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

[Mathieu Desnoyers]

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.

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