netdev - Re: [RFC PATCH v2 06/11] page-pool: add device memory support

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Message-ID: <CAF=yD-LiR1J7iaGCfc+YKNB7W7QO3XvZGfgOSKjpGur_JZFsGQ@mail.gmail.com>
Date: Sat, 19 Aug 2023 12:11:40 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc: brouer@...hat.com, Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Arnd Bergmann <arnd@...db.de>, David Ahern <dsahern@...nel.org>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Hari Ramakrishnan <rharix@...gle.com>, 
	Dan Williams <dan.j.williams@...el.com>, Andy Lutomirski <luto@...nel.org>, stephen@...workplumber.org, 
	sdf@...gle.com
Subject: Re: [RFC PATCH v2 06/11] page-pool: add device memory support

> > Any regression in page pool can be avoided in the common case that
> > does not use device mem by placing that behind a static_branch. Would
> > that address your performance concerns?
> >
>
> No. This will not help.
>
> The problem is that every where in the page_pool code it is getting
> polluted with:
>
>    if (page_is_page_pool_iov(page))
>      call-some-iov-func-instead()
>
> Like: the very central piece of getting the refcnt:
>
> +static inline int page_pool_page_ref_count(struct page *page)
> +{
> +       if (page_is_page_pool_iov(page))
> +               return page_pool_iov_refcount(page_to_page_pool_iov(page));
> +
> +       return page_ref_count(page);
> +}
>
>
> The fast-path of the PP is used for XDP_DROP scenarios, and is currently
> around 14 cycles (tsc). Thus, any extra code in this code patch will
> change the fast-path.

With static_branch disabled, it would only insert a NOP?