[PATCH 6/9] drm/amdgpu: Set/clear CPU_ACCESS_REQUIRED flag on page fault and CS

[Christian König]

Am 23.06.2017 um 19:39 schrieb John Brooks:
> 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.

And that is a clear NAK from my side.

CPU_ACCESS_REQUIRED is a permanent limitation to where the buffer should 
be placed.

Ignoring that and changing the IOCTL interface like this is clearly not 
acceptable.

Christian.

>
> 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 a BO to invisible
> VRAM, which is likely to cause a page fault that moves it right back to
> GTT. When this happens too much, 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, measured with jiffies), 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>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c     |  1 +
>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.c | 46 ++++++++++++++++++++++--------
>   drivers/gpu/drm/amd/amdgpu/amdgpu_object.h |  1 +
>   3 files changed, 36 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> index 2fad8bd..73d6882 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> @@ -320,6 +320,7 @@ static int amdgpu_cs_bo_validate(struct amdgpu_cs_parser *p,
>   	else
>   		domain = bo->allowed_domains;
>   
> +	amdgpu_bo_clear_cpu_access_required(bo);
>   retry:
>   	amdgpu_ttm_placement_from_domain(bo, domain);
>   	initial_bytes_moved = atomic64_read(&adev->num_bytes_moved);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> index 31d1f21..a7d48a7 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
> @@ -967,8 +967,8 @@ int amdgpu_bo_fault_reserve_notify(struct ttm_buffer_object *bo)
>   {
>   	struct amdgpu_device *adev = amdgpu_ttm_adev(bo->bdev);
>   	struct amdgpu_bo *abo;
> -	unsigned long offset, size, lpfn;
> -	int i, r;
> +	unsigned long offset, size;
> +	int r;
>   
>   	if (!amdgpu_ttm_bo_is_amdgpu_bo(bo))
>   		return 0;
> @@ -991,18 +991,9 @@ 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;
>   	amdgpu_ttm_placement_from_domain(abo, AMDGPU_GEM_DOMAIN_VRAM |
>   					 AMDGPU_GEM_DOMAIN_GTT);
> -	lpfn =	adev->mc.visible_vram_size >> PAGE_SHIFT;
> -	for (i = 0; i < abo->placement.num_placement; i++) {
> -		/* Try to move the BO into visible VRAM */
> -		if ((abo->placements[i].flags & TTM_PL_FLAG_VRAM) &&
> -		    (!abo->placements[i].lpfn ||
> -		     abo->placements[i].lpfn > lpfn))
> -			abo->placements[i].lpfn = lpfn;
> -	}
> -	abo->placement.busy_placement = abo->placement.placement;
> -	abo->placement.num_busy_placement = abo->placement.num_placement;
>   	r = ttm_bo_validate(bo, &abo->placement, false, false);
>   	if (unlikely(r != 0))
>   		return r;
> @@ -1057,3 +1048,34 @@ u64 amdgpu_bo_gpu_offset(struct amdgpu_bo *bo)
>   
>   	return bo->tbo.offset;
>   }
> +
> +/**
> + * amdgpu_bo_clear_cpu_access_required
> + * @bo: BO to update
> + *
> + * Clears CPU_ACCESS_REQUIRED flag if the BO hasn't had a page fault in a while
> + * and it didn't have a page fault too soon after the last time it was moved to
> + * VRAM.
> + *
> + * Caller should have bo reserved.
> + *
> + */
> +void amdgpu_bo_clear_cpu_access_required(struct amdgpu_bo *bo)
> +{
> +	const unsigned int page_fault_timeout_ms = 10000;
> +	const unsigned int min_period_ms = 500;
> +	unsigned int ms_since_pf, period_ms;
> +
> +	ms_since_pf = jiffies_to_msecs(jiffies - bo->last_page_fault_jiffies);
> +	period_ms = jiffies_to_msecs(abs(bo->last_page_fault_jiffies -
> +					 bo->last_cs_move_jiffies));
> +
> +	/*
> +	 * Try to avoid a revolving door between GTT and VRAM. Clearing the
> +	 * flag may move this BO back to VRAM, so don't clear it if it's likely
> +	 * to page fault and go right back to GTT.
> +	 */
> +	if ((!bo->last_page_fault_jiffies || !bo->last_cs_move_jiffies) ||
> +	    (ms_since_pf > page_fault_timeout_ms && period_ms > min_period_ms))
> +		bo->flags &= ~AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED;
> +}
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.h
> index 3824851..b0cb137 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.h
> @@ -182,6 +182,7 @@ int amdgpu_bo_restore_from_shadow(struct amdgpu_device *adev,
>   				  struct reservation_object *resv,
>   				  struct dma_fence **fence,
>   				  bool direct);
> +void amdgpu_bo_clear_cpu_access_required(struct amdgpu_bo *bo);
>   
>   
>   /*