[PATCH v4 10/14] drm/i915/ttm: hide shmem objects from TTM LRU

[Christian König]

Am 21.09.21 um 13:01 schrieb Matthew Auld:
> This is probably a NAK. But ideally we need to somehow prevent TTM from
> seeing shmem objects when doing its LRU swap walk. Since these are
> EXTERNAL they are ignored anyway, but keeping them in the LRU seems
> pretty wasteful.  Trying to use bo_pin() for this is all kinds of nasty
> since we need to be able to do the bo_unpin() from the unpopulate hook,
> but since that can be called from the BO destroy path we will likely go
> down in flames.
>
> An alternative is to maybe just add EXTERNAL objects to some
> bdev->external LRU in TTM, or just don't add them at all?

Yeah, that goes into the same direction as why I want to push the LRU 
into the resource for some time.

The problem is that the LRU is needed for multiple things. E.g. 
swapping, GART management, resource constrains, IOMMU teardown etc..

So for now I think that everything should be on the LRU even if it isn't 
valid to be there for some use case.

Regards,
Christian.

>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> Cc: Christian König <christian.koenig at amd.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_ttm.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> index 174aebe11264..b438ddb52764 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> @@ -800,6 +800,22 @@ static unsigned long i915_ttm_io_mem_pfn(struct ttm_buffer_object *bo,
>   	return ((base + sg_dma_address(sg)) >> PAGE_SHIFT) + ofs;
>   }
>   
> +static void i915_ttm_del_from_lru_notify(struct ttm_buffer_object *bo)
> +{
> +	struct i915_ttm_tt *i915_tt =
> +		container_of(bo->ttm, typeof(*i915_tt), ttm);
> +
> +	/* Idealy we need to prevent TTM from seeing shmem objects when doing
> +	 * its LRU swap walk. Since these are EXTERNAL they are ignored anyway,
> +	 * but keeping them in the LRU is pretty waseful. Trying to use bo_pin()
> +	 * for this is very nasty since we need to be able to do the bo_unpin()
> +	 * from the unpopulate hook, but since that can be called from the BO
> +	 * destroy path we will go down in flames.
> +	 */
> +	if (bo->ttm && ttm_tt_is_populated(bo->ttm) && i915_tt->is_shmem)
> +		list_del_init(&bo->lru);
> +}
> +
>   static struct ttm_device_funcs i915_ttm_bo_driver = {
>   	.ttm_tt_create = i915_ttm_tt_create,
>   	.ttm_tt_populate = i915_ttm_tt_populate,
> @@ -810,6 +826,7 @@ static struct ttm_device_funcs i915_ttm_bo_driver = {
>   	.move = i915_ttm_move,
>   	.swap_notify = i915_ttm_swap_notify,
>   	.delete_mem_notify = i915_ttm_delete_mem_notify,
> +	.del_from_lru_notify = i915_ttm_del_from_lru_notify,
>   	.io_mem_reserve = i915_ttm_io_mem_reserve,
>   	.io_mem_pfn = i915_ttm_io_mem_pfn,
>   };