[RFC PATCH] powerpc/ftrace: Refactoring and support for -fpatchable-function-entry

[Christophe Leroy]

Le 20/05/2023 à 12:34, Christophe Leroy a écrit :
> 
> 
> Le 19/05/2023 à 21:26, Naveen N Rao a écrit :
>> [Vous ne recevez pas souvent de courriers de naveen at kernel.org. 
>> Découvrez pourquoi ceci est important à 
>> https://aka.ms/LearnAboutSenderIdentification ]
>>
>> Refactor ftrace code and move to using ftrace_replace_code() to help
>> simplify and make the code more maintainable.
>>
>> - The existing ftrace.c code is moved to a separate file so that ppc64
>>    elfv1 and clang -pg only support continue. This makes it possible to
>>    converge ppc32 and ppc64 support further.
>> - Drop code to re-purpose compiler-generated long branches for ftrace
>>    use in support of large kernels. We still retain the ftrace stubs at
>>    the end of .text, so we now support kernels upto ~64MB.
>> - Add ftrace_init_nop() to keep boot-time validations and init separate
>>    from runtime.
>> - Implement ftrace_replace_code() to simplify overall ftrace setup. This
>>    will be especially useful when adding ability to nop out 'mflr r0'
>>    later, and for other subsequent ftrace features.
>> - Add support for -fpatchable-function-entry. On ppc64, this needs gcc
>>    v13.1 so that the nops are generated at LEP. This also moves ppc32 to
>>    using the same two-instruction sequence as that of ppc64.
>>
>> This applies atop patches 1-3 of Nick's series for elfv2 conversion, as
>> well as Nick's patch enabling -mprofile-kernel for elfv2 BE:
>> - https://lore.kernel.org/all/20230505071850.228734-1-npiggin@gmail.com/
>> - https://lore.kernel.org/all/20230506011814.8766-1-npiggin@gmail.com/
>>
>> This builds for me and passes a quick test, posting this as an early
>> RFC.
>>
>> Signed-off-by: Naveen N Rao <naveen at kernel.org>
> 
> Looks good, works on PPC32 but I observed some performance degradation, 
> around 25% more time needed to activate function tracer and around 10% 
> more time needed to de-activate function tracer (by writting 
> function/nop into /sys/kernel/debug/tracing/current_tracer.


perf record with your patch applied:

     20.59%  echo     [kernel.kallsyms]      [k] ftrace_check_record
     15.71%  echo     [kernel.kallsyms]      [k] patch_instruction
      6.75%  echo     [kernel.kallsyms]      [k] ftrace_replace_code
      4.30%  echo     [kernel.kallsyms]      [k] __ftrace_hash_rec_update
      3.96%  echo     [kernel.kallsyms]      [k] 
__rb_reserve_next.constprop.0
      3.20%  echo     [kernel.kallsyms]      [k] ftrace_get_call_inst.isra.0
      2.62%  echo     [kernel.kallsyms]      [k] ftrace_get_addr_new
      2.44%  echo     [kernel.kallsyms]      [k] ftrace_rec_iter_next
      2.15%  echo     [kernel.kallsyms]      [k] function_trace_call
      2.09%  echo     [kernel.kallsyms]      [k] rb_commit
      1.92%  echo     [kernel.kallsyms]      [k] ring_buffer_unlock_commit
      1.69%  echo     [kernel.kallsyms]      [k] ring_buffer_lock_reserve
      1.63%  echo     [kernel.kallsyms]      [k] copy_page
      1.45%  echo     [kernel.kallsyms]      [k] 
ftrace_create_branch_inst.constprop.0
      1.40%  echo     [kernel.kallsyms]      [k] unmap_page_range
      1.34%  echo     [kernel.kallsyms]      [k] mas_next_entry
      1.28%  echo     ld-2.23.so             [.] do_lookup_x
      1.22%  echo     [kernel.kallsyms]      [k] ftrace_call
      1.05%  echo     [kernel.kallsyms]      [k] trace_function
      0.99%  echo     [kernel.kallsyms]      [k] ftrace_caller
      0.81%  echo     [kernel.kallsyms]      [k] ftrace_rec_iter_record

perf record without your patch:

     22.58%  echo     [kernel.kallsyms]  [k] patch_instruction
     17.85%  echo     [kernel.kallsyms]  [k] ftrace_check_record
     11.65%  echo     [kernel.kallsyms]  [k] ftrace_replace_code
      6.76%  echo     [kernel.kallsyms]  [k] ftrace_make_call
      6.68%  echo     [kernel.kallsyms]  [k] __ftrace_hash_rec_update
      3.50%  echo     [kernel.kallsyms]  [k] ftrace_get_addr_curr
      3.42%  echo     [kernel.kallsyms]  [k] ftrace_get_addr_new
      2.36%  echo     [kernel.kallsyms]  [k] copy_page
      1.22%  echo     [kernel.kallsyms]  [k] __rb_reserve_next.constprop.0
      1.22%  echo     ld-2.23.so         [.] do_lookup_x
      1.06%  echo     [kernel.kallsyms]  [k] ftrace_lookup_ip
      0.73%  echo     ld-2.23.so         [.] _dl_relocate_object
      0.65%  echo     [kernel.kallsyms]  [k] flush_dcache_icache_page
      0.65%  echo     [kernel.kallsyms]  [k] function_trace_call