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SubjectRe: [PATCH v2 0/7] Implement inline static calls on PPC32 - v2
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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@csgroup.eu> 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@csgroup.eu/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
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