Re: [PATCH v22 00/20] tracing: fprobe: function_graph: Multi-function graph and fprobe on fgraph

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Dec 26, 2024 at 02:11:16PM +0900, Masami Hiramatsu (Google) wrote:
> Hi,
> 
> Here is the 22nd version of the series to re-implement the fprobe on
> function-graph tracer. The previous version is;
> 
> https://lore.kernel.org/all/173379652547.973433.2311391879173461183.stgit@devnote2/
> 
> This version is rebased on v6.13-rc4 with fixes on [3/20] for x86-32 and
> [5/20] for build error.


hi,
I ran the bench and I'm seeing native_sched_clock being used
again kretprobe_multi bench:

     5.85%  bench            [kernel.kallsyms]                                        [k] native_sched_clock
            |
            ---native_sched_clock
               sched_clock
               |
                --5.83%--trace_clock_local
                          ftrace_return_to_handler
                          return_to_handler
                          syscall
                          bpf_prog_test_run_opts
                          trigger_producer_batch
                          start_thread
                          __GI___clone3

I recall we tried to fix that before with [1] change, but that replaced
later with [2] changes

When I remove the trace_clock_local call in __ftrace_return_to_handler
than the kretprobe-multi gets much faster (see last block below), so it
seems worth to make it optional

there's some decrease in kprobe_multi benchmark compared to base numbers,
which I'm not sure yet why, but other than that it seems ok

base:
        kprobe         :   12.873 ± 0.011M/s
        kprobe-multi   :   13.088 ± 0.052M/s
        kretprobe      :    6.339 ± 0.003M/s
        kretprobe-multi:    7.240 ± 0.002M/s

fprobe_on_fgraph:
        kprobe         :   12.816 ± 0.002M/s
        kprobe-multi   :   12.126 ± 0.004M/s
        kretprobe      :    6.305 ± 0.018M/s
        kretprobe-multi:    7.740 ± 0.003M/s

removed native_sched_clock call:
        kprobe         :   12.850 ± 0.006M/s
        kprobe-multi   :   12.115 ± 0.006M/s
        kretprobe      :    6.270 ± 0.017M/s
        kretprobe-multi:    9.190 ± 0.005M/s


happy new year ;-) thanks,

jirka


[1] https://lore.kernel.org/bpf/172615389864.133222.14452329708227900626.stgit@devnote2/
[2] https://lore.kernel.org/all/20240914214805.779822616@goodmis.org/

