linux-kernel - Re: [PATCH net-next v3 0/5] page

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Date:   Tue, 11 May 2021 11:41:23 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Shay Agroskin <shayagr@...zon.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>,
        Will Deacon <will@...nel.org>,
        Michel Lespinasse <walken@...gle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Roman Gushchin <guro@...com>, Hugh Dickins <hughd@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
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        Aleksandr Nogikh <nogikh@...gle.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
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        Miaohe Lin <linmiaohe@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        bpf@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v3 0/5] page_pool: recycle buffers

Hi Shay,

On Sun, May 09, 2021 at 08:11:35AM +0300, Shay Agroskin wrote:
> 
> Jesper Dangaard Brouer <brouer@...hat.com> writes:
> 
> > On Fri, 7 May 2021 16:28:30 +0800
> > Yunsheng Lin <linyunsheng@...wei.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.
>