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
>
