
+}
 
-	snprintf(pidbuf, sizeof(pidbuf), "%d", getpid());
-	return cg_write(cgroup, "cgroup.procs", pidbuf);
+int cg_enter_current_thread(const char *cgroup)
+{
+	return cg_write(cgroup, "cgroup.threads", "0");
 }
 
 int cg_run(const char *cgroup,
@@ -410,10 +412,14 @@ int set_oom_adj_score(int pid, int score)
 	return 0;
 }
 
-char proc_read_text(int pid, const char *item, char *buf, size_t size)
+ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t size)
 {
 	char path[PATH_MAX];
 
+	if (!pid)
+		snprintf(path, sizeof(path), "/proc/%s/%s",
+			 thread ? "thread-self" : "self", item);
+	else
 		snprintf(path, sizeof(path), "/proc/%d/%s", pid, item);
 
 	return read_text(path, buf, size);

tools/testing/selftests/cgroup/cgroup_util.h

 /* SPDX-License-Identifier: GPL-2.0 */
+#include <stdbool.h>
 #include <stdlib.h>
 
 #define PAGE_SIZE 4096
@@ -35,6 +36,7 @@ extern int cg_run(const char *cgroup,
 		  void *arg);
 extern int cg_enter(const char *cgroup, int pid);
 extern int cg_enter_current(const char *cgroup);
+extern int cg_enter_current_thread(const char *cgroup);
 extern int cg_run_nowait(const char *cgroup,
 			 int (*fn)(const char *cgroup, void *arg),
 			 void *arg);
@@ -45,4 +47,4 @@ extern int is_swap_enabled(void);
 extern int set_oom_adj_score(int pid, int score);
 extern int cg_wait_for_proc_count(const char *cgroup, int count);
 extern int cg_killall(const char *cgroup);
-extern char proc_read_text(int pid, const char *item, char *buf, size_t size);
+extern ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t size);

tools/testing/selftests/cgroup/test_freezer.c

@@ -701,7 +701,7 @@ static int proc_check_stopped(int pid)
 	char buf[PAGE_SIZE];
 	int len;
 
-	len = proc_read_text(pid, "stat", buf, sizeof(buf));
+	len = proc_read_text(pid, 0, "stat", buf, sizeof(buf));
 	if (len == -1) {
 		debug("Can't get %d stat\n", pid);
 		return -1;