Re: [PATCH bpf-next] bpf: Cache the last valid build_id.

[Andrii Nakryiko]

On Wed, Feb 23, 2022 at 2:20 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> For binaries that are statically linked, consecutive stack frames are
> likely to be in the same VMA and therefore have the same build id.
> As an optimization for this case, we can cache the previous frame's
> VMA, if the new frame has the same VMA as the previous one, reuse the
> previous one's build id. We are holding the MM locks as reader across
> the entire loop, so we don't need to worry about VMA going away.
>
> Tested through "stacktrace_build_id" and "stacktrace_build_id_nmi" in
> test_progs.
>
> Suggested-by: Greg Thelen <gthelen@xxxxxxxxxx>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
>  kernel/bpf/stackmap.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 22c8ae94e4c1..280b9198af27 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -132,7 +132,8 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>         int i;
>         struct mmap_unlock_irq_work *work = NULL;
>         bool irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
> -       struct vm_area_struct *vma;
> +       struct vm_area_struct *vma, *prev_vma = NULL;
> +       const char *prev_build_id;
>
>         /* If the irq_work is in use, fall back to report ips. Same
>          * fallback is used for kernel stack (!user) on a stackmap with
> @@ -151,6 +152,11 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>
>         for (i = 0; i < trace_nr; i++) {
>                 vma = find_vma(current->mm, ips[i]);

as a further optimization, shouldn't we first check if ips[i] is
within prev_vma and avoid rbtree walk altogether? Would this work:

if (prev_vma && range_in_vma(prev_vma, ips[i])) {
   /* reuse build_id */
}
vma = find_vma(current->mm, ips[i]);


?

> +               if (vma && vma == prev_vma) {
> +                       memcpy(id_offs[i].build_id, prev_build_id,
> +                              BUILD_ID_SIZE_MAX);
> +                       goto build_id_valid;
> +               }
>                 if (!vma || build_id_parse(vma, id_offs[i].build_id, NULL)) {
>                         /* per entry fall back to ips */
>                         id_offs[i].status = BPF_STACK_BUILD_ID_IP;
> @@ -158,9 +164,12 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>                         memset(id_offs[i].build_id, 0, BUILD_ID_SIZE_MAX);
>                         continue;
>                 }
> +build_id_valid:
>                 id_offs[i].offset = (vma->vm_pgoff << PAGE_SHIFT) + ips[i]
>                         - vma->vm_start;
>                 id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
> +               prev_vma = vma;
> +               prev_build_id = id_offs[i].build_id;
>         }
>         bpf_mmap_unlock_mm(work, current->mm);
>  }
> --
> 2.35.1.473.g83b2b277ed-goog
>