Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updatedpatch
From: Maynard Johnson
Date: Thu Feb 08 2007 - 10:04:28 EST
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@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Maynard Johnson <mpjohn@xxxxxxxxxx>
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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