Re: [PATCH 4/4] perf record: Handle argument change in sched_switch

[Andrii Nakryiko]

On Wed, Apr 27, 2022 at 11:15 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Tue, Apr 26, 2022 at 4:55 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Fri, Apr 22, 2022 at 3:49 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > >
> > > Recently sched_switch tracepoint added a new argument for prev_state,
> > > but it's hard to handle the change in a BPF program.  Instead, we can
> > > check the function prototype in BTF before loading the program.
> > >
> > > Thus I make two copies of the tracepoint handler and select one based
> > > on the BTF info.
> > >
> > > Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > ---
> > >  tools/perf/util/bpf_off_cpu.c          | 32 +++++++++++++++
> > >  tools/perf/util/bpf_skel/off_cpu.bpf.c | 55 ++++++++++++++++++++------
> > >  2 files changed, 76 insertions(+), 11 deletions(-)
> > >
> >
> > [...]
> >
> > >
> > > +SEC("tp_btf/sched_switch")
> > > +int on_switch3(u64 *ctx)
> > > +{
> > > +       struct task_struct *prev, *next;
> > > +       int state;
> > > +
> > > +       if (!enabled)
> > > +               return 0;
> > > +
> > > +       /*
> > > +        * TP_PROTO(bool preempt, struct task_struct *prev,
> > > +        *          struct task_struct *next)
> > > +        */
> > > +       prev = (struct task_struct *)ctx[1];
> > > +       next = (struct task_struct *)ctx[2];
> >
> >
> > you don't have to have two BPF programs for this, you can use
> > read-only variable to make this choice.
> >
> > On BPF side
> >
> > const volatile bool has_prev_state = false;
> >
> > ...
> >
> > if (has_prev_state) {
> >     prev = (struct task_struct *)ctx[2];
> >     next = (struct task_struct *)ctx[3];
> > } else {
> >     prev = (struct task_struct *)ctx[1];
> >     next = (struct task_struct *)ctx[2];
> > }
> >
> >
> > And from user-space side you do your detection and before skeleton is loaded:
> >
> > skel->rodata->has_prev_state = <whatever you detected>
>
> Nice, thanks for the tip!
>
> Actually I tried something similar but it was with a variable (in bss)
> so the verifier in an old kernel rejected it due to invalid arg access.
>
> I guess now the const makes the verifier ignore the branch as if
> it's dead but the compiler still generates the code, right?


yes, exactly

>
> >
> > But I'm still hoping that this prev_state argument can be moved to the
> > end ([0]) to make all this unnecessary.
> >
> >   [0] https://lore.kernel.org/lkml/93a20759600c05b6d9e4359a1517c88e06b44834.camel@xxxxxx/
>
> Yeah, that would make life easier. :)
>
> Thanks,
> Namhyung