linux-kernel - Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated patch

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Message-Id: <1170888537.7831.14.camel@concordia.ozlabs.ibm.com>
Date:	Thu, 08 Feb 2007 09:48:57 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Maynard Johnson <maynardj@...ibm.com>
Cc:	Carl Love <cel@...ibm.com>, linuxppc-dev@...abs.org,
	linux-kernel@...r.kernel.org, cbe-oss-dev@...abs.org
Subject: Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updated
	patch

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@...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@...ibm.com>
> >Signed-off-by: Maynard Johnson <mpjohn@...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

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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