netdev - Re: [PATCH v2] net: ip/tnl: Set iph->id only when don't fragment is not set

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Date:   Wed, 27 Nov 2019 13:19:42 +0100
From:   Oliver Herms <oliver.peter.herms@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>
Cc:     yoshfuji@...ux-ipv6.org, kuznet@....inr.ac.ru,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: ip/tnl: Set iph->id only when don't fragment is
 not set

On 27.11.19 01:28, Eric Dumazet wrote:
> 
> 
> On 11/26/19 3:32 PM, Oliver Herms wrote:
>> Using a simple incrementation here, as with sockets, would solve my problem well enough.
>>
> 
> I have to ask : Are you aware that linux is SMP OS ?
> 
> If on a mostly idle host, two packets need a different ID, using a " simple incrementation" 
> wont fit the need.
> 
> sockets are protected against concurrent increments by their lock.
> 
I know and I'm not going to mess around with TCP.
I've double checked and found that for non IP tunnel traffic (UDP, TCP, etc.) the cheap function
ip_select_ident_segs() is being used. That is absolutely fine. Nothing to optimize here.

For IP tunnels __ip_select_ident is being called.
And that one is way more expensive than ip_select_ident_segs().

ip_select_ident_segs() increments a counter (yes, I'm aware it is protected by lock).
If somehow __ip_select_ident could be refactored to work in a similar fashion that
would solve my problem.

Thanks
Oliver