netdev - Re: [net-next, RFC, 4/8] net: core: add recycle capabilities on skbs via page

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Date:   Sat, 8 Dec 2018 11:26:56 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        ard.biesheuvel@...aro.org, Jason Wang <jasowang@...hat.com>,
        BjörnTöpel <bjorn.topel@...el.com>, w@....eu,
        Saeed Mahameed <saeedm@...lanox.com>,
        mykyta.iziumtsev@...il.com,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [net-next, RFC, 4/8] net: core: add recycle capabilities on skbs
 via page_pool API



On 12/08/2018 06:57 AM, Ilias Apalodimas wrote:
> Hi Eric, 
>>>> This patch is changing struct sk_buff, and is thus per-definition
>>>> controversial.
>>>>
>>>> Place a new member 'mem_info' of type struct xdp_mem_info, just after
>>>> members (flags) head_frag and pfmemalloc, And not in between
>>>> headers_start/end to ensure skb_copy() and pskb_copy() work as-is.
>>>> Copying mem_info during skb_clone() is required.  This makes sure that
>>>> pages are correctly freed or recycled during the altered
>>>> skb_free_head() invocation.
>>>
>>> I read this to mean that this 'info' isn't accessed/needed until skb
>>> is freed.  Any reason its not added at the end?
>>>
>>> This would avoid moving other fields that are probably accessed
>>> more frequently during processing.
>>>
>>
>> But I do not get why the patch is needed.
>>
>> Adding extra cost for each skb destruction is costly.
>>
>> I though XDP was all about _not_ having skbs.
> 
> You hit the only point i don't personally like in the code, xdp info in the 
> skb. Considering the benefits i am convinced this is ok and here's why:
> 
>> Please let's do not slow down the non XDP stack only to make XDP more appealing.
> 
> We are not trying to do that, on the contrary. The patchset has nothing towards
> speeding XDP and slowing down anything else. The patchset speeds up the 
> mvneta driver on the default network stack. The only change that was needed was
> to adapt the driver to using the page_pool API. The speed improvements we are
> seeing on specific workloads (i.e 256b < packet < 400b) are almost 3x.
> 
> Lots of high speed drivers are doing similar recycling tricks themselves (and
> there's no common code, everyone is doing something similar though). All we are
> trying to do is provide a unified API to make that easier for the rest. Another
> advantage is that if the some drivers switch to the API, adding XDP
> functionality on them is pretty trivial.
> 
> Moreover our tests are only performed on systems without or with SMMU disabled.
> On a platform i have access to, enabling and disabling the SMMU has some
> performance impact. By keeping the buffers mapped we believe this impact
> will be substantially less (i'll come back with results once i have them on
> this).
> 
> I do understand your point, but the potential advantages on my head
> overwight that by a lot.
> 
> I got other concerns on the patchset though. Like how much memory is it 'ok' to
> keep mapped keeping in mind we are using the streaming DMA API. Are we going to
> affect anyone else negatively by doing so ?
> 


I want to make sure you guys thought about splice() stuff, and skb_try_coalesce(),
and GRO, and skb cloning, and ...

I do not see how an essential property of page fragments would be attached
to sk_buff, without adding extra code all over the places.

Page recycling in drivers is fine, but should operate on pages that the driver owns.

Messing with skb->head seems not needed for performance, since skb->head can be regular
slab allocated.