Re: [PATCH net-next v5 3/5] page_pool: Allow drivers to hint on SKB recycling

[Ilias Apalodimas]

On Mon, May 17, 2021 at 07:10:09PM +0800, Yunsheng Lin wrote:
> On 2021/5/17 17:36, Ilias Apalodimas wrote:
>  >>
> >> Even if when skb->pp_recycle is 1, pages allocated from page allocator directly
> >> or page pool are both supported, so it seems page->signature need to be reliable
> >> to indicate a page is indeed owned by a page pool, which means the skb->pp_recycle
> >> is used mainly to short cut the code path for skb->pp_recycle is 0 case, so that
> >> the page->signature does not need checking?
> > 
> > Yes, the idea for the recycling bit, is that you don't have to fetch the page
> > in cache do do more processing (since freeing is asynchronous and we
> > can't have any guarantees on what the cache will have at that point).  So we
> > are trying to affect the existing release path a less as possible. However it's
> > that new skb bit that triggers the whole path.
> > 
> > What you propose could still be doable though.  As you said we can add the
> > page pointer to struct page when we allocate a page_pool page and never
> > reset it when we recycle the buffer. But I don't think there will be any
> > performance impact whatsoever. So I prefer the 'visible' approach, at least for
> 
> setting and unsetting the page_pool ptr every time the page is recycled may
> cause a cache bouncing problem when rx cleaning and skb releasing is not
> happening on the same cpu.

In our case since the skb is asynchronous and not protected by a NAPI context,
the buffer wont end up in the 'fast' page pool cache.  So we'll recycle by
calling page_pool_recycle_in_ring() not page_pool_recycle_in_cache().  Which
means that the page you recycled will be re-filled later, in batches, when
page_pool_refill_alloc_cache() is called to refill the fast cache.  I am not i
saying it might not happen, but I don't really know if it's going to make a
difference or not.  So I just really prefer taking this as is and perhaps
later, when 40/100gbit drivers start using it we can justify the optimization
(along with supporting the split page model).

Thanks
/Ilias

> 
> > the first iteration.
> > 
> > Thanks
> > /Ilias
> >  
> > 
> > .
> > 
>