Messages in this thread | | | From | Logan Gunthorpe <> | Date | Mon, 3 May 2021 11:04:18 -0600 | Subject | Re: [PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg |
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Oops missed a comment:
On 2021-05-02 5:28 p.m., John Hubbard wrote: >> 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.
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.
Logan
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