On 02/11/2019 12:53 AM, Tariq Toukan wrote:
>
> Hi,
>
> It's great to use the struct page to store its dma mapping, but I am
> worried about extensibility.
> page_pool is evolving, and it would need several more per-page fields.
> One of them would be pageref_bias, a planned optimization to reduce the
> number of the costly atomic pageref operations (and replace existing
> code in several drivers).
>
But the point about pageref_bias is to place it in a different cache line than
"struct page"
The major cost is having a cache line bouncing between producer and consumer.
pageref_bias means the producer only have to read the "struct page" and not
dirty it
in the case the page can be recycled.
> I would replace this dma field with a pointer to an extensible struct,
> that would contain the dma mapping (and other stuff in the near future).
> This pointer fits perfectly with the existing unsigned long private;
> they can share the memory, for both 32- and 64-bits systems.
>
> The only downside is one more pointer de-reference. This should be perf
> tested.
> However, when introducing the page refcnt bias optimization into
> page_pool, I believe the perf gain would be guaranteed.
Only in some cases perhaps (when the cache line can be dirtied without
performance hit)
