netdev - Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB recycling

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Date:   Sun, 6 Jun 2021 16:56:34 +0300
From:   Tariq Toukan <ttoukan.linux@...il.com>
To:     Matteo Croce <mcroce@...ux.microsoft.com>,
        David Ahern <dsahern@...il.com>
Cc:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Yunsheng Lin <linyunsheng@...wei.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>,
        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>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Cong Wang <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        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>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v6 3/5] page_pool: Allow drivers to hint on SKB
 recycling



On 6/5/2021 7:34 PM, Matteo Croce wrote:
> On Sat, Jun 5, 2021 at 6:06 PM David Ahern <dsahern@...il.com> wrote:
>>
>> On 6/4/21 2:42 AM, Ilias Apalodimas wrote:
>>> [...]
>>>>> +   /* Driver set this to memory recycling info. Reset it on recycle.
>>>>> +    * This will *not* work for NIC using a split-page memory model.
>>>>> +    * The page will be returned to the pool here regardless of the
>>>>> +    * 'flipped' fragment being in use or not.
>>>>> +    */
>>>>
>>>> I am not sure I understand how does the last part of comment related
>>>> to the code below, as there is no driver using split-page memory model
>>>> will reach here because those driver will not call skb_mark_for_recycle(),
>>>> right?
>>>>
>>>
>>> Yes the comment is there to prohibit people (mlx5 only actually) to add the
>>> recycling bit on their driver.  Because if they do it will *probably* work
>>> but they might get random corrupted packets which will be hard to debug.
>>>
>>
>> What's the complexity for getting it to work with split page model?
>> Since 1500 is the default MTU, requiring a page per packet means a lot
>> of wasted memory.
> 
> We could create a new memory model, e.g. MEM_TYPE_PAGE_SPLIT, and
> restore the behavior present in the previous versions of this serie,
> which is, save xdp_mem_info in struct page.
> As this could slightly impact the performances, this can be added in a
> future change when the drivers which are doing it want to use this
> recycling api.
> 

page-split model doesn't only help reduce memory waste, but increase 
cache-locality, especially for aggregated GRO SKBs.

I'm looking forward to integrating the page-pool SKB recycling API into 
mlx5e datapath. For this we need it to support the page-split model.

Let's see what's missing and how we can help making this happen.

Regards,
Tariq