[xdp-hints] 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 12:00 PM Stanislav Fomichev <sdf@google.com> wrote:
>
> On Wed, Mar 22, 2023 at 9:07 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > 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.
>
> But that's not for the production? xdp_hw_metadata is a small tool to
> verify that the metadata being dumped is correct (during the
> development).
> We have a proper (less verbose) selftest in
> {progs,prog_tests}/xdp_metadata.c (over veth).
> This xdp_hw_metadata was supposed to be used for running it against
> the real hardware, so having as much debugging at hand as possible
> seems helpful? (at least it was helpful to me when playing with mlx4)

The only use of bpf_printk is for debugging of bpf progs themselves.
It should not be used in any tool.