netdev - Re: Question about veth

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Message-ID: <1485198462.16328.208.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 23 Jan 2017 11:07:42 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Xiangning Yu <yuxiangning@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: Question about veth_xmit()

On Mon, 2017-01-23 at 10:46 -0800, Xiangning Yu 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?
> 
> IMHO this might lead to some latency issue under certain workload,
> can we change the call to dev_forward_skb() to something like this?
> 
>         if (likely(__dev_forward_skb(rcv, skb) == NET_RX_SUCCESS)) {
>                 local_bh_disable();
>                 netif_receive_skb(skb);
>                 local_bh_enable();
> 
> Could you please shed some light on this change? And please feel free
> to correct my if my understanding is wrong.

How veth would have different latency requirement than loopback device ?

Calling netif_receive_skb() is considered too dangerous here (or from
any ndo_start_xmit()) because of possible kernel stack exhaustion.