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

[Arnaldo Carvalho de Melo]

Em Fri, May 05, 2023 at 01:48:52PM -0700, Namhyung Kim escreveu:
> On Fri, May 5, 2023 at 1:46 PM Ian Rogers <irogers@xxxxxxxxxx> 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/

Those are upstream already:

⬢[acme@toolbox perf-tools]$ git log --oneline torvalds/master | grep -m1 -B1 "perf lock contention: Fix struct rq lock access"
b9f82b5c63bf5390 perf lock contention: Rework offset calculation with BPF CO-RE
e53de7b65a3ca59a perf lock contention: Fix struct rq lock access
⬢[acme@toolbox perf-tools]$

> > but I think that is just in perf-tools-next.
 
> Right, but we might still need the empty rq definition.

Yeah, without the empty struct diff libbpf complains about a mismatch of
just a forward declaration as the type for 'runqueues' on the
lock_contention.bpf.c file while the kernel has a the type as 'struct
rq':

[root@quaco ~]# perf lock con -ab sleep 1
libbpf: extern (var ksym) 'runqueues': incompatible types, expected [95] fwd rq, but kernel has [55509] struct rq
libbpf: failed to load object 'lock_contention_bpf'
libbpf: failed to load BPF skeleton 'lock_contention_bpf': -22
Failed to load lock-contention BPF skeleton
lock contention BPF setup failed
[root@quaco ~]#

Adding:

struct rq {};

libbpf is happy:

[root@quaco ~]# perf lock con -ab sleep 1
 contended   total wait     max wait     avg wait         type   caller

         2     50.64 us     25.38 us     25.32 us     spinlock   tick_do_update_jiffies64+0x25
         1     26.18 us     26.18 us     26.18 us     spinlock   tick_do_update_jiffies64+0x25
[root@quaco ~]#

- Arnaldo