netdev - Re: [PATCH net-next] virtio-net: page_to_skb() use build

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Message-ID: <84995a39-c588-3aa0-98ff-f16087dbdff0@gmail.com>
Date:   Wed, 7 Apr 2021 19:13:35 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] virtio-net: page_to_skb() use build_skb when
 there's sufficient tailroom



On 4/7/21 7:49 AM, Xuan Zhuo wrote:
> In page_to_skb(), if we have enough tailroom to save skb_shared_info, we
> can use build_skb to create skb directly. No need to alloc for
> additional space. And it can save a 'frags slot', which is very friendly
> to GRO.
> 
> Here, if the payload of the received package is too small (less than
> GOOD_COPY_LEN), we still choose to copy it directly to the space got by
> napi_alloc_skb. So we can reuse these pages.
> 
> Testing Machine:
>     The four queues of the network card are bound to the cpu1.
> 
> Test command:
>     for ((i=0;i<5;++i)); do sockperf tp --ip 192.168.122.64 -m 1000 -t 150& done
> 
> The size of the udp package is 1000, so in the case of this patch, there
> will always be enough tailroom to use build_skb. The sent udp packet
> will be discarded because there is no port to receive it. The irqsoftd
> of the machine is 100%, we observe the received quantity displayed by
> sar -n DEV 1:
> 
> no build_skb:  956864.00 rxpck/s
> build_skb:    1158465.00 rxpck/s
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Suggested-by: Jason Wang <jasowang@...hat.com>
> ---

This will generate merge conflicts.

Please wait that commit 0f6925b3e8da ("virtio_net: Do not pull payload in skb->head") has reached net-next.

Also are we sure pages are always writable, and not shared ?