Re: [PATCH v5 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot

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

On Thu, Sep 2, 2021 at 9:58 AM Song Liu <songliubraving@fb.com> wrote:
>
> Changes v4 => v5
> 1. Modify perf_snapshot_branch_stack_t to save some memcpy. (Andrii)
> 2. Minor fixes in selftests. (Andrii)
>
> Changes v3 => v4:
> 1. Do not reshuffle intel_pmu_disable_all(). Use some inline to save LBR
>    entries. (Peter)
> 2. Move static_call(perf_snapshot_branch_stack) to the helper. (Alexei)
> 3. Add argument flags to bpf_get_branch_snapshot. (Andrii)
> 4. Make MAX_BRANCH_SNAPSHOT an enum (Andrii). And rename it as
>    PERF_MAX_BRANCH_SNAPSHOT
> 5. Make bpf_get_branch_snapshot similar to bpf_read_branch_records.
>    (Andrii)
> 6. Move the test target function to bpf_testmod. Updated kallsyms_find_next
>    to work properly with modules. (Andrii)
>
> Changes v2 => v3:
> 1. Fix the use of static_call. (Peter)
> 2. Limit the use to perfmon version >= 2. (Peter)
> 3. Modify intel_pmu_snapshot_branch_stack() to use intel_pmu_disable_all
>    and intel_pmu_enable_all().
>
> Changes v1 => v2:
> 1. Rename the helper as bpf_get_branch_snapshot;
> 2. Fix/simplify the use of static_call;
> 3. Instead of percpu variables, let intel_pmu_snapshot_branch_stack output
>    branch records to an output argument of type perf_branch_snapshot.
>
> Branch stack can be very useful in understanding software events. For
> example, when a long function, e.g. sys_perf_event_open, returns an errno,
> it is not obvious why the function failed. Branch stack could provide very
> helpful information in this type of scenarios.
>
> This set adds support to read branch stack with a new BPF helper
> bpf_get_branch_trace(). Currently, this is only supported in Intel systems.
> It is also possible to support the same feaure for PowerPC.
>
> The hardware that records the branch stace is not stopped automatically on
> software events. Therefore, it is necessary to stop it in software soon.
> Otherwise, the hardware buffers/registers will be flushed. One of the key
> design consideration in this set is to minimize the number of branch record
> entries between the event triggers and the hardware recorder is stopped.
> Based on this goal, current design is different from the discussions in
> original RFC [1]:
>  1) Static call is used when supported, to save function pointer
>     dereference;
>  2) intel_pmu_lbr_disable_all is used instead of perf_pmu_disable(),
>     because the latter uses about 10 entries before stopping LBR.
>
> With current code, on Intel CPU, LBR is stopped after 10 branch entries
> after fexit triggers:
>
> ID: 0 from intel_pmu_lbr_disable_all+58 to intel_pmu_lbr_disable_all+93
> ID: 1 from intel_pmu_lbr_disable_all+54 to intel_pmu_lbr_disable_all+58
> ID: 2 from intel_pmu_snapshot_branch_stack+88 to intel_pmu_lbr_disable_all+0
> ID: 3 from bpf_get_branch_snapshot+77 to intel_pmu_snapshot_branch_stack+0
> ID: 4 from __brk_limit+478052814 to bpf_get_branch_snapshot+0
> ID: 5 from __brk_limit+478036039 to __brk_limit+478052760
> ID: 6 from __bpf_prog_enter+34 to __brk_limit+478036027
> ID: 7 from migrate_disable+60 to __bpf_prog_enter+9
> ID: 8 from __bpf_prog_enter+4 to migrate_disable+0
> ID: 9 from __brk_limit+478036022 to __bpf_prog_enter+0
> ID: 10 from bpf_testmod_loop_test+22 to __brk_limit+478036003
> ID: 11 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
> ID: 12 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
> ID: 13 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
> ...
>
> [1] https://lore.kernel.org/bpf/20210818012937.2522409-1-songliubraving@fb.com/
>
> Song Liu (3):
>   perf: enable branch record for software events
>   bpf: introduce helper bpf_get_branch_snapshot
>   selftests/bpf: add test for bpf_get_branch_snapshot
>

Besides the BPF helper comment nit, looks good to me. For the series:

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>  arch/x86/events/intel/core.c                  |  26 ++++-
>  arch/x86/events/intel/ds.c                    |   8 --
>  arch/x86/events/perf_event.h                  |  10 +-
>  include/linux/perf_event.h                    |  23 ++++
>  include/uapi/linux/bpf.h                      |  22 ++++
>  kernel/bpf/trampoline.c                       |   3 +-
>  kernel/events/core.c                          |   2 +
>  kernel/trace/bpf_trace.c                      |  33 ++++++
>  tools/include/uapi/linux/bpf.h                |  22 ++++
>  .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  19 +++-
>  .../selftests/bpf/prog_tests/core_reloc.c     |  14 +--
>  .../bpf/prog_tests/get_branch_snapshot.c      | 100 ++++++++++++++++++
>  .../selftests/bpf/prog_tests/module_attach.c  |  39 -------
>  .../selftests/bpf/progs/get_branch_snapshot.c |  40 +++++++
>  tools/testing/selftests/bpf/test_progs.c      |  39 +++++++
>  tools/testing/selftests/bpf/test_progs.h      |   2 +
>  tools/testing/selftests/bpf/trace_helpers.c   |  37 +++++++
>  tools/testing/selftests/bpf/trace_helpers.h   |   5 +
>  18 files changed, 378 insertions(+), 66 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/get_branch_snapshot.c
>  create mode 100644 tools/testing/selftests/bpf/progs/get_branch_snapshot.c
>
> --
> 2.30.2