linux-kernel - Re: BPF skels in perf .Re: [GIT PULL] perf tools changes for v6.4

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Message-ID: <ZFQrT42SyEbCj4om@kernel.org>
Date:   Thu, 4 May 2023 19:01:51 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Song Liu <song@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Kate Carcia <kcarcia@...hat.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Changbin Du <changbin.du@...wei.com>,
        Hao Luo <haoluo@...gle.com>, Ian Rogers <irogers@...gle.com>,
        James Clark <james.clark@....com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Roman Lozko <lozko.roma@...il.com>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: BPF skels in perf .Re: [GIT PULL] perf tools changes for v6.4

Em Thu, May 04, 2023 at 06:48:50PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Thu, May 04, 2023 at 04:07:29PM -0300, Arnaldo Carvalho de Melo escreveu:
> > Em Thu, May 04, 2023 at 11:50:07AM -0700, Andrii Nakryiko escreveu:
> > > On Thu, May 4, 2023 at 10:52 AM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > > > Andrii, can you add some more information about the usage of vmlinux.h
> > > > instead of using kernel headers?
> >  
> > > I'll just say that vmlinux.h is not a hard requirement to build BPF
> > > programs, it's more a convenience allowing easy access to definitions
> > > of both UAPI and kernel-internal structures for tracing needs and
> > > marking them relocatable using BPF CO-RE machinery. Lots of real-world
> > > applications just check-in pregenerated vmlinux.h to avoid build-time
> > > dependency on up-to-date host kernel and such.
> >  
> > > If vmlinux.h generation and usage is causing issues, though, given
> > > that perf's BPF programs don't seem to be using many different kernel
> > > types, it might be a better option to just use UAPI headers for public
> > > kernel type definitions, and just define CO-RE-relocatable minimal
> > > definitions locally in perf's BPF code for the other types necessary.
> > > E.g., if perf needs only pid and tgid from task_struct, this would
> > > suffice:
> >  
> > > struct task_struct {
> > >     int pid;
> > >     int tgid;
> > > } __attribute__((preserve_access_index));
> > 
> > Yeah, that seems like a way better approach, no vmlinux involved, libbpf
> > CO-RE notices that task_struct changed from this two integers version
> > (of course) and does the relocation to where it is in the running kernel
> > by using /sys/kernel/btf/vmlinux.
> 
> Doing it for one of the skels, build tested, runtime untested, but not
> using any vmlinux, BTF to help, not that bad, more verbose, but at least
> we state what are the fields we actually use, have those attribute
> documenting that those offsets will be recorded for future use, etc.
> 
> Namhyung, can you please check that this works?

Second case was simpler:

diff --git a/tools/perf/util/bpf_skel/bperf_follower.bpf.c b/tools/perf/util/bpf_skel/bperf_follower.bpf.c
index f193998530d431d8..1ab06f2ff5ad7548 100644
--- a/tools/perf/util/bpf_skel/bperf_follower.bpf.c
+++ b/tools/perf/util/bpf_skel/bperf_follower.bpf.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
 // Copyright (c) 2021 Facebook
-#include "vmlinux.h"
+#include <linux/types.h>
+#include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 #include "bperf_u.h"