netdev - RE: [PATCH] net: add per device sg_max

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Date:	Fri, 8 Jan 2016 10:33:11 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Hans Westgaard Ry' <hans.westgaard.ry@...cle.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Jiri Pirko <jiri@...lanox.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	"Eric W. Biederman " <ebiederm@...ssion.com>,
	Salam Noureddine <noureddine@...sta.com>,
	Jarod Wilson <jarod@...hat.com>,
	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Julian Anastasov <ja@....bg>,
	Ying Xue <ying.xue@...driver.com>,
	Craig Gallek <kraig@...gle.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"\" hannes\"@stressinduktion.org " <hannes@...essinduktion.org>,
	Edward Jee <edjee@...gle.com>,
	Julia Lawall <julia.lawall@...6.fr>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haakon Bugge <haakon.bugge@...cle.com>,
	Knut Omang <knut.omang@...cle.com>,
	Wei Lin Guay <wei.lin.guay@...cle.com>,
	Santosh Shilimkar <santosh.shilimkar@...cle.com>,
	Yuval Shaia <yuval.shaia@...cle.com>
Subject: RE: [PATCH] net: add per device sg_max_frags for skb

From: Hans Westgaard
> Sent: 08 January 2016 09:56
...
> >> The patch allows the device to limit the maximum number fragments used
> >> in one skb.
> >
> > This doesn't seem to me to be the correct way to fix this.
> > Anything that adds an extra fragment (in this case IPoIB) should allow
> > for the skb already having the maximum number of fragments.
> > Fully linearising the skb is overkill, but I think the first fragment
> > can be added to the linear part of the skb.
> >
> > 	David
> >
> >
> When IpoIB handles a skb-request it converts fragments to SGEs to
> be handled by a HCA.
> The problem arises when the HCA have a limited number of SGEs less than
> MAX_SKB_FRAGS.
> (it gets a little worse since IPoIB need to yet another segment)
> I have not found any easy way of fixing this with currenct codebase.

I think one of the xen ethernet interfaces had a similar problem.

Just reduce the number of fragments by copying two (or more) of them
into a single fragment.
In effect, anything that reduces the number of fragments will do a copy.

	David