Re: AVX "Sandy Bridge" hardware issue?

[Andy Lutomirski <luto@MIT.EDU>]

On 07/12/2011 04:16 PM, MK wrote:
> Hi gang! I'd forgotten how busy this list is, I hope someone can help
> me out.
>
> I have a small VPS slice, run under openVZ, that I use for testing and
> personal projects.  Recently, the provider migrated to new Xeon "Sandy
> Bridge" processors, which according to wikipedia are the first and
> thus far only commercially available processors using AVX.
>
> After the migration, I had a number of apache mod_perl applications
> break due to SIGILL.   Reproducible test case:
>
> use Apache2::Const qw(SERVER_ERROR)
>
> sub handler {
>       return SERVER_ERROR;
> };
>
> Apache2::Const is the indirect culprit here; if I remove it and just
> return 500 the module works.  Note that this is not a perl error. A
> backtrace from running apache under gdb, triggering the issue, is here:
>
> http://pastebin.com/16SrEzHM
>
> I posted this to the mod_perl list and someone pointed me to a
> backtrace identical in its final contexts, from a glibc bug
> reported last year:
>
> http://sourceware.org/bugzilla/show_bug.cgi?format=multiple&id=12113
>
> Which involves AVX hardware.  The VPS provider has provided me with a
> bare Fedora 14 slice for debugging this issue, and the "small
> reproducer" available from the above bug report, verified by Ulrich
> Drepper, does reproduce the issue.
>
> So I filed a glibc bug with fedora to that effect:
>
> https://bugzilla.redhat.com/show_bug.cgi?id=720176
>
> In which Andreas Schwab points out (rightly or wrongly) that according
> to the /proc/cpuinfo from the slice, the processor actually does not
> support AVX.  However,  the "model name", "Intel(R) Xeon(R) CPU
> E31230", is according to this a Sandy Bridge processor with AVX:
>
> http://en.wikipedia.org/wiki/Sandy_Bridge#Server_processors
>
> And while I do not have access to the hardware, the provider is very
> unequivocal about the fact that these are Sandy Bridges, which
> apparently include AVX.
>
> So I am looking for a next step to take in debugging this.  The kernel
> used on the slice (nb, openVZ does not allow for rolling your own) is
> 2.6.32 built with gcc 4.1.2.  I think this may be prior to AVX support
> in the kernel and gcc, but the glibc is 2.13, which apparently includes
> it.
>
> Does anyone have any idea why I would get this identical backtrace, and
> a failed reproducer test, on hardware which supposedly supports AVX
> (but not according to the kernel in /proc/cpuinfo)?

I was bored and read the manual.  It looks like glibc is buggy: it 
checks whether the CPU supports AVX but not whether the OS enables AVX.

http://sourceware.org/bugzilla/show_bug.cgi?id=13007

That being said, you should still bug your provider for a better kernel. 
  AVX is useful and should be enabled.

--Andy