[dpdk-dev] AVX512 bug on SkyLake

[Ferruh Yigit]

On 11/8/2018 11:01 PM, Yongseok Koh wrote:
> 
>> On Nov 8, 2018, at 9:21 AM, Ferruh Yigit <ferruh.yigit at intel.com> wrote:
>>
>> 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://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.dpdk.org%2Fshow_bug.cgi%3Fid%3D97&data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&sdata=2o%2Fg203aWrKCYg16S6oI4BcS41igpLu1DloS%2FrRnknc%3D&reserved=0
>>> The bug is also pointing to an email:
>>> 	https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmails.dpdk.org%2Farchives%2Fdev%2F2018-September%2F111522.html&data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&sdata=NCFKxaREd69iZ8eyFKg%2FWBP73CLTXkxrNQQeii%2Bbsao%3D&reserved=0
>>>
>>> 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?
> 
> Nothing's impossible but even with my quick search in gcc.gnu.org,
> I could find the following documents mention mavx512f support:
> 
> GCC 4.9.0
> April 22, 2014 (changes, documentation)
>  
> GCC 5.1
> April 22, 2015 (changes, documentation)
>  
> GCC 6.4
> July 4, 2017 (changes, documentation)
>  
> GCC 7.1
> May 2, 2017 (changes, documentation)
>  
> GCC 8.1
> May 2, 2018 (changes, documentation)
> 
> We altogether have to put quite large resource to verify all of the versions.
>  
> I assumed older than gcc 7 would have the same issue. I know it was a speculation
> but like I mentioned I wanted to be more conservative. I didn't mean this is a permanent fix.
> For two months, we couldn't have any tangible solution (actually nobody cared including myself),
> so I submitted the patch to temporarily disable mavx512f.
> 
> I'm still not sure what the best option is...

For permanent fix we need more information, currently we can't re-produce this
defect. Since you can reproduce it we need your support.

Right now we don't know if this is compiler issue or code defect in rte_memcpy()
or something else.

It is easy to disable mavx512f as temporarily solution but it is coming with the
cost of the performance drop, also without knowing the actual root cause I
wouldn't say this is being conservative, actual issue may be just hidden with
this change.

I think as first thing we need to find a way to reproduce this issue in any
other way than using mlx5 PMD. So that we can put more organized effort to fix this.
I attached a simple unit test for rte_memcpy(), if this is a rte_memcpy() with
avx512f defect as claimed, you should be able to see the issue with that, right?
Did you able to find a chance to test it? Do you observer any crash there?

> 
> Thanks,
> Yongseok
> 
>>
>>> 	- 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?
>