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

[Namhyung Kim]

On Thu, Sep 22, 2022 at 4:55 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> 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?

Ouch!  Will send v4.

>
> 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

Yeah, that would be much easier to use.

Thanks,
Namhyung


>
> 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
> >