linux-kernel - Re: [PATCH v2] net:Add sysctl_tcp_sg_max_skb

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Date:	Wed, 27 Jan 2016 19:12:12 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	"open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	Haakon Bugge <haakon.bugge@...cle.com>
Subject: Re: [PATCH v2] net:Add sysctl_tcp_sg_max_skb_frags

On 27.01.2016 16:15, Eric Dumazet wrote:
> On Wed, 2016-01-27 at 14:20 +0100, Hans Westgaard Ry wrote:
>> Devices may have limits on the number of fragments in an skb they support.
>> Current codebase uses a constant as maximum for number of fragments one
>> skb can hold and use.
>> When enabling scatter/gather and running traffic with many small messages
>> the codebase uses the maximum number of fragments and may thereby violate
>> the max for certain devices.
>> The patch introduces a global variable as max number of fragments in
>> scatter/gather.
>
>
> Principle looks good, but we have to ask if other skb providers [1] will
> add other sysctl, or if we could share a common one ?
>
> If it is a common one, it should be /proc/sys/net/core/... instead
> of /proc/sys/net/ipv4/tcp_....
 >
> Other providers include :
>
> 1) GRO stack
> 2) callers of sock_alloc_send_pskb(), alloc_skb_with_frags(),
> sock_alloc_send_skb() ..

I agree, this knob should get a generic name and live in a generic net/ 
directory to control this globally, so things don't break during 
forwarding etc.

It does not solve the problem completely, e.g. when VMs send gso packets 
through a vhost-net onto IPoIB, no?

Thanks,
Hannes