[PATCH 4/5] drm/amdgpu: Set/clear CPU_ACCESS_REQUIRED flag on page fault and CS

[Michel Dänzer]

On 28/06/17 11:33 AM, John Brooks wrote:
> When the AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED flag is given by userspace,
> it should only be treated as a hint to initially place a BO somewhere CPU
> accessible, rather than having a permanent effect on BO placement.
> 
> Instead of the flag being set in stone at BO creation, set the flag when a
> page fault occurs so that it goes somewhere CPU-visible, and clear it when
> the BO is requested by the GPU.
> 
> However, clearing the CPU_ACCESS_REQUIRED flag may move BOs in GTT to
> invisible VRAM, where they may promptly generate another page fault. When
> BOs are constantly moved back and forth like this, it is highly detrimental
> to performance. Only clear the flag on CS if:
> 
> - The BO wasn't page faulted for a certain amount of time (currently 10
>   seconds), and
> - its last page fault didn't occur too soon (currently 500ms) after its
>   last CS request, or vice versa.
> 
> Setting the flag in amdgpu_fault_reserve_notify() also means that we can
> remove the loop to restrict lpfn to the end of visible VRAM, because
> amdgpu_ttm_placement_init() will do it for us.
> 
> Signed-off-by: John Brooks <john at fastquake.com>

[...]

> @@ -967,15 +967,9 @@ int amdgpu_bo_fault_reserve_notify(struct ttm_buffer_object *bo)
>  
>  	/* hurrah the memory is not visible ! */
>  	atomic64_inc(&adev->num_vram_cpu_page_faults);
> +	abo->flags |= AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED;

Talking about this on IRC reminded me that I think we should set the
flag in this function regardless of where the BO is currently located.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer