netdev - Re: Question about veth

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Date:   Mon, 23 Jan 2017 14:05:45 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Xiangning Yu <yuxiangning@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Question about veth_xmit()

On Mon, 2017-01-23 at 13:46 -0800, Xiangning Yu wrote:
> On Mon, Jan 23, 2017 at 12:56 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Mon, Jan 23, 2017 at 10:46 AM, Xiangning Yu <yuxiangning@...il.com> wrote:
> >> Hi netdev folks,
> >>
> >> It looks like we call dev_forward_skb() in veth_xmit(), which calls
> >> netif_rx() eventually.
> >>
> >> While netif_rx() will enqueue the skb to the CPU RX backlog before the
> >> actual processing takes place. So, this actually means a TX skb has to
> >> wait some un-related RX skbs to finish. And this will happen twice for
> >> a single ping, because the veth device always works as a pair?
> >
> > For me it is more like for the completeness of network stack of each
> > netns. The /proc net.core.netdev_max_backlog etc. are per netns, which
> > means each netns, as an independent network stack, should respect it
> > too.
> >
> > Since you care about latency, why not tune net.core.dev_weight for your
> > own netns?
> 
> I haven't tried that yet, thank you for the hint! Though normally one
> of the veth device will be in the global namespace.

Well, per cpu backlog are not per net ns, but per cpu.

So Cong suggestion is not going to work.