Re: [PATCH v2 0/7] Implement inline static calls on PPC32 - v2

[Christophe Leroy]

Le 09/07/2022 à 08:52, Ard Biesheuvel a écrit :
> Hello Christophe,
> 
> On Fri, 8 Jul 2022 at 19:32, Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> wrote:
>>
>> This series applies on top of the series v3 "objtool: Enable and
>> implement --mcount option on powerpc" [1] rebased on powerpc-next branch
>>
>> A few modifications are done to core parts to enable powerpc
>> implementation:
>> - R_X86_64_PC32 is abstracted to R_REL32 so that it can then be
>> redefined as R_PPC_REL32.
>> - A call to static_call_init() is added to start_kernel() to avoid
>> every architecture to have to call it
>> - Trampoline address is provided to arch_static_call_transform() even
>> when setting a site to fallback on a call to the trampoline when the
>> target is too far.
>>
>> [1] https://lore.kernel.org/lkml/70b6d08d-aced-7f4e-b958-a3c7ae1a9319@xxxxxxxxxx/T/#rb3a073c54aba563a135fba891e0c34c46e47beef
>>
>> Christophe Leroy (7):
>>    powerpc: Add missing asm/asm.h for objtool
>>    objtool/powerpc: Activate objtool on PPC32
>>    objtool: Add architecture specific R_REL32 macro
>>    objtool/powerpc: Add necessary support for inline static calls
>>    init: Call static_call_init() from start_kernel()
>>    static_call_inline: Provide trampoline address when updating sites
>>    powerpc/static_call: Implement inline static calls
>>
> 
> Could you quantify the performance gains of moving from out-of-line,
> patched tail-call branch instructions to full-fledged inline static
> calls? On x86, the retpoline problem makes this glaringly obvious, but
> on other architectures, the complexity of supporting this model may
> outweigh the performance advantages.

Surprisingly, I get worst performance with inline static call than with 
out of line static call:

No static call:

root@vgoip:~# perf stat -r 10 ./hackbench 1
Running with 1*40 (== 40) tasks.
Time: 17.186
Running with 1*40 (== 40) tasks.
Time: 16.738
Running with 1*40 (== 40) tasks.
Time: 16.579
Running with 1*40 (== 40) tasks.
Time: 16.838
Running with 1*40 (== 40) tasks.
Time: 16.652
Running with 1*40 (== 40) tasks.
Time: 17.380
Running with 1*40 (== 40) tasks.
Time: 16.630
Running with 1*40 (== 40) tasks.
Time: 16.850
Running with 1*40 (== 40) tasks.
Time: 17.161
Running with 1*40 (== 40) tasks.
Time: 16.722

  Performance counter stats for './hackbench 1' (10 runs):

           17019.55 msec task-clock                #    0.980 CPUs 
utilized            ( +-  0.51% )
               4847      context-switches          #  282.280 /sec 
               ( +-  6.32% )
                  0      cpu-migrations            #    0.000 /sec
               1249      page-faults               #   72.739 /sec 
               ( +-  0.49% )
         2245344976      cycles                    #    0.131 GHz 
               ( +-  0.51% )
          727437072      instructions              #    0.32  insn per 
cycle           ( +-  0.40% )
    <not supported>      branches
    <not supported>      branch-misses

            17.3585 +- 0.0909 seconds time elapsed  ( +-  0.52% )


Outline static call:

root@vgoip:~# perf stat -r 10 ./hackbench 1
Running with 1*40 (== 40) tasks.
Time: 15.892
Running with 1*40 (== 40) tasks.
Time: 15.731
Running with 1*40 (== 40) tasks.
Time: 15.507
Running with 1*40 (== 40) tasks.
Time: 16.269
Running with 1*40 (== 40) tasks.
Time: 15.934
Running with 1*40 (== 40) tasks.
Time: 16.048
Running with 1*40 (== 40) tasks.
Time: 15.700
Running with 1*40 (== 40) tasks.
Time: 16.063
Running with 1*40 (== 40) tasks.
Time: 15.852
Running with 1*40 (== 40) tasks.
Time: 15.941

  Performance counter stats for './hackbench 1' (10 runs):

           16227.32 msec task-clock                #    0.992 CPUs 
utilized            ( +-  0.42% )
               3732      context-switches          #  230.525 /sec 
               ( +-  6.42% )
                  0      cpu-migrations            #    0.000 /sec
               1244      page-faults               #   76.842 /sec 
               ( +-  0.11% )
         2141094288      cycles                    #    0.132 GHz 
               ( +-  0.42% )
          712598441      instructions              #    0.33  insn per 
cycle           ( +-  0.29% )
    <not supported>      branches
    <not supported>      branch-misses

            16.3539 +- 0.0675 seconds time elapsed  ( +-  0.41% )


Inline static call:

root@vgoip:~# perf stat -r 10 ./hackbench 1
Running with 1*40 (== 40) tasks.
Time: 17.512
Running with 1*40 (== 40) tasks.
Time: 17.240
Running with 1*40 (== 40) tasks.
Time: 16.901
Running with 1*40 (== 40) tasks.
Time: 17.125
Running with 1*40 (== 40) tasks.
Time: 17.262
Running with 1*40 (== 40) tasks.
Time: 17.298
Running with 1*40 (== 40) tasks.
Time: 17.182
Running with 1*40 (== 40) tasks.
Time: 16.988
Running with 1*40 (== 40) tasks.
Time: 17.102
Running with 1*40 (== 40) tasks.
Time: 16.669

  Performance counter stats for './hackbench 1' (10 runs):

           16976.76 msec task-clock                #    0.964 CPUs 
utilized            ( +-  0.44% )
               4760      context-switches          #  273.007 /sec 
               ( +-  4.93% )
                  0      cpu-migrations            #    0.000 /sec
               1252      page-faults               #   71.808 /sec 
               ( +-  0.35% )
         2239986112      cycles                    #    0.128 GHz 
               ( +-  0.44% )
          721540184      instructions              #    0.31  insn per 
cycle           ( +-  0.31% )
    <not supported>      branches
    <not supported>      branch-misses

            17.6126 +- 0.0762 seconds time elapsed  ( +-  0.43% )


Summary:

No static calls:
            17.3585 +- 0.0909 seconds time elapsed  ( +-  0.52% )
Out-of-line static calls:
            16.3539 +- 0.0675 seconds time elapsed  ( +-  0.41% )
Inline static calls:
            17.6126 +- 0.0762 seconds time elapsed  ( +-  0.43% )

Is there anything wrong with inline statica calls ?

Christophe