Re: [PATCH bpf-next 0/4] Fix perf_buffer creation on systems with offline CPUs

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Dec 16, 2019 at 6:44 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On Wed, Dec 11, 2019 at 05:35:20PM -0800, Andrii Nakryiko wrote:
> > This patch set fixes perf_buffer__new() behavior on systems which have some of
> > the CPUs offline/missing (due to difference between "possible" and "online"
> > sets). perf_buffer will create per-CPU buffer and open/attach to corresponding
> > perf_event only on CPUs present and online at the moment of perf_buffer
> > creation. Without this logic, perf_buffer creation has no chances of
> > succeeding on such systems, preventing valid and correct BPF applications from
> > starting.
>
> Once CPU goes back online and processes BPF events, any attempt to push into
> perf RB via bpf_perf_event_output() with flag BPF_F_CURRENT_CPU would silently

bpf_perf_event_output() will return error code in such case, so it's
not exactly undetectable by application.


> get discarded. Should rather perf API be fixed instead of plain skipping as done
> here to at least allow creation of ring buffer for BPF to avoid such case?

Can you elaborate on what perf API fix you have in mind? Do you mean
for perf to allow attaching ring buffer to offline CPU or something
else?

>
> > Andrii Nakryiko (4):
> >   libbpf: extract and generalize CPU mask parsing logic
> >   selftests/bpf: add CPU mask parsing tests
> >   libbpf: don't attach perf_buffer to offline/missing CPUs
> >   selftests/bpf: fix perf_buffer test on systems w/ offline CPUs
> >
> >  tools/lib/bpf/libbpf.c                        | 157 ++++++++++++------
> >  tools/lib/bpf/libbpf_internal.h               |   2 +
> >  .../selftests/bpf/prog_tests/cpu_mask.c       |  78 +++++++++
> >  .../selftests/bpf/prog_tests/perf_buffer.c    |  29 +++-
> >  4 files changed, 213 insertions(+), 53 deletions(-)
> >  create mode 100644 tools/testing/selftests/bpf/prog_tests/cpu_mask.c
> >
> > --
> > 2.17.1
> >