kselftests: cgroup: add freezer controller self-tests

This patch implements 9 tests for the freezer controller for
cgroup v2:
1) a simple test, which aims to freeze and unfreeze a cgroup with 100
processes
2) a more complicated tree test, which creates a hierarchy of cgroups,
puts some processes in some cgroups, and tries to freeze and unfreeze
different parts of the subtree
3) a forkbomb test: the test aims to freeze a forkbomb running in a
cgroup, kill all tasks in the cgroup and remove the cgroup without
the unfreezing.
4) rmdir test: the test creates two nested cgroups, freezes the parent
one, checks that the child can be successfully removed, and a new
child can be created
5) migration tests: the test checks migration of a task between
frozen cgroups: from a frozen to a running, from a running to a
frozen, and from a frozen to a frozen.
6) ptrace test: the test checks that it's possible to attach to
a process in a frozen cgroup, get some information and detach, and
the cgroup will remain frozen.
7) stopped test: the test checks that it's possible to freeze a cgroup
with a stopped task
8) ptraced test: the test checks that it's possible to freeze a cgroup
with a ptraced task
9) vfork test: the test checks that it's possible to freeze a cgroup
with a parent process waiting for the child process in vfork()

Expected output:
  $ ./test_freezer
  ok 1 test_cgfreezer_simple
  ok 2 test_cgfreezer_tree
  ok 3 test_cgfreezer_forkbomb
  ok 4 test_cgrreezer_rmdir
  ok 5 test_cgfreezer_migrate
  ok 6 test_cgfreezer_ptrace
  ok 7 test_cgfreezer_stopped
  ok 8 test_cgfreezer_ptraced
  ok 9 test_cgfreezer_vfork

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: kernel-team@fb.com
Cc: linux-kselftest@vger.kernel.org

tools/testing/selftests/cgroup/.gitignore

@@ -1,2 +1,3 @@
 test_memcontrol
 test_core
+test_freezer

tools/testing/selftests/cgroup/Makefile

@@ -5,8 +5,10 @@ all:
 
 TEST_GEN_PROGS = test_memcontrol
 TEST_GEN_PROGS += test_core
+TEST_GEN_PROGS += test_freezer
 
 include ../lib.mk
 
 $(OUTPUT)/test_memcontrol: cgroup_util.c
 $(OUTPUT)/test_core: cgroup_util.c
+$(OUTPUT)/test_freezer: cgroup_util.c

tools/testing/selftests/cgroup/cgroup_util.c

@@ -74,6 +74,16 @@ char *cg_name_indexed(const char *root, const char *name, int index)
 	return ret;
 }
 
+char *cg_control(const char *cgroup, const char *control)
+{
+	size_t len = strlen(cgroup) + strlen(control) + 2;
+	char *ret = malloc(len);
+
+	snprintf(ret, len, "%s/%s", cgroup, control);
+
+	return ret;
+}
+
 int cg_read(const char *cgroup, const char *control, char *buf, size_t len)
 {
 	char path[PATH_MAX];
@@ -196,7 +206,32 @@ int cg_create(const char *cgroup)
 	return mkdir(cgroup, 0644);
 }
 
-static int cg_killall(const char *cgroup)
+int cg_wait_for_proc_count(const char *cgroup, int count)
+{
+	char buf[10 * PAGE_SIZE] = {0};
+	int attempts;
+	char *ptr;
+
+	for (attempts = 10; attempts >= 0; attempts--) {
+		int nr = 0;
+
+		if (cg_read(cgroup, "cgroup.procs", buf, sizeof(buf)))
+			break;
+
+		for (ptr = buf; *ptr; ptr++)
+			if (*ptr == '\n')
+				nr++;
+
+		if (nr >= count)
+			return 0;
+
+		usleep(100000);
+	}
+
+	return -1;
+}
+
+int cg_killall(const char *cgroup)
 {
 	char buf[PAGE_SIZE];
 	char *ptr = buf;
@@ -238,6 +273,14 @@ retry:
 	return ret;
 }
 
+int cg_enter(const char *cgroup, int pid)
+{
+	char pidbuf[64];
+
+	snprintf(pidbuf, sizeof(pidbuf), "%d", pid);
+	return cg_write(cgroup, "cgroup.procs", pidbuf);
+}
+
 int cg_enter_current(const char *cgroup)
 {
 	char pidbuf[64];
@@ -367,3 +410,12 @@ int set_oom_adj_score(int pid, int score)
 	close(fd);
 	return 0;
 }
+
+char proc_read_text(int pid, const char *item, char *buf, size_t size)
+{
+	char path[PATH_MAX];
+
+	snprintf(path, sizeof(path), "/proc/%d/%s", pid, item);
+
+	return read_text(path, buf, size);
+}

tools/testing/selftests/cgroup/cgroup_util.h

@@ -18,6 +18,7 @@ static inline int values_close(long a, long b, int err)
 extern int cg_find_unified_root(char *root, size_t len);
 extern char *cg_name(const char *root, const char *name);
 extern char *cg_name_indexed(const char *root, const char *name, int index);
+extern char *cg_control(const char *cgroup, const char *control);
 extern int cg_create(const char *cgroup);
 extern int cg_destroy(const char *cgroup);
 extern int cg_read(const char *cgroup, const char *control,
@@ -32,6 +33,7 @@ extern int cg_write(const char *cgroup, const char *control, char *buf);
 extern int cg_run(const char *cgroup,
 		  int (*fn)(const char *cgroup, void *arg),
 		  void *arg);
+extern int cg_enter(const char *cgroup, int pid);
 extern int cg_enter_current(const char *cgroup);
 extern int cg_run_nowait(const char *cgroup,
 			 int (*fn)(const char *cgroup, void *arg),
@@ -41,3 +43,6 @@ extern int alloc_pagecache(int fd, size_t size);
 extern int alloc_anon(const char *cgroup, void *arg);
 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);