Re: [PATCH v6 4/6] drm/drm_file: Add DRM obj's RSS reporting function for fdinfo

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 20/09/2023 00:34, Adrián Larumbe wrote:
> Some BO's might be mapped onto physical memory chunkwise and on demand,
> like Panfrost's tiler heap. In this case, even though the
> drm_gem_shmem_object page array might already be allocated, only a very
> small fraction of the BO is currently backed by system memory, but
> drm_show_memory_stats will then proceed to add its entire virtual size to
> the file's total resident size regardless.
> 
> This led to very unrealistic RSS sizes being reckoned for Panfrost, where
> said tiler heap buffer is initially allocated with a virtual size of 128
> MiB, but only a small part of it will eventually be backed by system memory
> after successive GPU page faults.
> 
> Provide a new DRM object generic function that would allow drivers to
> return a more accurate RSS size for their BOs.
> 
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Reviewed-by: Steven Price <steven.price@arm.com>
> ---
>   drivers/gpu/drm/drm_file.c | 5 ++++-
>   include/drm/drm_gem.h      | 9 +++++++++
>   2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
> index 883d83bc0e3d..762965e3d503 100644
> --- a/drivers/gpu/drm/drm_file.c
> +++ b/drivers/gpu/drm/drm_file.c
> @@ -944,7 +944,10 @@ void drm_show_memory_stats(struct drm_printer *p, struct drm_file *file)
>   		}
>   
>   		if (s & DRM_GEM_OBJECT_RESIDENT) {
> -			status.resident += obj->size;
> +			if (obj->funcs && obj->funcs->rss)
> +				status.resident += obj->funcs->rss(obj);
> +			else
> +				status.resident += obj->size;

Presumably you'd want the same smaller size in both active and 
purgeable? Or you can end up with more in those two than in rss which 
would look odd.

Also, alternative to adding a new callback could be adding multiple 
output parameters to the existing obj->func->status() which maybe ends 
up simpler due fewer callbacks?

Like:

  s = obj->funcs->status(obj, &supported_status, &rss)

And adjust the code flow to pick up the rss if driver signaled it 
supports reporting it.

Regards,

Tvrtko

>   		} else {
>   			/* If already purged or not yet backed by pages, don't
>   			 * count it as purgeable:
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index bc9f6aa2f3fe..16364487fde9 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -208,6 +208,15 @@ struct drm_gem_object_funcs {
>   	 */
>   	enum drm_gem_object_status (*status)(struct drm_gem_object *obj);
>   
> +	/**
> +	 * @rss:
> +	 *
> +	 * Return resident size of the object in physical memory.
> +	 *
> +	 * Called by drm_show_memory_stats().
> +	 */
> +	size_t (*rss)(struct drm_gem_object *obj);
> +
>   	/**
>   	 * @vm_ops:
>   	 *