Re: [PATCH v6 bpf-next 0/3] bpf: introduce bpf_get_branch_snapshot

[Song Liu]

Hi Peter, 

Do you have further comments/concerns on v6? If not, could you please 
reply with your Reviewed-by or Acked-by?

Thanks,
Song

> On Sep 7, 2021, at 1:29 PM, Song Liu <songliubraving@xxxxxx> wrote:
> 
> 
> 
>> On Sep 7, 2021, at 1:27 PM, Song Liu <songliubraving@xxxxxx> wrote:
> 
> Forgot to add changes:
> 
> Changes v5 => v6
> 1. Add local_irq_save/restore to intel_pmu_snapshot_branch_stack. 
>   (Peter)
> 2. Remove buf and size check in bpf_get_branch_snapshot, move flags 
>   check to later fo the function. (Peter, Andrii)
> 3. Revise comments for bpf_get_branch_snapshot in bpf.h (Andrii)
> 
>> 
>> 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+102 to intel_pmu_lbr_disable_all+0
>> ID: 3 from bpf_get_branch_snapshot+18 to intel_pmu_snapshot_branch_stack+0
>> ID: 4 from bpf_get_branch_snapshot+18 to bpf_get_branch_snapshot+0
>> ID: 5 from __brk_limit+474918983 to bpf_get_branch_snapshot+0
>> ID: 6 from __bpf_prog_enter+34 to __brk_limit+474918971
>> 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 bpf_testmod_loop_test+20 to __bpf_prog_enter+0
>> ID: 10 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
>> 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@xxxxxx/
>> 
>> 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
>> 
>> arch/x86/events/intel/core.c                  |  29 ++++-
>> 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                      |  30 ++++++
>> 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
>