Re: [PATCH 2/4] perf tools: Add cgroup_is_v2() helper

[Ian Rogers <irogers@google.com>]

On Fri, Jun 25, 2021 at 12:18 AM Namhyung Kim <namhyung@kernel.org> wrote:
>
> 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>
> ---
>  tools/perf/util/cgroup.c | 19 +++++++++++++++++++
>  tools/perf/util/cgroup.h |  2 ++
>  2 files changed, 21 insertions(+)
>
> diff --git a/tools/perf/util/cgroup.c b/tools/perf/util/cgroup.c
> index ef18c988c681..e819a4f30fc2 100644
> --- a/tools/perf/util/cgroup.c
> +++ b/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;
> diff --git a/tools/perf/util/cgroup.h b/tools/perf/util/cgroup.h
> index 707adbe25123..1549ec2fd348 100644
> --- a/tools/perf/util/cgroup.h
> +++ b/tools/perf/util/cgroup.h
> @@ -47,4 +47,6 @@ int read_cgroup_id(struct cgroup *cgrp)
>  }
>  #endif  /* HAVE_FILE_HANDLE */
>
> +int cgroup_is_v2(const char *subsys);
> +

I think this is okay. It may make sense to have this in
tools/lib/api/fs/fs.h, for example fs__valid_mount is already checking
magic numbers. Perhaps we can avoid a statfs call, but it'd need some
reorganization of the fs.h code.

Acked-by: Ian Rogers <irogers@google.com>

Thanks,
Ian

>  #endif /* __CGROUP_H__ */
> --
> 2.32.0.93.g670b81a890-goog
>