Re: [PATCH RFC/RFT] perf bpf skels: Stop using vmlinux.h generated from BTF, use subset of used structs + CO-RE. was Re: BPF skels in perf .Re: [GIT PULL] perf tools changes for v6.4

[Andrii Nakryiko]

On Fri, May 5, 2023 at 2:15 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Fri, May 05, 2023 at 01:46:30PM -0700, Ian Rogers wrote:
> > On Fri, May 5, 2023 at 1:43 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> > >
> > > On Fri, May 05, 2023 at 10:04:47AM -0700, Ian Rogers wrote:
> > > > On Fri, May 5, 2023 at 9:56 AM Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > > > >
> > > > > Em Fri, May 05, 2023 at 10:33:15AM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > > > Em Fri, May 05, 2023 at 01:03:14AM +0200, Jiri Olsa escreveu:
> > > > > > That with the preserve_access_index isn't needed, we need just the
> > > > > > fields that we access in the tools, right?
> > > > >
> > > > > I'm now doing build test this in many distro containers, without the two
> > > > > reverts, i.e. BPF skels continue as opt-out as in my pull request, to
> > > > > test build and also for the functionality tests on the tools using such
> > > > > bpf skels, see below, no touching of vmlinux nor BTF data during the
> > > > > build.
> > > > >
> > > > > - Arnaldo
> > > > >
> > > > > From 882adaee50bc27f85374aeb2fbaa5b76bef60d05 Mon Sep 17 00:00:00 2001
> > > > > From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > > > > Date: Thu, 4 May 2023 19:03:51 -0300
> > > > > Subject: [PATCH 1/1] perf bpf skels: Stop using vmlinux.h generated from BTF,
> > > > >  use subset of used structs + CO-RE
> > > > >
> > > > > Linus reported a build break due to using a vmlinux without a BTF elf
> > > > > section to generate the vmlinux.h header with bpftool for use in the BPF
> > > > > tools in tools/perf/util/bpf_skel/*.bpf.c.
> > > > >
> > > > > Instead add a vmlinux.h file with the structs needed with the fields the
> > > > > tools need, marking the structs with __attribute__((preserve_access_index)),
> > > > > so that libbpf's CO-RE code can fixup the struct field offsets.
> > > > >
> > > > > In some cases the vmlinux.h file that was being generated by bpftool
> > > > > from the kernel BTF information was not needed at all, just including
> > > > > linux/bpf.h, sometimes linux/perf_event.h was enough as non-UAPI
> > > > > types were not being used.
> > > > >
> > > > > To keep te patch small, include those UAPI headers from the trimmed down
> > > > > vmlinux.h file, that then provides the tools with just the structs and
> > > > > the subset of its fields needed for them.
> > > > >
> > > > > Testing it:
> > > > >
> > > > >   # perf lock contention -b find / > /dev/null
> > >
> > > I tested perf lock con -abv -L rcu_state sleep 1
> > > and needed fix below
> > >
> > > jirka
> >
> > I thought this was fixed by:
> > https://lore.kernel.org/lkml/20230427234833.1576130-1-namhyung@xxxxxxxxxx/
> > but I think that is just in perf-tools-next.
>
> ah ok, missed that one

Please try validating with veristat to check if all of perf's .bpf.o
files are successful. Veristat is part of selftests and can be built
with just `make -C tools/testing/selftests/bpf veristat`. After that;

 sudo ~/bin/veristat tools/perf/util/bpf_skel/.tmp/*.bpf.o

This is a surer way to check that BPF object files are ok at least on
your currently running kernel, than trying to exercise each BPF
program through perf commands.

>
> thanks,
> jirka