Re: AVX512 bug on SkyLake

[Christian Ehrhardt <christian.ehrhardt@canonical.com>]

On Fri, Nov 9, 2018 at 12:01 AM Yongseok Koh <yskoh@mellanox.com> wrote:
>
>
> > On Nov 8, 2018, at 9:21 AM, Ferruh Yigit <ferruh.yigit@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.
> >>

Thanks Thomas for looping us in!

> >> 07/11/2018 10:04, Wiles, Keith:
> >>>> On Nov 6, 2018, at 9:30 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
> >>>>> On Nov 5, 2018, at 6:06 AM, Wiles, Keith <keith.wiles@intel.com> wrote:
> >>>>>> On Nov 2, 2018, at 9:04 PM, Yongseok Koh <yskoh@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&amp;data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&amp;sdata=2o%2Fg203aWrKCYg16S6oI4BcS41igpLu1DloS%2FrRnknc%3D&amp;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&amp;data=02%7C01%7Cyskoh%40mellanox.com%7C90ff6c361faf422b976108d6459eb490%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C636772945282345908&amp;sdata=NCFKxaREd69iZ8eyFKg%2FWBP73CLTXkxrNQQeii%2Bbsao%3D&amp;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...
>

What I wonder in all of this as I don't understand that part of it yet is this.
I assume you are building on Ubuntu as that is your gcc reference.
FYI: as people asked for bug references, there also is [1] which seems
pretty much the same issue.

It builds with mostly defaults, that means per
mk/machine/default/rte.vars.mk and similar it sets -march=corei7

But when I look at what that implies all avx512 is disabled
$ gcc -Q --help=target -m64 -march=corei7 | grep avx512f
 -mavx512f                             [disabled]

So I wonder what/why -mno-avx512f should help at all.
I used the full list of gcc args we have for the build (e.g. [2] of a
18.05 build), but that doesn't change that (mostly -W, -I and -D).
So I wonder, did people do a custom build and bump up march or enable
-mavx512f on their own to hit that?
Or are we facing a real gcc issue where " -mavx512f [disabled]" is not
the same as -mno-avx512f ?
Maybe someone who hit the bug could clarify that please?

BTW: per reports I've seen it also seems to apply to the latest
compiler update of the same series - at least it was said to be fully
updated, that would be 7.3.0-27ubuntu1~18.04
But this is 2nd grade information as I don't have a system with the
right combo MLX5+Skylake available atm, so I can't confirm for sure
:-/

[1]: https://bugs.launchpad.net/ubuntu/+source/dpdk/+bug/1799397
[2]: https://launchpadlibrarian.net/373589345/buildlog_ubuntu-bionic-amd64.dpdk_18.05-1~ubuntu0.18.04.1_BUILDING.txt.gz