Re: [Intel-gfx] [PATCH 1/2] drm/i915/gt: Prefer recycling an idle fence

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> If we want to reuse a fence that is in active use by the GPU, we have to
> wait an uncertain amount of time, but if we reuse an inactive fence, we
> can change it right away. Loop through the list of available fences
> twice, ignoring any active fences on the first pass.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c | 22 ++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> index 7fb36b12fe7a..a357bb431815 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ggtt_fencing.c
> @@ -320,13 +320,31 @@ void i915_vma_revoke_fence(struct i915_vma *vma)
>  		fence_write(fence);
>  }
>  
> +static bool fence_is_active(const struct i915_fence_reg *fence)
> +{
> +	return fence->vma && i915_vma_is_active(fence->vma);
> +}
> +
>  static struct i915_fence_reg *fence_find(struct i915_ggtt *ggtt)
>  {
> -	struct i915_fence_reg *fence;
> +	struct i915_fence_reg *active = NULL;
> +	struct i915_fence_reg *fence, *fn;
>  
> -	list_for_each_entry(fence, &ggtt->fence_list, link) {
> +	list_for_each_entry_safe(fence, fn, &ggtt->fence_list, link) {
>  		GEM_BUG_ON(fence->vma && fence->vma->fence != fence);
>  
> +		if (fence == active) /* now seen this fence twice */
> +			active = ERR_PTR(-EAGAIN);
> +
> +		/* Prefer idle fences so we do not have to wait on the GPU */
> +		if (active != ERR_PTR(-EAGAIN) && fence_is_active(fence)) {
> +			if (!active)
> +				active = fence;
> +
> +			list_move_tail(&fence->link, &ggtt->fence_list);
> +			continue;
> +		}
> +
>  		if (atomic_read(&fence->pin_count))
>  			continue;
>  
> -- 
> 2.20.1
>
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