perf tools: Add cgroup_is_v2() helper

The cgroup_is_v2() is to check if the given subsystem is mounted on
cgroup v2 or not.  It'll be used by BPF cgroup code later.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210625071826.608504-3-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/cgroup.c

@@ -9,6 +9,7 @@
 #include <linux/zalloc.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <sys/statfs.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
@@ -70,6 +71,24 @@ int read_cgroup_id(struct cgroup *cgrp)
 }
 #endif  /* HAVE_FILE_HANDLE */
 
+#ifndef CGROUP2_SUPER_MAGIC
+#define CGROUP2_SUPER_MAGIC  0x63677270
+#endif
+
+int cgroup_is_v2(const char *subsys)
+{
+	char mnt[PATH_MAX + 1];
+	struct statfs stbuf;
+
+	if (cgroupfs_find_mountpoint(mnt, PATH_MAX + 1, subsys))
+		return -1;
+
+	if (statfs(mnt, &stbuf) < 0)
+		return -1;
+
+	return (stbuf.f_type == CGROUP2_SUPER_MAGIC);
+}
+
 static struct cgroup *evlist__find_cgroup(struct evlist *evlist, const char *str)
 {
 	struct evsel *counter;

tools/perf/util/cgroup.h

@@ -48,4 +48,6 @@ static inline int read_cgroup_id(struct cgroup *cgrp __maybe_unused)
 }
 #endif  /* HAVE_FILE_HANDLE */
 
+int cgroup_is_v2(const char *subsys);
+
 #endif /* __CGROUP_H__ */