Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

[Namhyung Kim]

On Fri, Aug 26, 2022 at 2:26 PM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > On Aug 26, 2022, at 2:12 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > On Fri, Aug 26, 2022 at 1:59 PM Song Liu <songliubraving@xxxxxx> wrote:
> >>
> >>
> >>
> >>> On Aug 26, 2022, at 12:30 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >>>
> >>> On Fri, Aug 26, 2022 at 11:45 AM Song Liu <songliubraving@xxxxxx> wrote:
> >>>
> >>>>> And actually, we can just read ctx->data and get the raw record,
> >>>>> right..?
> >>>>
> >>>> Played with this for a little bit. ctx->data appears to be not
> >>>> reliable sometimes. I guess (not 100% sure) this is because we
> >>>> call bpf program before event->orig_overflow_handler. We can
> >>>> probably add a flag to specify we want to call orig_overflow_handler
> >>>> first.
> >>>
> >>> I'm not sure.  The sample_data should be provided by the caller
> >>> of perf_event_overflow.  So I guess the bpf program should see
> >>> a valid ctx->data.
> >>
> >> Let's dig into this. Maybe we need some small changes in
> >> pe_prog_convert_ctx_access.
> >
> > Sure, can you explain the problem in detail and share your program?
>
> I push the code to
>
>  https://git.kernel.org/pub/scm/linux/kernel/git/song/linux.git/log/?h=test-perf-event
>
> The code is in tools/bpf/perf-test/.
>
> The problem is we cannot get reliable print of data->cpu_entry in
> /sys/kernel/tracing/trace.

Ah, right.  I've realized that the sample data is passed before full
initialized.  Please see perf_sample_data_init().  The other members
are initialized right before written to the ring buffer in the
orig_overflow_handler (__perf_event_output).

That explains why pe_prog_convert_ctx_access() handles
data and period specially.  We need to handle it first.

Thanks,
Namhyung