Re: [RESEND PATCH v5 1/4] perf/bpf: Call bpf handler directly, not through overflow machinery

[Kyle Huey]

On Thu, Apr 11, 2024 at 8:11 AM Kyle Huey <me@xxxxxxxxxxxx> wrote:
>
> On Wed, Apr 10, 2024 at 12:32 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> >
> > * Kyle Huey <me@xxxxxxxxxxxx> wrote:
> >
> > > To ultimately allow bpf programs attached to perf events to completely
> > > suppress all of the effects of a perf event overflow (rather than just the
> > > sample output, as they do today), call bpf_overflow_handler() from
> > > __perf_event_overflow() directly rather than modifying struct perf_event's
> > > overflow_handler. Return the bpf program's return value from
> > > bpf_overflow_handler() so that __perf_event_overflow() knows how to
> > > proceed. Remove the now unnecessary orig_overflow_handler from struct
> > > perf_event.
> > >
> > > This patch is solely a refactoring and results in no behavior change.
> > >
> > > Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> > > Suggested-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > Acked-by: Song Liu <song@xxxxxxxxxx>
> > > Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > > ---
> > >  include/linux/perf_event.h |  6 +-----
> > >  kernel/events/core.c       | 28 +++++++++++++++-------------
> > >  2 files changed, 16 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > > index d2a15c0c6f8a..c7f54fd74d89 100644
> > > --- a/include/linux/perf_event.h
> > > +++ b/include/linux/perf_event.h
> > > @@ -810,7 +810,6 @@ struct perf_event {
> > >       perf_overflow_handler_t         overflow_handler;
> > >       void                            *overflow_handler_context;
> > >  #ifdef CONFIG_BPF_SYSCALL
> > > -     perf_overflow_handler_t         orig_overflow_handler;
> > >       struct bpf_prog                 *prog;
> > >       u64                             bpf_cookie;
> > >  #endif
> >
> > Could we reduce the #ifdeffery please?
>
> Not easily.
>
> > On distros CONFIG_BPF_SYSCALL is almost always enabled, so it's not like
> > this truly saves anything on real systems.
> >
> > I'd suggest making the perf_event::prog and perf_event::bpf_cookie fields
> > unconditional.
>
> That's not sufficient. See below.
>
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > +static int bpf_overflow_handler(struct perf_event *event,
> > > +                             struct perf_sample_data *data,
> > > +                             struct pt_regs *regs);
> > > +#endif
> >
> > If the function definitions are misordered then first do a patch that moves
> > the function earlier in the file, instead of slapping a random prototype
> > into a random place.
>
> Ok.
>
> > > -     READ_ONCE(event->overflow_handler)(event, data, regs);
> > > +#ifdef CONFIG_BPF_SYSCALL
> > > +     if (!(event->prog && !bpf_overflow_handler(event, data, regs)))
> > > +#endif
> > > +             READ_ONCE(event->overflow_handler)(event, data, regs);
> >
> > This #ifdef would go away too - on !CONFIG_BPF_SYSCALL event->prog should
> > always be NULL.
>
> bpf_overflow_handler() is also #ifdef CONFIG_BPF_SYSCALL. It uses
> bpf_prog_active, so that would need to be moved out of the ifdef,
> which would require moving the DEFINE_PER_CPU out of bpf/syscall.c ...
> or I'd have to add a !CONFIG_BPF_SYSCALL definition of
> bpf_overflow_handler() that only returns 1 and never actually gets
> called because the condition short-circuits on event->prog. Neither
> seems like it makes my patch or the code simpler, especially since
> this weird ifdef-that-applies-only-to-the-condition goes away in Part
> 3 where I actually change the behavior.

After fiddling with this I think the stub definition of
bpf_overflow_handler() is fine. The other CONFIG_BPF_SYSCALL functions
in this file already have similar stubs. I'll send a new patch set.

- Kyle

> It feels like the root of your objection is that CONFIG_BPF_SYSCALL
> exists at all. I could remove it in a separate patch if there's
> consensus about that.
>
>
>
>
> > Please keep the #ifdeffery reduction and function-moving patches separate
> > from these other changes.
> >
> > Thanks,
> >
> >         Ingo
>
> - Kyle