[PATCH] drm/ttm: Fix TTM BO accounting

[Felix Kuehling]

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

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;
>>  }

-- 
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