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

On 03/24, Andrii Nakryiko wrote:

On Tue, Mar 24, 2020 at 4:31 PM Stanislav Fomichev [off-list ref] wrote:

quoted

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 <redacted>
---
 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.

quoted

@@ -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.

quoted

-               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.