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

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Date:   Sun, 9 May 2021 08:11:35 +0300
From:   Shay Agroskin <shayagr@...zon.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
CC:     Yunsheng Lin <linyunsheng@...wei.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        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>,
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        Michel Lespinasse <walken@...gle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
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        Alexander Lobakin <alobakin@...me>,
        "Cong Wang" <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
        Aleksandr Nogikh <nogikh@...gle.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
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        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


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.

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.