[dpdk-dev] AVX512 bug on SkyLake

[Ferruh Yigit]

On 11/8/2018 3:59 PM, Thomas Monjalon wrote:
> Hi,
> 
> We need to gather more information about this bug.
> More below.
> 
> 07/11/2018 10:04, Wiles, Keith:
>>> On Nov 6, 2018, at 9:30 PM, Yongseok Koh <yskoh at mellanox.com> wrote:
>>>> On Nov 5, 2018, at 6:06 AM, Wiles, Keith <keith.wiles at intel.com> wrote:
>>>>> On Nov 2, 2018, at 9:04 PM, Yongseok Koh <yskoh at mellanox.com> wrote:
>>>>>
>>>>> This is a workaround to prevent a crash, which might be caused by
>>>>> optimization of newer gcc (7.3.0) on Intel Skylake.
>>>>
>>>> Should the code below not also test for the gcc version and
>>>> the Sky Lake processor, maybe I am wrong but it seems it is
>>>> turning AVX512 for all GCC builds
>>>
>>> I didn't want to check gcc version as 7.3.0 is very new. Only gcc 8 is newly up since then (gcc 8.2).
>>> Also, I wasn't able to test every gcc versions and I wanted to be a bit conservative for this crash.
>>> Performance drop (if any) by disabling a new (experimental) feature would be less risky than unaccountable crash.
>>> And, it does disable the feature only if CONFIG_RTE_ENABLE_AVX512=n. Please refer to v3.
>>
>> Are you not turning off all of the GCC versions for AVX512.
>> And you can test for range or greater then GCC version and
>> it just seems like we are turning off every gcc version, is that true?
> 
> Do we know exactly which GCC versions are affected?
> 
>>>> Also bug 97 seems a bit obscure reference, maybe you know
>>>> the bug report, but more details would be good?
>>>
>>> I sent out the report to dev list two month ago.
>>> And I created the Bug 97 in order to reference it
>>> in the commit message.
>>> I didn't want to repeat same message here and there,
>>> but it would've been better to have some sort of summary
>>> of the Bug, although v3 has a few more words.
>>> However, v3 has been merged.
>>
>> Still this is too obscure if nothing else give a link to
>> a specific bug not just 97.
> 
> The URL is
> 	https://bugs.dpdk.org/show_bug.cgi?id=97
> The bug is also pointing to an email:
> 	https://mails.dpdk.org/archives/dev/2018-September/111522.html
> 
> Summary:
> 	- CPU: Intel Skylake
> 	- Linux environment: Ubuntu 18.04
> 	- Compiler: gcc-7.3 (Ubuntu 7.3.0-16ubuntu3)

Is it possible to test a few other gcc versions to check if the issue is
specific to this compiler version?

> 	- Scenario: testpmd crashes when it starts forwarding
> 	- Behaviour: AVX2 version of rte_memcpy() optimized with 512b instructions
> 	- Fix: disable AVX512 optimization with -mno-avx512f
> 
> It seems to have been reproduced only when using mlx5 PMD so far.
> Any other experience?
> 
>