Re: [PATCH 2/2] perf lock contention: Rework offset calculation with BPF CO-RE

[Ian Rogers <irogers@google.com>]

On Thu, Apr 27, 2023 at 4:48 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> It seems BPF CO-RE reloc doesn't work well with the pattern that gets
> the field-offset only.  Use offsetof() to make it explicit so that
> the compiler would generate the correct code.
>
> Fixes: 0c1228486bef ("perf lock contention: Support pre-5.14 kernels")
> Co-developed-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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

Thanks,
Ian

> ---
>  tools/perf/util/bpf_skel/lock_contention.bpf.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 30c193078bdb..8d3cfbb3cc65 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -429,21 +429,21 @@ struct rq___new {
>  SEC("raw_tp/bpf_test_finish")
>  int BPF_PROG(collect_lock_syms)
>  {
> -       __u64 lock_addr;
> +       __u64 lock_addr, lock_off;
>         __u32 lock_flag;
>
> +       if (bpf_core_field_exists(struct rq___new, __lock))
> +               lock_off = offsetof(struct rq___new, __lock);
> +       else
> +               lock_off = offsetof(struct rq___old, lock);
> +
>         for (int i = 0; i < MAX_CPUS; i++) {
>                 struct rq *rq = bpf_per_cpu_ptr(&runqueues, i);
> -               struct rq___new *rq_new = (void *)rq;
> -               struct rq___old *rq_old = (void *)rq;
>
>                 if (rq == NULL)
>                         break;
>
> -               if (bpf_core_field_exists(rq_new->__lock))
> -                       lock_addr = (__u64)&rq_new->__lock;
> -               else
> -                       lock_addr = (__u64)&rq_old->lock;
> +               lock_addr = (__u64)(void *)rq + lock_off;
>                 lock_flag = LOCK_CLASS_RQLOCK;
>                 bpf_map_update_elem(&lock_syms, &lock_addr, &lock_flag, BPF_ANY);
>         }
> --
> 2.40.1.495.gc816e09b53d-goog
>