netdev - Re: Netperf UDP issue with connected sockets

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Date:   Wed, 16 Nov 2016 16:34:09 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Rick Jones <rick.jones2@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Netperf UDP issue with connected sockets

On Wed, 2016-11-16 at 23:40 +0100, Jesper Dangaard Brouer wrote:

> Using -R 1 does not seem to help remove __ip_select_ident()
> 
> Samples: 56K of event 'cycles', Event count (approx.): 78628132661
>   Overhead  Command        Shared Object        Symbol
> +    9.11%  netperf        [kernel.vmlinux]     [k] __ip_select_ident
> +    6.98%  netperf        [kernel.vmlinux]     [k] _raw_spin_lock
> +    6.21%  swapper        [mlx5_core]          [k] mlx5e_poll_tx_cq
> +    5.03%  netperf        [kernel.vmlinux]     [k] copy_user_enhanced_fast_string
> +    4.69%  netperf        [kernel.vmlinux]     [k] __ip_make_skb
> +    4.63%  netperf        [kernel.vmlinux]     [k] skb_set_owner_w
> +    4.15%  swapper        [kernel.vmlinux]     [k] __slab_free
> +    3.80%  netperf        [mlx5_core]          [k] mlx5e_sq_xmit
> +    2.00%  swapper        [kernel.vmlinux]     [k] sock_wfree
> +    1.94%  netperf        netperf              [.] send_data
> +    1.92%  netperf        netperf              [.] send_omni_inner

Check "ss -nu"  ?

You will see if sockets are connected (present in ss output or not)

UDP being connected does not prevent __ip_select_ident() being used.

    if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) {

So you need IP_DF being set, and skb->ignore_df being 0

-> time to try IP_MTU_DISCOVER ;)