On 5/3/21 10:04 AM, Logan Gunthorpe wrote:

Oops missed a comment:

On 2021-05-02 5:28 p.m., John Hubbard wrote:

quoted

quoted

   int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
   		enum dma_data_direction dir, unsigned long attrs)
   {
-	int i;
+	struct pci_p2pdma_map_state p2pdma_state = {};

Is it worth putting this stuff on the stack--is there a noticeable
performance improvement from caching the state? Because if it's
invisible, then simplicity is better. I suspect you're right, and that
it *is* worth it, but it's good to know for real.

I haven't measured it (it would be hard to measure), but I think it's
fairly clear here. Without the state, xa_load() would need to be called
on *every* page in an SGL that maps only P2PDMA memory from one device.
With the state, it only needs to be called once. xa_load() is cheap, but
it is not that cheap.

OK, thanks for spelling it out for me. :)

There's essentially the same optimization in get_user_pages for
ZONE_DEVICE pages. So, if it is necessary there, it should be necessary
here.

Right, that's a pretty solid example.

thanks,
-- 
John Hubbard
NVIDIA