[PATCH] drm/ttm: Fix TTM BO accounting

[Thomas Hellstrom]

On 04/12/2016 06:39 PM, Felix Kuehling wrote:
> This is the implementation of ttm_round_pot:
>
> size_t ttm_round_pot(size_t size)
> {
>         if ((size & (size - 1)) == 0)
>                 return size;
>         else if (size > PAGE_SIZE)
>                 return PAGE_ALIGN(size);
>         else {
>                 size_t tmp_size = 4;
>
>                 while (tmp_size < size)
>                         tmp_size <<= 1;
>
>                 return tmp_size;
>         }
>         return 0;
> }
>
> As you can see, for size > PAGE_SIZE it just returns PAGE_ALIGN(size).
> It does not actually round to a power of two in this case.
>
> Regards,
>   Felix

Ah yes. I should've refreshed my memory.

Reviewed-by: Thomas Hellstrom <thellstrom at vmware.com>




>
> On 16-04-12 12:21 PM, Thomas Hellstrom wrote:
>> On 04/12/2016 05:57 PM, Alex Deucher wrote:
>>> From: Felix Kuehling <Felix.Kuehling at amd.com>
>>>
>>> TTM BO accounting is out of sync with how memory is really allocated
>>> in ttm[_dma]_tt_alloc_page_directory. This resulted in excessive
>>> estimated overhead with many small allocations.
>>>
>>> ttm_dma_tt_alloc_page_directory makes a single allocation for three
>>> arrays: pages, DMA and CPU addresses. It uses drm_calloc_large, which
>>> uses kmalloc internally for allocations smaller than PAGE_SIZE.
>>> ttm_round_pot should be a good approximation of its memory usage both
>>> above and below PAGE_SIZE.
>> I think for allocations larger than PAGE_SIZE,
>> ttm_round_pot() will overestimate. You should probably use the smaller
>> of the two.
>>
>> /Thomas
>>
>>
>>
>>> Signed-off-by: Felix Kuehling <Felix.Kuehling at amd.com>
>>> Reviewed-by: Monk Liu <monk.liu at amd.com>
>>> Reviewed-by: Christian König <christian.koenig at amd.com>
>>> ---
>>>  drivers/gpu/drm/ttm/ttm_bo.c | 5 ++---
>>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>>> index 4cbf265..870a87a 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>>> @@ -1215,7 +1215,7 @@ size_t ttm_bo_acc_size(struct ttm_bo_device *bdev,
>>>  	size_t size = 0;
>>>  
>>>  	size += ttm_round_pot(struct_size);
>>> -	size += PAGE_ALIGN(npages * sizeof(void *));
>>> +	size += ttm_round_pot(npages * sizeof(void *));
>>>  	size += ttm_round_pot(sizeof(struct ttm_tt));
>>>  	return size;
>>>  }
>>> @@ -1229,8 +1229,7 @@ size_t ttm_bo_dma_acc_size(struct ttm_bo_device *bdev,
>>>  	size_t size = 0;
>>>  
>>>  	size += ttm_round_pot(struct_size);
>>> -	size += PAGE_ALIGN(npages * sizeof(void *));
>>> -	size += PAGE_ALIGN(npages * sizeof(dma_addr_t));
>>> +	size += ttm_round_pot(npages * (2*sizeof(void *) + sizeof(dma_addr_t)));
>>>  	size += ttm_round_pot(sizeof(struct ttm_dma_tt));
>>>  	return size;
>>>  }