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

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Date:   Tue, 17 Dec 2019 14:00:10 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 0/4] Fix perf_buffer creation on systems with
 offline CPUs

On 12/16/19 6:59 PM, Andrii Nakryiko wrote:
> On Mon, Dec 16, 2019 at 6:44 AM Daniel Borkmann <daniel@...earbox.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.

Yeah, true, given there would be no element in the perf map at that slot, the
program would receive -ENOENT and we could account for missed events via per
CPU map or such.

>> 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?

Yes, was wondering about the former, meaning, possibility to attach ring buffer
to offline CPU.

>>> 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
>>>