Re: [PATCH bpf-next V2 3/6] selftests/bpf: xdp_hw_metadata RX hash return code info

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Mar 22, 2023 at 9:05 AM Jesper Dangaard Brouer
<jbrouer@redhat.com> wrote:
>
>
>
> On 21/03/2023 19.47, Stanislav Fomichev wrote:
> > On Tue, Mar 21, 2023 at 6:47 AM Jesper Dangaard Brouer
> > <brouer@redhat.com> wrote:
> >>
> >> When driver developers add XDP-hints kfuncs for RX hash it is
> >> practical to print the return code in bpf_printk trace pipe log.
> >>
> >> Print hash value as a hex value, both AF_XDP userspace and bpf_prog,
> >> as this makes it easier to spot poor quality hashes.
> >>
> >> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> >> ---
> >>   .../testing/selftests/bpf/progs/xdp_hw_metadata.c  |    9 ++++++---
> >>   tools/testing/selftests/bpf/xdp_hw_metadata.c      |    5 ++++-
> >>   2 files changed, 10 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> >> index 40c17adbf483..ce07010e4d48 100644
> >> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> >> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> >> @@ -77,10 +77,13 @@ int rx(struct xdp_md *ctx)
> >>                  meta->rx_timestamp = 0; /* Used by AF_XDP as not avail signal */
> >>          }
> >>
> >> -       if (!bpf_xdp_metadata_rx_hash(ctx, &meta->rx_hash))
> >> -               bpf_printk("populated rx_hash with %u", meta->rx_hash);
> >> -       else
> >> +       ret = bpf_xdp_metadata_rx_hash(ctx, &meta->rx_hash);
> >> +       if (ret >= 0) {
> >> +               bpf_printk("populated rx_hash with 0x%08X", meta->rx_hash);
> >> +       } else {
> >> +               bpf_printk("rx_hash not-avail errno:%d", ret);
> >>                  meta->rx_hash = 0; /* Used by AF_XDP as not avail signal */
> >> +       }
> >>
> >>          return bpf_redirect_map(&xsk, ctx->rx_queue_index, XDP_PASS);
> >>   }
> >> diff --git a/tools/testing/selftests/bpf/xdp_hw_metadata.c b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> >> index 400bfe19abfe..f3ec07ccdc95 100644
> >> --- a/tools/testing/selftests/bpf/xdp_hw_metadata.c
> >> +++ b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> >> @@ -3,6 +3,9 @@
> >>   /* Reference program for verifying XDP metadata on real HW. Functional test
> >>    * only, doesn't test the performance.
> >>    *
> >> + * BPF-prog bpf_printk info outout can be access via
> >> + * /sys/kernel/debug/tracing/trace_pipe
> >
> > s/outout/output/
> >
>
> Fixed in V3
>
> > But let's maybe drop it? If you want to make it more usable, let's
> > have a separate patch to enable tracing and periodically dump it to
> > the console instead (as previously discussed).
>
> Cat'ing /sys/kernel/debug/tracing/trace_pipe work for me regardless of
> setting in
> /sys/kernel/debug/tracing/events/bpf_trace/bpf_trace_printk/enable
>
> We likely need a followup patch that adds a BPF config switch that can
> disable bpf_printk calls, because this adds overhead and thus affects
> the timestamps.

No. This is by design.
Do not use bpf_printk* in production.