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

[Greg Thelen <gthelen@google.com>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Wed, Feb 23, 2022 at 2:20 PM Hao Luo <haoluo@google.com> 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@google.com>
>> Signed-off-by: Hao Luo <haoluo@google.com>
>> ---
>>  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]);
>
>
> ?

Yes, that's a nice addition. Good idea.

>> +               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
>>