The SOURCE_DATE_EPOCH environment variable is used in reproducible
builds to set build timestamps and is generally synonymous with
reproducible builds: instead of making users figure out how to pass
pahole flags (e.g. explicitly setting PAHOLE_FLAGS in linux) just assume
that the user wants a reproducible build if this variable is set,
printing a debug message in verbose mode.
The impact on build time appears to be minimal enough even if we
incorrectly make this decision (on a Ryzen 7 7735HS):
$ hyperfine 'sh pahole-test -j --reproducible_build' 'sh pahole-test -j1' 'sh pahole-test -j' -p 'cp .tmp_vmlinux.orig .tmp_vmlinux.btf' -m 4
Benchmark 1: sh pahole-test -j --reproducible_build
Time (mean ± σ): 3.991 s ± 0.045 s [User: 7.223 s, System: 3.741 s]
Range (min … max): 3.950 s … 4.042 s 4 runs
Benchmark 2: sh pahole-test -j1
Time (mean ± σ): 7.083 s ± 0.095 s [User: 4.805 s, System: 2.242 s]
Range (min … max): 6.964 s … 7.191 s 4 runs
Benchmark 3: sh pahole-test -j
Time (mean ± σ): 3.858 s ± 0.089 s [User: 13.447 s, System: 7.078 s]
Range (min … max): 3.763 s … 3.978 s 4 runs
Summary
sh pahole-test -j ran
1.03 ± 0.03 times faster than sh pahole-test -j --reproducible_build
1.84 ± 0.05 times faster than sh pahole-test -j1
Where pahole-test is the pahole command of the linux build (minus -j):
LLVM_OBJCOPY=objcopy pahole -J --btf_gen_floats --lang_exclude=rust --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf "$@"
And .tmp_vmlinux.orig was the .tmp_vmlinux.btf binary before being
processed:
ld -m elf_x86_64 -z noexecstack --no-warn-rwx-segments --emit-relocs --discard-none -z max-page-size=0x200000 --build-id=sha1 --orphan-handling=warn --script=./arch/x86/kernel/vmlinux.lds -o .tmp_vmlinux.btf --whole-archive vmlinux.a .vmlinux.export.o init/version-timestamp.o --no-whole-archive --start-group --end-group
Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
---
v1 -> v2:
- added message in verbose mode if setting reproducible build through
this.
I wasn't quite sure what is least intrusive message-wise -- most
verbose messages just go to stdout, but using pahole in a script to
extract some c structs would see this mangled in output which is
obviously not great. Since --reproducible_build only impacts BTF
generation at this time, I'm only checking when generating BTF to
avoid such needless messages -- btf generation always goes through a
file so there is no risk of mangling the output in this case.
pahole.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/pahole.c b/pahole.c
index c866095c8be6..2792e11265d6 100644
--- a/pahole.c
+++ b/pahole.c
@@ -3705,6 +3705,15 @@ int main(int argc, char *argv[])
goto out;
}
+ // SOURCE_DATE_EPOCH being set means whoever called us likeely tries
+ // to do a reproducible build. The setting only impacts btf_encode code
+ // so only check in this case to avoid needless messages.
+ if (btf_encode && getenv("SOURCE_DATE_EPOCH")) {
+ if (!conf_load.reproducible_build && global_verbose)
+ printf("pahole: auto-enabling reproducible build (SOURCE_DATE_EPOCH set)\n");
+ conf_load.reproducible_build = true;
+ }
+
if (languages.str && parse_languages())
return rc;
--
2.45.1
