Re: [PATCH v3] perf tools: Get a perf cgroup more portably in BPF

[Ian Rogers]

On Thu, Sep 22, 2022 at 1:53 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> The perf_event_cgrp_id can be different on other configurations.
> To be more portable as CO-RE, it needs to get the cgroup subsys id
> using the bpf_core_enum_value() helper.
>
> Suggested-by: Ian Rogers <irogers@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> v3 changes)
>  * check compiler features for enum value
>
> v2 changes)
>  * fix off_cpu.bpf.c too
>  * get perf_subsys_id only once
>
>  tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 11 ++++++++++-
>  tools/perf/util/bpf_skel/off_cpu.bpf.c      | 12 ++++++++----
>  2 files changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> index 292c430768b5..8e7520e273db 100644
> --- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> +++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> @@ -48,6 +48,7 @@ const volatile __u32 num_cpus = 1;
>
>  int enabled = 0;
>  int use_cgroup_v2 = 0;
> +int perf_subsys_id = -1;
>
>  static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
>  {
> @@ -58,7 +59,15 @@ static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
>         int level;
>         int cnt;
>
> -       cgrp = BPF_CORE_READ(p, cgroups, subsys[perf_event_cgrp_id], cgroup);
> +       if (perf_subsys_id == -1) {
> +#if __has_builtin(__builtin_preserve_enum_value)
> +               perf_subsys_id = bpf_core_enum_value(enum cgroup_subsys_id,
> +                                                    perf_event_cgrp_id);
> +#else
> +               perf_subsys_id = perf_event_cgrp_id;
> +#endif
> +       }
> +       cgrp = BPF_CORE_READ(p, cgroups, subsys[perf_subsys_id], cgroup);
>         level = BPF_CORE_READ(cgrp, level);
>
>         for (cnt = 0; i < MAX_LEVELS; i++) {
> diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> index c4ba2bcf179f..e917ef7b8875 100644
> --- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
> +++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
> @@ -94,6 +94,8 @@ const volatile bool has_prev_state = false;
>  const volatile bool needs_cgroup = false;
>  const volatile bool uses_cgroup_v1 = false;
>
> +int perf_subsys_id = -1;
> +
>  /*
>   * Old kernel used to call it task_struct->state and now it's '__state'.
>   * Use BPF CO-RE "ignored suffix rule" to deal with it like below:
> @@ -119,11 +121,13 @@ static inline __u64 get_cgroup_id(struct task_struct *t)
>  {
>         struct cgroup *cgrp;
>
> -       if (uses_cgroup_v1)
> -               cgrp = BPF_CORE_READ(t, cgroups, subsys[perf_event_cgrp_id], cgroup);
> -       else
> -               cgrp = BPF_CORE_READ(t, cgroups, dfl_cgrp);
> +       if (!uses_cgroup_v1)
> +               return BPF_CORE_READ(t, cgroups, dfl_cgrp, kn, id);
> +
> +       if (perf_subsys_id == -1)
> +               perf_subsys_id = bpf_core_enum_value(enum cgroup_subsys_id, perf_event_cgrp_id);

Should the "#if __has_builtin(__builtin_preserve_enum_value)" test also be here?

It feels a shame that bpf_core_enum_value isn't defined something like:

#if __has_builtin(__builtin_preserve_enum_value)
#define bpf_core_enum_value(enum_type, enum_value) \
__builtin_preserve_enum_value(*(typeof(enum_type) *)enum_value,
BPF_ENUMVAL_VALUE)
#else
#define bpf_core_enum_value(enum_type, enum_value) enum_value
#endif

for backward clang compatibility, but I could see why an error would
be preferable.

Thanks,
Ian

>
> +       cgrp = BPF_CORE_READ(t, cgroups, subsys[perf_subsys_id], cgroup);
>         return BPF_CORE_READ(cgrp, kn, id);
>  }
>
> --
> 2.37.3.998.g577e59143f-goog
>