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

[Adrián Larumbe]

On 20.09.2023 16:53, Tvrtko Ursulin wrote:
>
>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@xxxxxxxxxxxxx>
>> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>> Reviewed-by: Steven Price <steven.price@xxxxxxx>
>> ---
>>   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.

I didn't think of this. I guess when an object is both resident and purgeable,
then its RSS and purgeable sizes should be the same.

>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.

I personally find having a separate object callback more readable in this case.
There's also the question of what output parameter value would be used as a token
that the relevant BO doesn't have an RSS different from its virtual
size. I guess '0' would be alright, but this is on the assumption that this
could never be a legitimate BO virtual size across all DRM drivers. I guess
most of them round the size up to the nearest page multiple at BO creation
time.

>
>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:
>>   	 *