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

[Andrii Nakryiko]

On Tue, Sep 7, 2021 at 1:31 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)
>

Looks great, thanks! Looking forward to being able to use it. Please
consider following up with migrate_disable() inlining as well.

For the series:

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> >
> > 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
>