Re: [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:
 

quoted

Carl Love wrote:

   

quoted

Subject: Add support to OProfile for profiling Cell BE SPUs

From: Maynard Johnson <redacted>

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 <redacted>
Signed-off-by: Maynard Johnson <redacted>

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.

 

quoted

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