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

[Namhyung Kim <namhyung@kernel.org>]

On Thu, May 04, 2023 at 06:48:50PM -0300, Arnaldo Carvalho de Melo wrote:
> 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@kernel.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?

Yep, it works great!

  $ sudo ./perf stat -a --bpf-counters --for-each-cgroup /,user.slice,system.slice sleep 1
  
   Performance counter stats for 'system wide':
  
           64,110.41 msec cpu-clock                        /                #   64.004 CPUs utilized
              15,787      context-switches                 /                #  246.247 /sec
                  72      cpu-migrations                   /                #    1.123 /sec
               1,236      page-faults                      /                #   19.279 /sec
         848,608,137      cycles                           /                #    0.013 GHz                         (83.23%)
         106,928,070      stalled-cycles-frontend          /                #   12.60% frontend cycles idle        (83.23%)
         209,204,795      stalled-cycles-backend           /                #   24.65% backend cycles idle         (83.23%)
         645,183,025      instructions                     /                #    0.76  insn per cycle
                                                    #    0.32  stalled cycles per insn     (83.24%)
         141,776,876      branches                         /                #    2.211 M/sec                       (83.63%)
           3,001,078      branch-misses                    /                #    2.12% of all branches             (83.44%)
               66.67 msec cpu-clock                        user.slice       #    0.067 CPUs utilized
                 695      context-switches                 user.slice       #   10.424 K/sec
                  22      cpu-migrations                   user.slice       #  329.966 /sec
               1,202      page-faults                      user.slice       #   18.028 K/sec
         150,514,330      cycles                           user.slice       #    2.257 GHz                         (90.17%)
          13,504,605      stalled-cycles-frontend          user.slice       #    8.97% frontend cycles idle        (69.71%)
          38,859,376      stalled-cycles-backend           user.slice       #   25.82% backend cycles idle         (95.28%)
         189,382,145      instructions                     user.slice       #    1.26  insn per cycle
                                                    #    0.21  stalled cycles per insn     (88.92%)
          36,019,878      branches                         user.slice       #  540.242 M/sec                       (90.16%)
             697,723      branch-misses                    user.slice       #    1.94% of all branches             (65.77%)
               44.33 msec cpu-clock                        system.slice     #    0.044 CPUs utilized
               2,382      context-switches                 system.slice     #   53.732 K/sec
                  42      cpu-migrations                   system.slice     #  947.418 /sec
                  34      page-faults                      system.slice     #  766.958 /sec
         100,383,549      cycles                           system.slice     #    2.264 GHz                         (87.27%)
          10,165,225      stalled-cycles-frontend          system.slice     #   10.13% frontend cycles idle        (71.73%)
          29,964,682      stalled-cycles-backend           system.slice     #   29.85% backend cycles idle         (84.94%)
         101,210,743      instructions                     system.slice     #    1.01  insn per cycle
                                                    #    0.30  stalled cycles per insn     (80.68%)
          19,893,831      branches                         system.slice     #  448.757 M/sec                       (86.94%)
             397,854      branch-misses                    system.slice     #    2.00% of all branches             (88.42%)
  
         1.001667221 seconds time elapsed
  
Thanks,
Namhyung


> diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> index 6a438e0102c5a2cb..f376d162549ebd74 100644
> --- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> +++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
> @@ -1,11 +1,40 @@
>  // SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>  // Copyright (c) 2021 Facebook
>  // Copyright (c) 2021 Google
> -#include "vmlinux.h"
> +#include <linux/types.h>
> +#include <linux/bpf.h>
>  #include <bpf/bpf_helpers.h>
>  #include <bpf/bpf_tracing.h>
>  #include <bpf/bpf_core_read.h>
>  
> +// libbpf's CO-RE will take care of the relocations so that these fields match
> +// the layout of these structs in the kernel where this ends up running on.
> +
> +struct cgroup_subsys_state {
> +	struct cgroup *cgroup;
> +} __attribute__((preserve_access_index));
> +
> +struct css_set {
> +	struct cgroup_subsys_state *subsys[13];
> +} __attribute__((preserve_access_index));
> +
> +struct task_struct {
> +	struct css_set *cgroups;
> +} __attribute__((preserve_access_index));
> +
> +struct kernfs_node {
> +	__u64 id;
> +}  __attribute__((preserve_access_index));
> +
> +struct cgroup {
> +	struct kernfs_node *kn;
> +	int                level;
> +}  __attribute__((preserve_access_index));
> +
> +enum cgroup_subsys_id {
> +	perf_event_cgrp_id  = 8,
> +};
> +
>  #define MAX_LEVELS  10  // max cgroup hierarchy level: arbitrary
>  #define MAX_EVENTS  32  // max events per cgroup: arbitrary
>  
> @@ -52,7 +81,7 @@ struct cgroup___new {
>  /* old kernel cgroup definition */
>  struct cgroup___old {
>  	int level;
> -	u64 ancestor_ids[];
> +	__u64 ancestor_ids[];
>  } __attribute__((preserve_access_index));
>  
>  const volatile __u32 num_events = 1;
>