btf_encoder: Support delaying function addition to check for function prototype inconsistencies
There are multiple sources of inconsistency that can result in functions
of the same name having multiple prototypes:
- multiple static functions in different CUs share the same name
- static and external functions share the same name
In addition a function may have optimized-out parameters in some
CUs but not others.
Here we attempt to catch such cases by finding inconsistencies across
CUs using the save/compare/merge mechanisms that were previously
introduced to handle optimized-out parameters.
For two instances of a function to be considered consistent:
- number of parameters must match
- parameter names must match
The latter is a less strong method than a full type comparison but
suffices to match functions.
To enable inconsistency checking, the --skip_encoding_btf_inconsistent_proto
option is introduced.
With it, and the --btf_gen_optimized options in place:
- 285 functions are omitted due to inconsistent function prototypes
- 2495 functions are omitted due to optimized-out parameters, of these
803 are functions with optimization-related suffixes ".isra", etc,
leaving 1692 other functions without such suffixes.
Below performance effects and variability in encoded BTF are detailed.
It can be seen that due to the approach used - where functions are
marked "generated" on a per-encoder basis, we see quite variable
numbers of multiply-defined functions in the baseline case, some
with inconsistent prototypes. With --skip_encoding_btf_inconsistent_proto
specified, this variability disappears, at the cost of a longer time
to carry out encoding due to the need to compare representations across
encoders at thread collection time.
Baseline:
Single-threaded:
$ time LLVM_OBJCOPY=objcopy pahole -J vmlinux
real 0m17.534s
user 0m17.019s
sys 0m0.514s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
51529
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
51529
2 threads:
$ time LLVM_OBJCOPY=objcopy pahole -J -j2 vmlinux
real 0m10.942s
user 0m17.309s
sys 0m0.592s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
51798
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
51529
4 threads:
$ time LLVM_OBJCOPY=objcopy pahole -J -j4 vmlinux
real 0m7.890s
user 0m18.067s
sys 0m0.661s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
52028
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
51529
Test:
Single-threaded:
$ time LLVM_OBJCOPY=objcopy pahole -J --skip_encoding_btf_inconsistent_proto --btf_gen_optimized vmlinux
real 0m19.216s
user 0m17.590s
sys 0m1.624s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
50246
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
50246
2 threads:
$ time LLVM_OBJCOPY=objcopy pahole -J -j2 --skip_encoding_btf_inconsistent_proto --btf_gen_optimized vmlinux
real 0m13.147s
user 0m18.179s
sys 0m3.486s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
50246
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
50246
4 threads:
$ time LLVM_OBJCOPY=objcopy pahole -J -j4 --skip_encoding_btf_inconsistent_proto --btf_gen_optimized vmlinux
real 0m11.090s
user 0m19.613s
sys 0m5.895s
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |wc -l
50246
$ bpftool btf dump file vmlinux|grep " FUNC "| awk '{print $3}'|sort |uniq|wc -l
50246
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Hao Luo <haoluo@google.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: Kui-Feng Lee <sinquersw@gmail.com>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Timo Beckers <timo@incline.eu>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/1675790102-23037-7-git-send-email-alan.maguire@oracle.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
4 files changed