> 
> Overview
> --------
> This series rewrites the fprobe on this function-graph.
> The purposes of this change are;
> 
>  1) Remove dependency of the rethook from fprobe so that we can reduce
>    the return hook code and shadow stack.
> 
>  2) Make 'ftrace_regs' the common trace interface for the function
>    boundary.
> 
> 1) Currently we have 2(or 3) different function return hook codes,
>  the function-graph tracer and rethook (and legacy kretprobe).
>  But since this  is redundant and needs double maintenance cost,
>  I would like to unify those. From the user's viewpoint, function-
>  graph tracer is very useful to grasp the execution path. For this
>  purpose, it is hard to use the rethook in the function-graph
>  tracer, but the opposite is possible. (Strictly speaking, kretprobe
>  can not use it because it requires 'pt_regs' for historical reasons.)
> 
> 2) Now the fprobe provides the 'pt_regs' for its handler, but that is
>  wrong for the function entry and exit. Moreover, depending on the
>  architecture, there is no way to accurately reproduce 'pt_regs'
>  outside of interrupt or exception handlers. This means fprobe should
>  not use 'pt_regs' because it does not use such exceptions.
>  (Conversely, kprobe should use 'pt_regs' because it is an abstract
>   interface of the software breakpoint exception.)
> 
> This series changes fprobe to use function-graph tracer for tracing
> function entry and exit, instead of mixture of ftrace and rethook.
> Unlike the rethook which is a per-task list of system-wide allocated
> nodes, the function graph's ret_stack is a per-task shadow stack.
> Thus it does not need to set 'nr_maxactive' (which is the number of
> pre-allocated nodes).
> Also the handlers will get the 'ftrace_regs' instead of 'pt_regs'.
> Since eBPF mulit_kprobe/multi_kretprobe events still use 'pt_regs' as
> their register interface, this changes it to convert 'ftrace_regs' to
> 'pt_regs'. Of course this conversion makes an incomplete 'pt_regs',
> so users must access only registers for function parameters or
> return value. 
> 
> Design
> ------
> Instead of using ftrace's function entry hook directly, the new fprobe
> is built on top of the function-graph's entry and return callbacks
> with 'ftrace_regs'.
> 
> Since the fprobe requires access to 'ftrace_regs', the architecture
> must support CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS and
> CONFIG_HAVE_FTRACE_GRAPH_FUNC, which enables to call function-graph
> entry callback with 'ftrace_regs', and also
> CONFIG_HAVE_FUNCTION_GRAPH_FREGS, which passes the ftrace_regs to
> return_to_handler.
> 
> All fprobes share a single function-graph ops (means shares a common
> ftrace filter) similar to the kprobe-on-ftrace. This needs another
> layer to find corresponding fprobe in the common function-graph
> callbacks, but has much better scalability, since the number of
> registered function-graph ops is limited.
> 
> In the entry callback, the fprobe runs its entry_handler and saves the
> address of 'fprobe' on the function-graph's shadow stack as data. The
> return callback decodes the data to get the 'fprobe' address, and runs
> the exit_handler.
> 
> The fprobe introduces two hash-tables, one is for entry callback which
> searches fprobes related to the given function address passed by entry
> callback. The other is for a return callback which checks if the given
> 'fprobe' data structure pointer is still valid. Note that it is
> possible to unregister fprobe before the return callback runs. Thus
> the address validation must be done before using it in the return
> callback.
> 
> Download
> --------
> This series can be applied against the v6.13-rc2 kernel.
> 
> This series can also be found below branch.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/log/?h=topic/fprobe-on-fgraph
> 
> Thank you,
> 
> ---
> 
> Masami Hiramatsu (Google) (19):
>       fgraph: Get ftrace recursion lock in function_graph_enter
>       fgraph: Pass ftrace_regs to entryfunc
>       fgraph: Replace fgraph_ret_regs with ftrace_regs
>       fgraph: Pass ftrace_regs to retfunc
>       fprobe: Use ftrace_regs in fprobe entry handler
>       fprobe: Use ftrace_regs in fprobe exit handler
>       tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs
>       tracing: Add ftrace_fill_perf_regs() for perf event
>       tracing/fprobe: Enable fprobe events with CONFIG_DYNAMIC_FTRACE_WITH_ARGS
>       bpf: Enable kprobe_multi feature if CONFIG_FPROBE is enabled
