Re: [PATCH] dma-buf: system_heap: Optimize sg_table-to-pages conversion in vmap

[Christian König <christian.koenig@amd.com>]

On 4/6/26 23:49, Barry Song (Xiaomi) wrote:
> From: Xueyuan Chen <Xueyuan.chen21@gmail.com>
> 
> Replace the heavy for_each_sgtable_page() iterator in system_heap_do_vmap()
> with a more efficient nested loop approach.
> 
> Instead of iterating page by page, we now iterate through the scatterlist
> entries via for_each_sgtable_sg(). Because pages within a single sg entry
> are physically contiguous, we can populate the page array with a in an
> inner loop using simple pointer math. This save a lot of time.
> 
> The WARN_ON check is also pulled out of the loop to save branch
> instructions.
> 
> Performance results mapping a 2GB buffer on Radxa O6:
> - Before: ~1440000 ns
> - After:  ~232000 ns
> (~84% reduction in iteration time, or ~6.2x faster)

Well real question is why do you care about the vmap performance?

That should basically only be used for fbdev emulation (except for VMGFX) and we absolutely don't care about performance there.

Regards,
Christian.

> 
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Benjamin Gaignard <benjamin.gaignard@collabora.com>
> Cc: Brian Starkey <Brian.Starkey@arm.com>
> Cc: John Stultz <jstultz@google.com>
> Cc: T.J. Mercier <tjmercier@google.com>
> Cc: Christian König <christian.koenig@amd.com>
> Signed-off-by: Xueyuan Chen <Xueyuan.chen21@gmail.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  drivers/dma-buf/heaps/system_heap.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
> index b3650d8fd651..769f01f0cc96 100644
> --- a/drivers/dma-buf/heaps/system_heap.c
> +++ b/drivers/dma-buf/heaps/system_heap.c
> @@ -224,16 +224,21 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer)
>  	int npages = PAGE_ALIGN(buffer->len) / PAGE_SIZE;
>  	struct page **pages = vmalloc(sizeof(struct page *) * npages);
>  	struct page **tmp = pages;
> -	struct sg_page_iter piter;
>  	void *vaddr;
> +	u32 i, j, count;
> +	struct page *base_page;
> +	struct scatterlist *sg;
>  
>  	if (!pages)
>  		return ERR_PTR(-ENOMEM);
>  
> -	for_each_sgtable_page(table, &piter, 0) {
> -		WARN_ON(tmp - pages >= npages);
> -		*tmp++ = sg_page_iter_page(&piter);
> +	for_each_sgtable_sg(table, sg, i) {
> +		base_page = sg_page(sg);
> +		count = sg->length >> PAGE_SHIFT;
> +		for (j = 0; j < count; j++)
> +			*tmp++ = base_page + j;
>  	}
> +	WARN_ON(tmp - pages != npages);
>  
>  	vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL);
>  	vfree(pages);