Re: [PATCH bpf-next] libbpf: don't allocate 16M for log buffer by default

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Mar 24, 2020 at 4:31 PM Stanislav Fomichev <sdf@google.com> wrote:
>
> For each prog/btf load we allocate and free 16 megs of verifier buffer.
> On production systems it doesn't really make sense because the
> programs/btf have gone through extensive testing and (mostly) guaranteed
> to successfully load.
>
> Let's switch to a much smaller buffer by default (128 bytes, sys_bpf
> doesn't accept smaller log buffer) and resize it if the kernel returns
> ENOSPC. On the first ENOSPC error we resize the buffer to BPF_LOG_BUF_SIZE
> and then, on each subsequent ENOSPC, we keep doubling the buffer.
>
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
>  tools/lib/bpf/btf.c             | 10 +++++++++-
>  tools/lib/bpf/libbpf.c          | 10 ++++++++--
>  tools/lib/bpf/libbpf_internal.h |  2 ++
>  3 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 3d1c25fc97ae..53c7efc3b347 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -657,13 +657,14 @@ int btf__finalize_data(struct bpf_object *obj, struct btf *btf)
>
>  int btf__load(struct btf *btf)
>  {
> -       __u32 log_buf_size = BPF_LOG_BUF_SIZE;
> +       __u32 log_buf_size = BPF_MIN_LOG_BUF_SIZE;
>         char *log_buf = NULL;
>         int err = 0;
>
>         if (btf->fd >= 0)
>                 return -EEXIST;
>
> +retry_load:
>         log_buf = malloc(log_buf_size);
>         if (!log_buf)
>                 return -ENOMEM;

I'd argue that on first try we shouldn't allocate log_buf at all, then
start allocating it using reasonable starting size (see below).

> @@ -673,6 +674,13 @@ int btf__load(struct btf *btf)
>         btf->fd = bpf_load_btf(btf->data, btf->data_size,
>                                log_buf, log_buf_size, false);
>         if (btf->fd < 0) {
> +               if (errno == ENOSPC) {
> +                       log_buf_size = max((__u32)BPF_LOG_BUF_SIZE,
> +                                          log_buf_size << 1);
> +                       free(log_buf);
> +                       goto retry_load;
> +               }
> +
>                 err = -errno;
>                 pr_warn("Error loading BTF: %s(%d)\n", strerror(errno), errno);
>                 if (*log_buf)
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 085e41f9b68e..793c81b35ccc 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4855,7 +4855,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
>  {
>         struct bpf_load_program_attr load_attr;
>         char *cp, errmsg[STRERR_BUFSIZE];
> -       int log_buf_size = BPF_LOG_BUF_SIZE;
> +       size_t log_buf_size = BPF_MIN_LOG_BUF_SIZE;
>         char *log_buf;
>         int btf_fd, ret;
>
> @@ -4911,7 +4911,13 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
>         }
>
>         if (errno == ENOSPC) {

same, doing if (!log_buf || errno == ENOSPC) should handle this
without any other major changes?

> -               log_buf_size <<= 1;
> +               if (errno == ENOSPC) {
> +                       log_buf_size = max((size_t)BPF_LOG_BUF_SIZE,
> +                                          log_buf_size << 1);
> +                       free(log_buf);
> +                       goto retry_load;
> +               }
> +
>                 free(log_buf);
>                 goto retry_load;
>         }
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 8c3afbd97747..2720f3366798 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -23,6 +23,8 @@
>  #define BTF_PARAM_ENC(name, type) (name), (type)
>  #define BTF_VAR_SECINFO_ENC(type, offset, size) (type), (offset), (size)
>
> +#define BPF_MIN_LOG_BUF_SIZE 128

This seems way too low, if there is some error it almost certainly
will be too short, probably for few iterations, just causing waste.
Let's make it something a bit more reasonable, like 32KB or something?

> +
>  #ifndef min
>  # define min(x, y) ((x) < (y) ? (x) : (y))
>  #endif
> --
> 2.25.1.696.g5e7596f4ac-goog
>