On 11/8/2018 11:01 PM, Yongseok Koh wrote: > >> On Nov 8, 2018, at 9:21 AM, Ferruh Yigit <ferruh.yi...@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 <ys...@mellanox.com> wrote: >>>>>> On Nov 5, 2018, at 6:06 AM, Wiles, Keith <keith.wi...@intel.com> wrote: >>>>>>> On Nov 2, 2018, at 9:04 PM, Yongseok Koh <ys...@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? >
