linux-kernel - Re: [PATCH v3] net:Add sysctl_max_skb

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Date:	Wed, 3 Feb 2016 15:03:50 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Hans Westgaard Ry <hans.westgaard.ry@...cle.com>,
	"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>,
	Tom Herbert <tom@...bertland.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Florian Westphal <fw@...len.de>, Jiri Pirko <jiri@...nulli.us>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Michal Hocko <mhocko@...e.com>,
	Linus Lüssing <linus.luessing@...3.blue>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Kodanev <alexey.kodanev@...cle.com>,
	Håkon Bugge <haakon.bugge@...cle.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>
Subject: Re: [PATCH v3] net:Add sysctl_max_skb_frags

On 03.02.2016 13:20, Herbert Xu wrote:
> On Wed, Feb 03, 2016 at 12:36:21PM +0100, Hannes Frederic Sowa wrote:
>>
>> Agreed that it feels like a hack, but a rather simple one. I would
>> consider this to be just a performance improvement. We certainly need
>> a slow-path when virtio drivers submit gso packets to the stack (and
>> already discussed with Hans). The sysctl can't help here. But without
>> the sysctl the packets would constantly hit the slow-path in case of
>> e.g. IPoIB and that would also be rather bad.
>
> So you want to penalise every NIC in the system if just one of
> them is broken? This is insane.  Just do the partial linearisation
> in that one driver that needs it and not only won't you have to
> penalise anyone else but you still get the best result for that
> driver that needs it.

Most normal Ethernet systems and drivers currently don't need tweating 
this knob at all, only some special kinds of installations. This patch 
referred to IPoIB as a possible user which drivers/firmware/cards seem 
to have this problem. Current behavior just leaves everything as-is.

If you use IPoIB you probably use it quite regular and linearizing an 
skbs *always* seems to be much more work than simply capping the number 
of frags globally.

> Besides, you have to implement the linearisation anyway because
> of virtualisation.

Yes, the slow-path is necessary. But instead of writing a new 
complicated linearizing function to just reduce the fragments we could 
also simply linearize it completely and ask the admin to also tune the 
vm guests.

I only see this tuning in kind in very specific environments where the 
admins now what they do.

Bye,
Hannes