[Mesa-dev] [PATCH] i965: Reuse the same single-page bo for all zero sized allocations

[Michal Srb]

Hi,

This is my first attempt to review patch for Mesa, so please take it with a 
grain of salt.

On úterý 14. srpna 2018 20:21:40 CEST Chris Wilson wrote:
> @@ -504,6 +506,24 @@ bo_alloc_internal(struct brw_bufmgr *bufmgr,
>     bool busy = false;
>     bool zeroed = false;
> 
> +   /* Reuse the same bo for all zero-sized requests */
> +   if (size == 0) {
> +      if (bufmgr->bo_zero == NULL) {
> +         bo = bo_alloc_internal(bufmgr, "zero", 4096,
> +                                BRW_MEMZONE_OTHER, BO_ALLOC_BUSY, 0, 0);
> +         if (!bo)
> +            return NULL;
> +
> +         bo->size = 0;

Doesn't this break something once the bo_zero is freed? Either after two 
threads raced and one has to cleanup or when the whole bufmgr is destroyed.

The bucket_for_size will choose different bucket for sizes 4096 and 0:
  size 4096 -> bucket index   0 -> some bucket (I assume)
  size    0 -> bucket index 119 -> null bucket (always too big)

So the get_bucket_allocator function will return some bucket when allocating 
and null bucket when freeing, so allocation goes thru bucket_vma_alloc and 
freeing thru util_vma_heap_free. I did not examine those functions closer, but 
it does sound like it would break something.

> +
> +         if (p_atomic_cmpxchg(&bufmgr->bo_zero, NULL, bo))
> +            brw_bo_unreference(bo);
> +      }

Michal