Re: [PATCH 4/4] drm/i915: Use __sg_alloc_table_from_pages for userptr allocations

[Chris Wilson]

Quoting Tvrtko Ursulin (2017-07-27 10:05:04)
> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> 
> With the addition of __sg_alloc_table_from_pages we can control
> the maximum coallescing size and eliminate a separate path for
> allocating backing store here.
> 
> Similar to 871dfbd67d4e ("drm/i915: Allow compaction upto
> SWIOTLB max segment size") this enables more compact sg lists to
> be created and so has a beneficial effect on workloads with many
> and/or large objects of this class.
> 
> v2:
>  * Rename helper to i915_sg_segment_size and fix swiotlb override.
>  * Commit message update.
> 
> v3:
>  * Actually include the swiotlb override fix.
> 
> v4:
>  * Regroup parameters a bit. (Chris Wilson)
> 
> v5:
>  * Rebase for swiotlb_max_segment.
>  * Add DMA map failure handling as in abb0deacb5a6
>    ("drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping").
> 
> v6: Handle swiotlb_max_segment() returning 1. (Joonas Lahtinen)
> 
> v7: Rebase.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> (v4)
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/i915/i915_drv.h         | 15 +++++++
>  drivers/gpu/drm/i915/i915_gem.c         |  6 +--
>  drivers/gpu/drm/i915/i915_gem_userptr.c | 79 ++++++++++++---------------------
>  3 files changed, 45 insertions(+), 55 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 2c7456f4ed38..6383940e8d55 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2749,6 +2749,21 @@ static inline struct scatterlist *__sg_next(struct scatterlist *sg)
>              (((__iter).curr += PAGE_SIZE) < (__iter).max) ||           \
>              ((__iter) = __sgt_iter(__sg_next((__iter).sgp), false), 0))
>  
> +static inline unsigned int i915_sg_segment_size(void)
> +{
> +       unsigned int size = swiotlb_max_segment();
> +
> +       if (size == 0)
> +               return SCATTERLIST_MAX_SEGMENT;
> +
> +       size = rounddown(size, PAGE_SIZE);

Looks like swiotbl_max_seqment() is always page aligned when not 1.
And it returns bytes, ok.

Given that you are using a pot, you can use round_down().

> +       /* swiotlb_max_segment_size can return 1 byte when it means one page. */
> +       if (size < PAGE_SIZE)
> +               size = PAGE_SIZE;
> +
> +       return size;
> +}
> +
>  static inline const struct intel_device_info *
>  intel_info(const struct drm_i915_private *dev_priv)
>  {
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 6faabf34f142..a60885d6231b 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2339,7 +2339,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>         struct sgt_iter sgt_iter;
>         struct page *page;
>         unsigned long last_pfn = 0;     /* suppress gcc warning */
> -       unsigned int max_segment;
> +       unsigned int max_segment = i915_sg_segment_size();
>         gfp_t noreclaim;
>         int ret;
>  
> @@ -2350,10 +2350,6 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
>         GEM_BUG_ON(obj->base.read_domains & I915_GEM_GPU_DOMAINS);
>         GEM_BUG_ON(obj->base.write_domain & I915_GEM_GPU_DOMAINS);
>  
> -       max_segment = swiotlb_max_segment();
> -       if (!max_segment)
> -               max_segment = rounddown(UINT_MAX, PAGE_SIZE);

Conversion to i915_sg_segment_size(), ok.

> -
>         st = kmalloc(sizeof(*st), GFP_KERNEL);
>         if (st == NULL)
>                 return ERR_PTR(-ENOMEM);
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index ccd09e8419f5..60c10d4118ae 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -399,64 +399,42 @@ struct get_pages_work {
>         struct task_struct *task;
>  };
>  
> -#if IS_ENABLED(CONFIG_SWIOTLB)
> -#define swiotlb_active() swiotlb_nr_tbl()
> -#else
> -#define swiotlb_active() 0
> -#endif

Converted to i915_sg_segment_size(), nice.

> -static int
> -st_set_pages(struct sg_table **st, struct page **pvec, int num_pages)
> -{
> -       struct scatterlist *sg;
> -       int ret, n;
> -
> -       *st = kmalloc(sizeof(**st), GFP_KERNEL);
> -       if (*st == NULL)
> -               return -ENOMEM;
> -
> -       if (swiotlb_active()) {
> -               ret = sg_alloc_table(*st, num_pages, GFP_KERNEL);
> -               if (ret)
> -                       goto err;
> -
> -               for_each_sg((*st)->sgl, sg, num_pages, n)
> -                       sg_set_page(sg, pvec[n], PAGE_SIZE, 0);
> -       } else {
> -               ret = sg_alloc_table_from_pages(*st, pvec, num_pages,
> -                                               0, num_pages << PAGE_SHIFT,
> -                                               GFP_KERNEL);
> -               if (ret)
> -                       goto err;
> -       }
> -
> -       return 0;
> -
> -err:
> -       kfree(*st);
> -       *st = NULL;
> -       return ret;
> -}
> -
>  static struct sg_table *
> -__i915_gem_userptr_set_pages(struct drm_i915_gem_object *obj,
> -                            struct page **pvec, int num_pages)
> +__i915_gem_userptr_alloc_pages(struct drm_i915_gem_object *obj,
> +                              struct page **pvec, int num_pages)
>  {
> -       struct sg_table *pages;
> +       unsigned int max_segment = i915_sg_segment_size();
> +       struct sg_table *st;
>         int ret;
>  
> -       ret = st_set_pages(&pages, pvec, num_pages);
> -       if (ret)
> +       st = kmalloc(sizeof(*st), GFP_KERNEL);
> +       if (!st)
> +               return ERR_PTR(-ENOMEM);
> +
> +alloc_table:
> +       ret = __sg_alloc_table_from_pages(st, pvec, num_pages,
> +                                         0, num_pages << PAGE_SHIFT,
> +                                         max_segment,
> +                                         GFP_KERNEL);
> +       if (ret) {
> +               kfree(st);
>                 return ERR_PTR(ret);
> +       }
>  
> -       ret = i915_gem_gtt_prepare_pages(obj, pages);
> +       ret = i915_gem_gtt_prepare_pages(obj, st);
>         if (ret) {
> -               sg_free_table(pages);
> -               kfree(pages);
> +               sg_free_table(st);
> +
> +               if (max_segment > PAGE_SIZE) {
> +                       max_segment = PAGE_SIZE;
> +                       goto alloc_table;
> +               }
> +
> +               kfree(st);
>                 return ERR_PTR(ret);
>         }

Much neater.

>  
> -       return pages;
> +       return st;
>  }
>  
>  static int
> @@ -540,7 +518,8 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
>                 struct sg_table *pages = ERR_PTR(ret);
>  
>                 if (pinned == npages) {
> -                       pages = __i915_gem_userptr_set_pages(obj, pvec, npages);
> +                       pages = __i915_gem_userptr_alloc_pages(obj, pvec,
> +                                                              npages);
>                         if (!IS_ERR(pages)) {
>                                 __i915_gem_object_set_pages(obj, pages);
>                                 pinned = 0;
> @@ -661,7 +640,7 @@ i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
>                 pages = __i915_gem_userptr_get_pages_schedule(obj);
>                 active = pages == ERR_PTR(-EAGAIN);
>         } else {
> -               pages = __i915_gem_userptr_set_pages(obj, pvec, num_pages);
> +               pages = __i915_gem_userptr_alloc_pages(obj, pvec, num_pages);
>                 active = !IS_ERR(pages);
>         }
>         if (active)

Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
-Chris