Re: [RFC 00/12] net: huge page backed page_pool

On 12/07/2023 13.47, Yunsheng Lin wrote:

On 2023/7/12 8:08, Jakub Kicinski wrote:

quoted

On Tue, 11 Jul 2023 17:49:19 +0200 Jesper Dangaard Brouer wrote:

quoted

I see you have discovered that the next bottleneck are the IOTLB misses.
One of the techniques for reducing IOTLB misses is using huge pages.
Called "super-pages" in article (below), and they report that this trick
doesn't work on AMD (Pacifica arch).

I think you have convinced me that the pp_provider idea makes sense for
*this* use-case, because it feels like natural to extend PP with
mitigations for IOTLB misses. (But I'm not 100% sure it fits Mina's
use-case).

We're on the same page then (no pun intended).

quoted

What is your page refcnt strategy for these huge-pages. I assume this
rely on PP frags-scheme, e.g. using page->pp_frag_count.
Is this correctly understood?

Oh, I split the page into individual 4k pages after DMA mapping.
There's no need for the host memory to be a huge page. I mean,
the actual kernel identity mapping is a huge page AFAIU, and the
struct pages are allocated, anyway. We just need it to be a huge
page at DMA mapping time.

So the pages from the huge page provider only differ from normal
alloc_page() pages by the fact that they are a part of a 1G DMA
mapping.

So, Jakub you are saying the PP refcnt's are still done "as usual" on 
individual pages.

If it is about DMA mapping, is it possible to use dma_map_sg()
to enable a big continuous dma map for a lot of discontinuous
4k pages to avoid allocating big huge page?

As the comment:
"The scatter gather list elements are merged together (if possible)
and tagged with the appropriate dma address and length."

https://elixir.free-electrons.com/linux/v4.16.18/source/arch/arm/mm/dma-mapping.c#L1805

This is interesting for two reasons.

(1) if this DMA merging helps IOTLB misses (?)

(2) PP could use dma_map_sg() to amortize dma_map call cost.

For case (2) __page_pool_alloc_pages_slow() already does bulk allocation
of pages (alloc_pages_bulk_array_node()), and then loops over the pages
to DMA map them individually.  It seems like an obvious win to use
dma_map_sg() here?

--Jesper