netdev - RE: [PATCH v3] net:Add sysctl_max_skb

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Date:	Wed, 3 Feb 2016 17:36:24 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Herbert Xu' <herbert@...dor.apana.org.au>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
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

From: Herbert Xu
> Sent: 03 February 2016 12:21
> 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.
> 
> Besides, you have to implement the linearisation anyway because
> of virtualisation.

And if a MAC driver needs to linearize a tx frame it might as well
copy it into a separately allocated tx buffer area.
Indeed it can copy fragments until the number left is less than the
fragment limit.

	David