[lttng-dev] lttng-consumerd crash on aarch64 due to x86 arch specific optimization

[Mathieu Desnoyers]

On 2023-01-31 11:08, Mathieu Desnoyers wrote:
> On 2023-01-30 01:50, Beckius, Mikael via lttng-dev wrote:
>> Hello Matthieu!
>>
>> I have looked at this in place of Anders and as far as I can tell this 
>> is not an arm64 issue but an arm issue. And even on arm 
>> __ARM_FEATURE_UNALIGNED is 1 so it seems the problem only occurs if 
>> size equals 8.
> 
> So for ARM, perhaps we should do the following in include/lttng/ust-arch.h:
> 
> #if defined(LTTNG_UST_ARCH_ARM) && defined(__ARM_FEATURE_UNALIGNED)
> #define LTTNG_UST_ARCH_HAS_EFFICIENT_UNALIGNED_ACCESS 1
> #endif
> 
> And refer to 
> https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html#ARM-Options
> 
> Based on that documentation, it is possible to build with 
> -mno-unaligned-access,
> and for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures,
> unaligned accesses are not enabled.
> 
> I would only push this kind of change into the master branch though, due to
> its impact and the fact that this is only a performance improvement.

But setting LTTNG_UST_ARCH_HAS_EFFICIENT_UNALIGNED_ACCESS 1 for arm32
when __ARM_FEATURE_UNALIGNED is defined would still cause issues for
8-byte lttng_inline_memcpy with my proposed patch right ?

AFAIU 32-bit arm with __ARM_FEATURE_UNALIGNED has unaligned accesses for
2 and 4 bytes accesses, but somehow traps for unaligned 8-bytes
accesses ?

Thanks,

Mathieu

> 
>>
>> In addition I did some performance testing of lttng_inline_memcpy by 
>> extracting it and adding it to a simple test program. It appears that 
>> the general performance increases on arm, arm64, arm on arm64 hardware 
>> and x86-64. But it also appears that on arm if you end up in memcpy 
>> the old code where you call memcpy directly is actually slightly faster.
> 
> Nothing unexpected here. Just make sure that your test program does not 
> call lttng_inline_memcpy
> with constant size values which end up optimizing away branches. In the 
> context where lttng_inline_memcpy
> is used, most of the time its arguments are not constants.
> 
>>
>> Skipping the memcpy fallback on arm for unaligned copies of sizes 2 
>> and 4 further improves the performance
> 
> This would be naturally done on your board if we conditionally
> set LTTNG_UST_ARCH_HAS_EFFICIENT_UNALIGNED_ACCESS 1 for 
> __ARM_FEATURE_UNALIGNED
> right ?
> 
> and setting LTTNG_UST_ARCH_HAS_EFFICIENT_UNALIGNED_ACCESS 1 yields the 
> best performance on arm64.
> 
> This could go into lttng-ust master branch as well, e.g.:
> 
> #if defined(LTTNG_UST_ARCH_AARCH64)
> #define LTTNG_UST_ARCH_HAS_EFFICIENT_UNALIGNED_ACCESS 1
> #endif
> 
> Thanks!
> 
> Mathieu
> 
>>
>> Micke
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>> lttng-dev at lists.lttng.org
>> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com