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 22/09/2023 11:58, Adrián Larumbe wrote:
> 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@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.
> 
> 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.

Okay. See how it will look once you need to apply it to resident and 
purgeable. I wonder if "driver knows better" will end up a dominant case 
and we do end up considering reversing the scheme (like ask the driver 
to fill in the meminfo record). TBH I do not remember all the flavours 
both Rob and I proposed at this point.

Regards,

Tvrtko

> 
>>
>> 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:
>>>    	 *