Re: [PATCH] drm/i915: Remove guard page insertion around unevictable nodes

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On Sun, 24 Jan 2021 at 13:57, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>
> Assume that unevictable nodes are not in the GTT and so we can ignore
> page boundary concerns, and so allow regular nodes to abutt against
> irregular unevictable nodes.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/i915_drv.h       |  2 --
>  drivers/gpu/drm/i915/i915_gem_evict.c |  6 ++++--
>  drivers/gpu/drm/i915/i915_vma.h       | 10 +++++++++-
>  drivers/gpu/drm/i915/i915_vma_types.h |  2 ++
>  4 files changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 99cf861df92d..69c5a185ecff 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -357,8 +357,6 @@ enum i915_cache_level {
>         I915_CACHE_WT, /* hsw:gt3e WriteThrough for scanouts */
>  };
>
> -#define I915_COLOR_UNEVICTABLE (-1) /* a non-vma sharing the address space */
> -
>  struct intel_fbc {
>         /* This is always the inner lock when overlapping with struct_mutex and
>          * it's the outer lock when overlapping with stolen_lock. */
> diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c
> index 4d2d59a9942b..aef88fdb9f66 100644
> --- a/drivers/gpu/drm/i915/i915_gem_evict.c
> +++ b/drivers/gpu/drm/i915/i915_gem_evict.c
> @@ -313,11 +313,13 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
>                  */
>                 if (i915_vm_has_cache_coloring(vm)) {
>                         if (node->start + node->size == target->start) {
> -                               if (node->color == target->color)
> +                               if (i915_node_color_matches(node,
> +                                                           target->color))
>                                         continue;
>                         }
>                         if (node->start == target->start + target->size) {
> -                               if (node->color == target->color)
> +                               if (i915_node_color_matches(node,
> +                                                           target->color))
>                                         continue;
>                         }
>                 }

Since we bail early on seeing COLOR_UNEVICTABLE, and since we have to
enlarge our search space by a page on both ends, do we need something
like:

@@ -291,18 +291,22 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,

                /* Always look at the page afterwards to avoid the end-of-GTT */
                end += I915_GTT_PAGE_SIZE;
+
+               /*
+                * The special head node is marked as I915_COLOR_UNEVICTABLE to
+                * prevent normal vma from using the last page, in order to
+                * prevent prefetching over the boundary. If we are dealing with
+                * a normal vma and we abutt with the head node we will always
+                * get -ENOSPC as per the color adjust later, so just bail
+                * early???
+                */
+               if (target->color != I915_COLOR_UNEVICTABLE && end >= vm->total)
+                       return -ENOSPC;
        }
        GEM_BUG_ON(start >= end);

        drm_mm_for_each_node_in_range(node, &vm->mm, start, end) {
-               /* If we find any non-objects (!vma), we cannot evict them */
-               if (node->color == I915_COLOR_UNEVICTABLE) {
-                       ret = -ENOSPC;
-                       break;
-               }
-
                GEM_BUG_ON(!drm_mm_node_allocated(node));
-               vma = container_of(node, typeof(*vma), node);

                /*
                 * If we are using coloring to insert guard pages between
@@ -310,6 +314,9 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
                 * to check whether the objects on either side of our range
                 * abutt and conflict. If they are in conflict, then we evict
                 * those as well to make room for our guard pages.
+                *
+                * Also consider I915_COLOR_UNEVICTABLE nodes, since we consider
+                * these special and permit abutting on either end.
                 */
                if (i915_vm_has_cache_coloring(vm)) {
                        if (node->start + node->size == target->start) {
@@ -324,6 +331,14 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
                        }
                }

+               /* If we find any non-objects (!vma), we cannot evict them */
+               if (node->color == I915_COLOR_UNEVICTABLE) {
+                       ret = -ENOSPC;
+                       break;
+               }
+
+               vma = container_of(node, typeof(*vma), node);
+
                if (i915_vma_is_pinned(vma)) {
                        ret = -ENOSPC;
                        break;


> diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
> index a64adc8c883b..dac953815118 100644
> --- a/drivers/gpu/drm/i915/i915_vma.h
> +++ b/drivers/gpu/drm/i915/i915_vma.h
> @@ -283,10 +283,18 @@ static inline bool i915_vma_is_bound(const struct i915_vma *vma,
>         return atomic_read(&vma->flags) & where;
>  }
>
> +static inline bool i915_node_color_matches(const struct drm_mm_node *node,
> +                                          unsigned long color)
> +{
> +       return (node->color | color) == I915_COLOR_UNEVICTABLE ||
> +               node->color == color;
> +}
> +
>  static inline bool i915_node_color_differs(const struct drm_mm_node *node,
>                                            unsigned long color)
>  {
> -       return drm_mm_node_allocated(node) && node->color != color;
> +       return drm_mm_node_allocated(node) &&
> +               !i915_node_color_matches(node, color);
>  }
>
>  /**
> diff --git a/drivers/gpu/drm/i915/i915_vma_types.h b/drivers/gpu/drm/i915/i915_vma_types.h
> index f5cb848b7a7e..e72a07692a64 100644
> --- a/drivers/gpu/drm/i915/i915_vma_types.h
> +++ b/drivers/gpu/drm/i915/i915_vma_types.h
> @@ -95,6 +95,8 @@ enum i915_cache_level;
>   *
>   */
>
> +#define I915_COLOR_UNEVICTABLE (-1) /* a non-vma sharing the address space */
> +
>  struct intel_remapped_plane_info {
>         /* in gtt pages */
>         unsigned int width, height, stride, offset;
> --
> 2.20.1
>
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> Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx
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