Re: [PATCH] dma-direct: avoid redundant memory sync for swiotlb

[Robin Murphy]

On 12/04/2022 12:38 pm, Chao Gao wrote:
> When we looked into FIO performance with swiotlb enabled in VM, we found
> swiotlb_bounce() is always called one more time than expected for each DMA
> read request.
>
> It turns out that the bounce buffer is copied to original DMA buffer twice
> after the completion of a DMA request (one is done by in
> dma_direct_sync_single_for_cpu(), the other by swiotlb_tbl_unmap_single()).
> But the content in bounce buffer actually doesn't change between the two
> rounds of copy. So, one round of copy is redundant.
>
> Pass DMA_ATTR_SKIP_CPU_SYNC flag to swiotlb_tbl_unmap_single() to
> skip the memory copy in it.

It's still a little suboptimal and non-obvious to call into SWIOTLB 
twice though - even better might be for SWIOTLB to call 
arch_sync_dma_for_cpu() at the appropriate place internally, then put 
the dma_direct_sync in an else path here. I'm really not sure why we 
have the current disparity between map and unmap in this regard... :/

Robin.

> This fix increases FIO 64KB sequential read throughput in a guest with
> swiotlb=force by 5.6%.
>
> Reported-by: Wang Zhaoyang1 <zhaoyang1.wang@xxxxxxxxx>
> Reported-by: Gao Liang <liang.gao@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> ---
>   kernel/dma/direct.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/dma/direct.h b/kernel/dma/direct.h
> index 4632b0f4f72e..8a6cd53dbe8c 100644
> --- a/kernel/dma/direct.h
> +++ b/kernel/dma/direct.h
> @@ -114,6 +114,7 @@ static inline void dma_direct_unmap_page(struct device *dev, dma_addr_t addr,
>   		dma_direct_sync_single_for_cpu(dev, addr, size, dir);
>   
>   	if (unlikely(is_swiotlb_buffer(dev, phys)))
> -		swiotlb_tbl_unmap_single(dev, phys, size, dir, attrs);
> +		swiotlb_tbl_unmap_single(dev, phys, size, dir,
> +					 attrs | DMA_ATTR_SKIP_CPU_SYNC);
>   }
>   #endif /* _KERNEL_DMA_DIRECT_H */