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