netdev - Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

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Message-ID: <CANn89i+WiW+1KGVKqLZ8xQpUaZYywonZLCnkVGzRawFv=YeLzg@mail.gmail.com>
Date:   Fri, 2 Apr 2021 14:52:49 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Paolo Abeni <pabeni@...hat.com>,
        Greg Thelen <gthelen@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, su-lifan@...ux.alibaba.com,
        "dust.li" <dust.li@...ux.alibaba.com>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH net] net: avoid 32 x truesize under-estimation for tiny skbs

On Fri, Apr 2, 2021 at 12:35 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
>
> Great!!
>
> test cmd:
>         sockperf tp --tcp --ip <ip> -m 8192 -t 300
>
>                 server softirq cpu | server sockperf cpu | rxpck/s | rxkB/s
> page_frag_alloc 100%.                86.1%                 2211785   3270145
> kmalloc         100%.                100%                  2226950   3292571
> copy ETH        100%                 69.3%                 2140552   3163408
>
> Compared with page_frag_alloc, the performance has also increased by 19.4%. The
> reason for such a big improvement I think is that although merge is turned on in
> my test environment, there is only one buffer for each packet received.
>
> So in the page_frag_alloc scenario, when performing gro merge, one skb will
> occupy two frags, one is used to store the data in the linear space, and the
> other is a frag in the skb frags. Now, the linear space only has the header, so
> one skb only occupies one frag space.
>
> In the case of using page_frag_alloc, an skb can only contain the original 8
> packets. Now a skb merges the original 17 packets

Exactly. Never put payload in skb->head, unless you are sure that :
1) All payload can be put there. (or it is filling a full page,
followed by remaining payload in further frags for big MTU)
2) skb->head is backed by a page fraf as well (skb->head_frag == 1)

>
> It's really exciting.
>
> I fixed the retval check of pskb_may_pull and added a pskb_may_pull call.
> Here is the patch I used for testing.
>
> Thanks very much.

Excellent, I am going to submit this patch officially, since it is not
risky for stable kernels.
I will give you credits as Reporter, and co-developer.

Thanks for your help !