Re: [PATCH 02/12] pahole: Disable BTF multithreaded encoded when doing reproducible builds

Andrii Nakryiko
2024-04-03T18:19:47Z

On Tue, Apr 2, 2024 at 12:40 PM Arnaldo Carvalho de Melo
<acme@kernel.org> wrote:
>
> From: Arnaldo Carvalho de Melo <acme@redhat.com>
>
> Reproducible builds need to produce BTF that have the same ids, which is
> not possible at the moment to do in parallel with libbpf, so serialize
> the encoding.
>
> The next patches will also make sure that DWARF while being read in
> parallel into internal representation for later BTF encoding has its CU
> (Compile Units) fed to the BTF encoder in the same order as it is in the
> DWARF file, this way we'll produce the same BTF output no matter how
> many threads are used to read BTF.
>
> Then we'll make sure we have tests in place that compare the output of
> parallel BTF encoding (well, just the DWARF loading part, maybe the BTF
> in the future), i.e. when using 'pahole -j' with the one obtained when
> doing single threaded encoding.
>
> Testing it on a:
>
> # grep -m1 "model name" /proc/cpuinfo
> model name : 13th Gen Intel(R) Core(TM) i7-1365U
> ~#
>
> I.e. 2 performance cores (4 threads) + 8 efficiency cores.
>
> From:
>
> $ perf stat -r5 pahole -j --btf_encode_detached=vmlinux.btf.parallel vmlinux
>
> Performance counter stats for 'pahole -j --btf_encode_detached=vmlinux.btf.parallel vmlinux' (5 runs):
>
> 17,187.27 msec task-clock:u # 6.153 CPUs utilized ( +- 0.34% )
> <SNIP>
> 2.7931 +- 0.0336 seconds time elapsed ( +- 1.20% )
>
> $
>
> To:
>
> $ perf stat -r5 pahole -j --reproducible_build --btf_encode_detached=vmlinux.btf.parallel.reproducible_build vmlinux
>
> Performance counter stats for 'pahole -j --reproducible_build --btf_encode_detached=vmlinux.btf.parallel.reproducible_build vmlinux' (5 runs):
>
> 14,654.06 msec task-clock:u # 3.507 CPUs utilized ( +- 0.45% )
> <SNIP>
> 4.1787 +- 0.0344 seconds time elapsed ( +- 0.82% )
>
> $
>
> Which is still a nice improvement over doing it completely serially:
>
> $ perf stat -r5 pahole --btf_encode_detached=vmlinux.btf.serial vmlinux
>
> Performance counter stats for 'pahole --btf_encode_detached=vmlinux.btf.serial vmlinux' (5 runs):
>
> 7,506.93 msec task-clock:u # 1.000 CPUs utilized ( +- 0.13% )
> <SNIP>
> 7.5106 +- 0.0115 seconds time elapsed ( +- 0.15% )
>
> $
>
> $ pahole vmlinux.btf.parallel > /tmp/parallel
> $ pahole vmlinux.btf.parallel.reproducible_build > /tmp/parallel.reproducible_build
> $ diff -u /tmp/parallel /tmp/parallel.reproducible_build | wc -l
> 269920
> $ pahole --sort vmlinux.btf.parallel > /tmp/parallel.sorted
> $ pahole --sort vmlinux.btf.parallel.reproducible_build > /tmp/parallel.reproducible_build.sorted
> $ diff -u /tmp/parallel.sorted /tmp/parallel.reproducible_build.sorted | wc -l
> 0
> $
>
> The BTF ids continue to be undeterministic, as we need to process the
> CUs (compile unites) in the same order that they are on vmlinux:
>
> $ bpftool btf dump file vmlinux.btf.serial > btfdump.serial
> $ bpftool btf dump file vmlinux.btf.parallel.reproducible_build > btfdump.parallel.reproducible_build
> $ bpftool btf dump file vmlinux.btf.parallel > btfdump.parallel
> $ diff -u btfdump.serial btfdump.parallel | wc -l
> 624144
> $ diff -u btfdump.serial btfdump.parallel.reproducible_build | wc -l
> 594622
> $ diff -u btfdump.parallel.reproducible_build btfdump.parallel | wc -l
> 623355
> $
>
> The BTF ids don't match, we'll get them to match at the end of this
> patch series:
>
> $ tail -5 btfdump.serial
> type_id=127124 offset=219200 size=40 (VAR 'rt6_uncached_list')
> type_id=11760 offset=221184 size=64 (VAR 'vmw_steal_time')
> type_id=13533 offset=221248 size=8 (VAR 'kvm_apic_eoi')
> type_id=13532 offset=221312 size=64 (VAR 'steal_time')
> type_id=13531 offset=221376 size=68 (VAR 'apf_reason')
> $ tail -5 btfdump.parallel.reproducible_build
> type_id=113812 offset=219200 size=40 (VAR 'rt6_uncached_list')
> type_id=87979 offset=221184 size=64 (VAR 'vmw_steal_time')
> type_id=127391 offset=221248 size=8 (VAR 'kvm_apic_eoi')
> type_id=127390 offset=221312 size=64 (VAR 'steal_time')
> type_id=127389 offset=221376 size=68 (VAR 'apf_reason')
> $
>
> Now to make it process the CUs in order, that should get everything
> straight without hopefully not degrading it further too much.
>
> Cc: Alan Maguire <alan.maguire@oracle.com>
> Cc: Kui-Feng Lee <kuifeng@fb.com>
> Cc: Thomas Weißschuh <linux@weissschuh.net>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> ---
> pahole.c | 25 ++++++++++++++++++++++---
> 1 file changed, 22 insertions(+), 3 deletions(-)
>

We can still produce per-thread smaller BTFs in parallel, that won't
hurt reproducibility. You only need to concatenate them in
reproducible order at the very end.

Or maybe it's already working like this, not sure, I'm a bit rusty in
pahole internals nowadays, sorry :)

> diff --git a/pahole.c b/pahole.c
> index 96e153432fa212a5..fcb4360f11debeb9 100644
> --- a/pahole.c
> +++ b/pahole.c
> @@ -3173,6 +3173,14 @@ struct thread_data {
> struct btf_encoder *encoder;
> }; …