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Date:   Tue, 11 May 2021 11:41:23 +0300
From:   Ilias Apalodimas <ilias.apalodimas@linaro.org>
To:     Shay Agroskin <shayagr@amazon.com>
Cc:     Jesper Dangaard Brouer <brouer@redhat.com>,
        Yunsheng Lin <linyunsheng@huawei.com>,
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Subject: Re: [PATCH net-next v3 0/5] page_pool: recycle buffers
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Hi Shay,

On Sun, May 09, 2021 at 08:11:35AM +0300, Shay Agroskin wrote:
> 
> Jesper Dangaard Brouer <brouer@redhat.com> writes:
> 
> > On Fri, 7 May 2021 16:28:30 +0800
> > Yunsheng Lin <linyunsheng@huawei.com> wrote:
> > 
> > > On 2021/5/7 15:06, Ilias Apalodimas wrote:
> > > > On Fri, May 07, 2021 at 11:23:28AM +0800, Yunsheng Lin wrote:  >>
> > > On 2021/5/6 20:58, Ilias Apalodimas wrote:  >>>>>>  >>>>>
> > ...
> > > > > > I think both choices are sane.  What I am trying to explain >
> > > here, is
> > > > regardless of what we choose now, we can change it in the > future
> > > without
> > > > affecting the API consumers at all.  What will change > internally
> > > is the way we
> > > > lookup the page pool pointer we are trying to recycle.
> > > 
> > > It seems the below API need changing?
> > > +static inline void skb_mark_for_recycle(struct sk_buff *skb, struct
> > > page *page,
> > > +					struct xdp_mem_info *mem)
> > 
> > I don't think we need to change this API, to support future memory
> > models.  Notice that xdp_mem_info have a 'type' member.
> 
> Hi,
> Providing that we will (possibly as a future optimization) store the pointer
> to the page pool in struct page instead of strcut xdp_mem_info, passing
> xdp_mem_info * instead of struct page_pool * would mean that for every
> packet we'll need to call
>             xa = rhashtable_lookup(mem_id_ht, &mem->id,
> mem_id_rht_params);
>             xa->page_pool;
> 
> which might pressure the Dcache to fetch a pointer that might be present
> already in cache as part of driver's data-structures.
> 
> I tend to agree with Yunsheng that it makes more sense to adjust the API for
> the clear use-case now rather than using xdp_mem_info indirection. It seems
> to me like
> the page signature provides the same information anyway and allows to
> support different memory types.

We've switched the patches already.  We didn't notice any performance boost
by doing so (tested on a machiattobin), but I agree as well.  As I
explained the only thing that will change if we ever the need the struct
xdp_mem_info in struct page is the internal contract between struct page
and the recycling function, so let's start clean and see if we ever need
that.


Cheers
/Ilias
> 
> Shay
> 
> > 
> > Naming in Computer Science is a hard problem ;-). Something that seems
> > to confuse a lot of people is the naming of the struct "xdp_mem_info".
> > Maybe we should have named it "mem_info" instead or "net_mem_info", as
> > it doesn't indicate that the device is running XDP.
> > 
> > I see XDP as the RX-layer before the network stack, that helps drivers
> > to support different memory models, also for handling normal packets
> > that doesn't get process by XDP, and the drivers doesn't even need to
> > support XDP to use the "xdp_mem_info" type.
>