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SubjectRe: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated patch
Michael,
Thanks very much for the advice. Both issues have been solved now, with
your help.

-Maynard

Michael Ellerman wrote:

>On Wed, 2007-02-07 at 09:41 -0600, Maynard Johnson wrote:
>
>
>>Carl Love wrote:
>>
>>
>>
>>>Subject: Add support to OProfile for profiling Cell BE SPUs
>>>
>>>From: Maynard Johnson <maynardj@us.ibm.com>
>>>
>>>This patch updates the existing arch/powerpc/oprofile/op_model_cell.c
>>>to add in the SPU profiling capabilities. In addition, a 'cell' subdirectory
>>>was added to arch/powerpc/oprofile to hold Cell-specific SPU profiling
>>>code.
>>>
>>>Signed-off-by: Carl Love <carll@us.ibm.com>
>>>Signed-off-by: Maynard Johnson <mpjohn@us.ibm.com>
>>>
>>>Index: linux-2.6.20-rc1/arch/powerpc/configs/cell_defconfig
>>>
>>>
>>>
>>>
>>I've discovered more problems with the kref handling for the cached_info
>>object that we store in the spu_context. :-(
>>
>>When the OProfile module initially determines that no cached_info yet
>>exists for a given spu_context, it creates the cached_info, inits the
>>cached_info's kref (which increments the refcount) and does a kref_get
>>(for SPUFS' ref) before passing the cached_info reference off to SUPFS
>>to store into the spu_context. When OProfile shuts down or the SPU job
>>ends, OProfile gives up its ref to the cached_info with kref_put. Then
>>when SPUFS destroys the spu_context, it also gives up its ref. HOWEVER
>>. . . . If OProfile shuts down while the SPU job is still active _and_
>>_then_ is restarted while the job is still active, OProfile will find
>>that the cached_info exists for the given spu_context, so it won't go
>>through the process of creating it and doing kref_init on the kref.
>>Under this scenario, OProfile does not own a ref of its own to the
>>cached_info, and should not be doing a kref_put when done using the
>>cached_info -- but it does, and so does SPUFS when the spu_context is
>>destroyed. The end result (with the code as currently written) is that
>>an extra kref_put is done when the refcount is already down to zero. To
>>fix this, OProfile needs to detect when it finds an existing cached_info
>>already stored in the spu_context. Then, instead of creating a new one,
>>it sets a reminder flag to be used later when it's done using the cached
>>info to indicate whether or not it needs to call kref_put.
>>
>>
>
>I think all you want to do is when oprofile finds the cached_info
>already existing, it does a kref_get(). After all it doesn't have a
>reference to it, so before it starts using it it must inc the ref count.
>
>
>
>>Unfortunately, there's another problem (one that should have been
>>obvious to me). The cached_info's kref "release" function is
>>destroy_cached_info(), defined in the OProfile module. If the OProfile
>>module is unloaded when SPUFS destroys the spu_context and calls
>>kref_put on the cached_info's kref -- KABOOM! The destroy_cached_info
>>function (the second arg to kref_put) is not in memory, so we get a
>>paging fault. I see a couple options to solve this:
>> 1) Don't store the cached_info in the spu_context. Basically, go
>>back to the simplistic model of creating/deleting the cached_info on
>>every SPU task activation/deactivation.
>> 2) If there's some way to do this, force the OProfile module to
>>stay loaded until SPUFS has destroyed its last spu_context that holds a
>>cached_info object.
>>
>>
>
>There is a mechanism for that, you just have each cached_info inc the
>module's refcount.
>
>Another option would be to have a mapping, in the oprofile code, from
>spu_contexts to cached_infos, ie. a hash table or something.
>
>cheers
>
>
>


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