Re: [Cbe-oss-dev] [RFC, PATCH] CELL Oprofile SPU profiling updatedpatch

[Maynard Johnson]

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.

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.

I thought about putting the cached_info's kref "release" function in 
SPUFS, but this just won't work. This implies that SPUFS needs to know 
about the structure of the cached_info, e.g., that it contains the 
vma_map member that needs to be freed.  But even with that information, 
it's not enough, since the vma_map member consists of list of vma_maps, 
which is why we have the vma_map_free() function.  So SPUFS would still 
need access to vma_map_free() from the OProfile module.

Opinions from others would be appreciated.

Thanks.
-Maynard

> +/* Container for caching information about an active SPU task.
> + * 
> + */
> +struct cached_info {
> +	struct vma_to_fileoffset_map * map;
> +	struct spu * the_spu;   /* needed to access pointer to local_store */
> +	struct kref cache_ref;
> +};
> +
> +static struct cached_info * spu_info[MAX_NUMNODES * 8];
> +
> +static void destroy_cached_info(struct kref * kref)
> +{
> +	struct cached_info * info;
> +	info = container_of(kref, struct cached_info, cache_ref);
> +	vma_map_free(info->map);
> +	kfree(info);
> +}
> +
> +/* Return the cached_info for the passed SPU number.
> + * 
> + */
> +static struct cached_info * get_cached_info(struct spu * the_spu, int spu_num)
> +{
> +	struct cached_info * ret_info = NULL;
> +	unsigned long flags = 0;
> +	if (spu_num >= num_spu_nodes) {
> +		printk(KERN_ERR "SPU_PROF: " 
> +		       "%s, line %d: Invalid index %d into spu info cache\n",
> +		       __FUNCTION__, __LINE__, spu_num); 
> +		goto out;
> +	}
> +	spin_lock_irqsave(&cache_lock, flags);
> +	if (!spu_info[spu_num] && the_spu)
> +		spu_info[spu_num] = (struct cached_info *)
> +			spu_get_profile_private(the_spu->ctx);
> +
> +	ret_info = spu_info[spu_num];
> +	spin_unlock_irqrestore(&cache_lock, flags);
> + out:
> +	return ret_info;
> +}
> +
> +
> +/* Looks for cached info for the passed spu.  If not found, the
> + * cached info is created for the passed spu.
> + * Returns 0 for success; otherwise, -1 for error.  
> + */ 
> +static int
> +prepare_cached_spu_info(struct spu * spu, unsigned int objectId)
> +{
> +	unsigned long flags = 0;
> +	struct vma_to_fileoffset_map * new_map;
> +	int retval = 0;
> +	struct cached_info * info = get_cached_info(spu, spu->number);
> +
> +	if (info) {
> +		pr_debug("Found cached SPU info.\n");
> +		goto out;
> +	}
> +
> +	/* Create cached_info and set spu_info[spu->number] to point to it.
> +	 * spu->number is a system-wide value, not a per-node value.
> +	 */
> +	info = kzalloc(sizeof(struct cached_info), GFP_KERNEL);
> +	if (!info) {
> +		printk(KERN_ERR "SPU_PROF: "
> +		       "%s, line %d: create vma_map failed\n",
> +		       __FUNCTION__, __LINE__);
> +		goto err_alloc;
> +	}
> +	new_map = create_vma_map(spu, objectId);
> +	if (!new_map) {
> +		printk(KERN_ERR "SPU_PROF: "
> +		       "%s, line %d: create vma_map failed\n",
> +		       __FUNCTION__, __LINE__);
> +		goto err_alloc;
> +	}
> +
> +	pr_debug("Created vma_map\n");
> +	info->map = new_map;
> +	info->the_spu = spu;
> +	kref_init(&info->cache_ref);
> +	spin_lock_irqsave(&cache_lock, flags);
> +	spu_info[spu->number] = info;
> +	spin_unlock_irqrestore(&cache_lock, flags);
> +	/* Increment count before passing off ref to SPUFS. */
> +	kref_get(&info->cache_ref);
> +	spu_set_profile_private(spu->ctx, info, &info->cache_ref,
> +				destroy_cached_info);
> +	goto out;
> +	
> +err_alloc:
> +	retval = -1;
> +out:
> +	return retval;
> +}
> +
> +/*
> + * NOTE:  The caller is responsible for locking the
> + *	  cache_lock prior to calling this function.
> + */
> +static int release_cached_info(int spu_index)
> +{
> +	int index, end;
> +	if (spu_index == RELEASE_ALL) {
> +		end = num_spu_nodes;
> +		index = 0;
> +	} else {
> +	        if (spu_index >= num_spu_nodes) {
> +        	        printk(KERN_ERR "SPU_PROF: "
> +			       "%s, line %d: Invalid index %d into spu info cache\n",
> +               	               __FUNCTION__, __LINE__, spu_index);
> +	                goto out;
> +	        }
> +		end = spu_index +1;
> +		index = spu_index;
> +	}
> +	for (; index < end; index++) {
> +		if (spu_info[index]) {
> +			kref_put(&spu_info[index]->cache_ref, destroy_cached_info);
> +			spu_info[index] = NULL;
> +		}
> +	}
> +
> +out:
> +	return 0;
> +}
> +
> Index: linux-2.6.20-rc1/arch/powerpc/platforms/cell/spufs/context.c
> ===================================================================
> --- linux-2.6.20-rc1.orig/arch/powerpc/platforms/cell/spufs/context.c	2007-02-05 14:42:04.359859432 -0600
> +++ linux-2.6.20-rc1/arch/powerpc/platforms/cell/spufs/context.c	2007-02-06 16:44:05.983965096 -0600
> @@ -22,6 +22,7 @@
>
> #include <linux/fs.h>
> #include <linux/mm.h>
> +#include <linux/module.h>
> #include <linux/slab.h>
> #include <asm/spu.h>
> #include <asm/spu_csa.h>
> @@ -71,6 +72,8 @@
> 	spu_fini_csa(&ctx->csa);
> 	if (ctx->gang)
> 		spu_gang_remove_ctx(ctx->gang, ctx);
> +	if (ctx->prof_priv_kref)
> +		kref_put(ctx->prof_priv_kref, ctx->prof_priv_release);
> 	kfree(ctx);
> }
>
> @@ -200,3 +203,29 @@
>
> 	downgrade_write(&ctx->state_sema);
> }
> +
> +/* This interface allows a profiler (e.g., OProfile) to store
> + * spu_context information needed for profiling, allowing it to
> + * be saved across context save/restore operation.
> + *
> + * Assumes the caller has already incremented the ref count to
> + * profile_info; then spu_context_destroy must call kref_put
> + * on prof_info_kref.
> + */
> +void spu_set_profile_private(struct spu_context * ctx, void * profile_info,
> +			     struct kref * prof_info_kref,
> +			     void (* prof_info_release) (struct kref * kref))
> +{
> +	ctx->profile_private = profile_info;
> +	ctx->prof_priv_kref = prof_info_kref;
> +	ctx->prof_priv_release = prof_info_release;
> +}
> +EXPORT_SYMBOL_GPL(spu_set_profile_private);
> +
> +void * spu_get_profile_private(struct spu_context * ctx)
> +{
> +	return ctx->profile_private;
> +}
> +EXPORT_SYMBOL_GPL(spu_get_profile_private);
> +
> +
>
>
>  


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