Re: [Patch] Losing many samples when profiling multiple JVMs

On 07/16/2015 10:27 AM, Will Schmidt wrote:
> On Wed, 2015-07-15 at 15:03 -0400, William Cohen wrote:
>>
>> I tried the test on rhel7 ppc64 and it doesn't work because of the way
>> that calls operates.  The ppc64 generated code is treating the address
>> returned by 
>> initialize_anon_func as a pointer to a descriptor rather than the
>> address that the branch should branch to.  The first 3 double word in
>> the generated anon_func are used as a descriptor and the branch goes
>> off to the wrong location
>> (https://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#FUNC-DES)
>>
>> Is this code working on ppc64le? How is the code being compiled on
>> your machine? Or does it have a similar problem?  Any suggestions on
>> how to best address this?
> 
> 
> Note that the PPC64le platform (which uses the ELF ABIv2) does not use
> function descriptors, so it is possible the code actually just works on
> ppc64le.
> I've not actually tried this patch, but saw the comment go past, and
> thought I should chime in in case folks were off scratching their heads
> in confusion.. :-)

Hi Will,

The problem wasn't with oprofile data collection; it was with some test code that was generating code and a pointer to that code.  On ppc64be that pointer was be interpreted as a pointer to the function descriptor and the beginning of the function was being interpreted as fields of the function descriptor causing the processor to jump off to the wrong location. I did find information about ELF ABIv2 saying it didn't use function descriptors. Last night I added an ifdef to the anon_remap.c test in the testsuite that should only be enable generating function descriptor generation for powerpc64be. I tested that the code worked on powerpc64be and x86-64. It would be good for someone with ppc64le access to verify that it still works for ppc64le.

  git clone git://git.code.sf.net/p/oprofile/oprofile-tests
  cd oprofile-tests/testsuite

as root:

  runtest --tool=oprofile



-Will Cohen


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