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

[Stanislav Fomichev <sdf@fomichev.me>]

On 03/24, Andrii Nakryiko wrote:
> 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).
Agreed, makes sense.

> > @@ -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?
Yeah, I don't see why it shouldn't. Let me try to see if I hit something
in the selftests with that approach.

> > -               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?
In this case, maybe start with the existing 16M BPF_LOG_BUF_SIZE?
My goal here is optimize for the successful case. If there is an error the
size shouldn't matter that much.