>       ftrace: Add CONFIG_HAVE_FTRACE_GRAPH_FUNC
>       fprobe: Rewrite fprobe on function-graph tracer
>       fprobe: Add fprobe_header encoding feature
>       tracing/fprobe: Remove nr_maxactive from fprobe
>       selftests: ftrace: Remove obsolate maxactive syntax check
>       selftests/ftrace: Add a test case for repeating register/unregister fprobe
>       Documentation: probes: Update fprobe on function-graph tracer
>       ftrace: Add ftrace_get_symaddr to convert fentry_ip to symaddr
>       bpf: Use ftrace_get_symaddr() for kprobe_multi probes
> 
> Sven Schnelle (1):
>       s390/tracing: Enable HAVE_FTRACE_GRAPH_FUNC
> 
> 
>  Documentation/trace/fprobe.rst                     |   42 +
>  arch/arm64/Kconfig                                 |    2 
>  arch/arm64/include/asm/Kbuild                      |    1 
>  arch/arm64/include/asm/ftrace.h                    |   51 +-
>  arch/arm64/kernel/asm-offsets.c                    |   12 
>  arch/arm64/kernel/entry-ftrace.S                   |   32 +
>  arch/arm64/kernel/ftrace.c                         |   78 ++
>  arch/loongarch/Kconfig                             |    4 
>  arch/loongarch/include/asm/fprobe.h                |   12 
>  arch/loongarch/include/asm/ftrace.h                |   32 -
>  arch/loongarch/kernel/asm-offsets.c                |   12 
>  arch/loongarch/kernel/ftrace_dyn.c                 |   10 
>  arch/loongarch/kernel/mcount.S                     |   17 -
>  arch/loongarch/kernel/mcount_dyn.S                 |   14 
>  arch/powerpc/Kconfig                               |    1 
>  arch/powerpc/include/asm/ftrace.h                  |   13 
>  arch/powerpc/kernel/trace/ftrace.c                 |    8 
>  arch/powerpc/kernel/trace/ftrace_64_pg.c           |   16 
>  arch/riscv/Kconfig                                 |    3 
>  arch/riscv/include/asm/Kbuild                      |    1 
>  arch/riscv/include/asm/ftrace.h                    |   45 +
>  arch/riscv/kernel/ftrace.c                         |   17 -
>  arch/riscv/kernel/mcount.S                         |   24 -
>  arch/s390/Kconfig                                  |    4 
>  arch/s390/include/asm/fprobe.h                     |   10 
>  arch/s390/include/asm/ftrace.h                     |   37 +
>  arch/s390/kernel/asm-offsets.c                     |    6 
>  arch/s390/kernel/entry.h                           |    1 
>  arch/s390/kernel/ftrace.c                          |   48 -
>  arch/s390/kernel/mcount.S                          |   23 -
>  arch/x86/Kconfig                                   |    4 
>  arch/x86/include/asm/Kbuild                        |    1 
>  arch/x86/include/asm/ftrace.h                      |   54 +-
>  arch/x86/kernel/ftrace.c                           |   47 +
>  arch/x86/kernel/ftrace_32.S                        |   13 
>  arch/x86/kernel/ftrace_64.S                        |   17 -
>  include/asm-generic/fprobe.h                       |   46 +
>  include/linux/fprobe.h                             |   62 +-
>  include/linux/ftrace.h                             |  116 +++
>  include/linux/ftrace_regs.h                        |    2 
>  kernel/trace/Kconfig                               |   22 -
>  kernel/trace/bpf_trace.c                           |   28 +
>  kernel/trace/fgraph.c                              |   65 +-
>  kernel/trace/fprobe.c                              |  664 +++++++++++++++-----
>  kernel/trace/ftrace.c                              |    6 
>  kernel/trace/trace.h                               |    6 
>  kernel/trace/trace_fprobe.c                        |  146 ++--
>  kernel/trace/trace_functions_graph.c               |   10 
>  kernel/trace/trace_irqsoff.c                       |    6 
>  kernel/trace/trace_probe_tmpl.h                    |    2 
>  kernel/trace/trace_sched_wakeup.c                  |    6 
>  kernel/trace/trace_selftest.c                      |   11 
>  lib/test_fprobe.c                                  |   51 --
>  samples/fprobe/fprobe_example.c                    |    4 
>  .../test.d/dynevent/add_remove_fprobe_repeat.tc    |   19 +
>  .../ftrace/test.d/dynevent/fprobe_syntax_errors.tc |    4 
>  56 files changed, 1318 insertions(+), 670 deletions(-)
>  create mode 100644 arch/loongarch/include/asm/fprobe.h
>  create mode 100644 arch/s390/include/asm/fprobe.h
>  create mode 100644 include/asm-generic/fprobe.h
>  create mode 100644 tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe_repeat.tc
> 
> --
> Masami Hiramatsu (Google) <mhiramat@kernel.org>