Re: [PATCH] perf record: Fix BTF type checks in the off-cpu profiling

[Namhyung Kim]

Hi Ian,

On Sun, Sep 24, 2023 at 11:03 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Fri, Sep 22, 2023 at 4:44 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > The BTF func proto for a tracepoint has one more argument than the
> > actual tracepoint function since it has a context argument at the
> > begining.  So it should compare to 5 when the tracepoint has 4
> > arguments.
> >
> >   typedef void (*btf_trace_sched_switch)(void *, bool, struct task_struct *, struct task_struct *, unsigned int);
> >
> > Also, recent change in the perf tool would use a hand-written minimal
> > vmlinux.h to generate BTF in the skeleton.  So it won't have the info
> > of the tracepoint.  Anyway it should use the kernel's vmlinux BTF to
> > check the type in the kernel.
> >
> > Fixes: b36888f71c85 ("perf record: Handle argument change in sched_switch")
> > Cc: Song Liu <song@xxxxxxxxxx>
> > Cc: Hao Luo <haoluo@xxxxxxxxxx>
> > CC: bpf@xxxxxxxxxxxxxxx
> > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks for the review!

>
> > ---
> >  tools/perf/util/bpf_off_cpu.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
> > index 01f70b8e705a..21f4d9ba023d 100644
> > --- a/tools/perf/util/bpf_off_cpu.c
> > +++ b/tools/perf/util/bpf_off_cpu.c
> > @@ -98,7 +98,7 @@ static void off_cpu_finish(void *arg __maybe_unused)
> >  /* v5.18 kernel added prev_state arg, so it needs to check the signature */
> >  static void check_sched_switch_args(void)
> >  {
> > -       const struct btf *btf = bpf_object__btf(skel->obj);
> > +       const struct btf *btf = btf__load_vmlinux_btf();
> >         const struct btf_type *t1, *t2, *t3;
> >         u32 type_id;
> >
> > @@ -116,7 +116,8 @@ static void check_sched_switch_args(void)
> >                 return;
> >
> >         t3 = btf__type_by_id(btf, t2->type);
> > -       if (t3 && btf_is_func_proto(t3) && btf_vlen(t3) == 4) {
> > +       /* btf_trace func proto has one more argument for the context */
> > +       if (t3 && btf_is_func_proto(t3) && btf_vlen(t3) == 5) {
> >                 /* new format: pass prev_state as 4th arg */
>
> nit: does this comment need updating?

No, it's the 4th arg and the sched_switch is called like.

        trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next,
prev_state);

Thanks,
Namhyung