linux-kernel - Re: [PATCH] net: add per device sg_max

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Date:	Fri, 8 Jan 2016 12:47:49 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Hans Westgaard Ry <hans.westgaard.ry@...cle.com>,
	David Laight <David.Laight@...LAB.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>,
	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

On 08.01.2016 10:55, Hans Westgaard Ry wrote:
>
>
> On 01/06/2016 02:59 PM, David Laight wrote:
>> From: Hans Westgaard Ry
>>> Sent: 06 January 2016 13:16
>>> 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 (MAX_SKB_FRAGS) 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 thereby
>>> violates the max for certain devices.
>>>
>>> An example of such a violation is when running IPoIB on a HCA
>>> supporting 16 SGE on an architecture with 4K pagesize. The
>>> MAX_SKB_FRAGS will be 17 (64K/4K+1) and because IPoIB adds yet another
>>> segment we end up with send_requests with 18 SGE resulting in
>>> kernel-panic.
>>>
>>> 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 because of the complex forwarding nature, a global counter which 
driver's can reduce during initialization time is the only solution I 
see right now without changing the layout of the skb later on.

Unfortunately this doesn't resolve the cases were virtual machines 
inject gso skbs, for those there still needs to be a slow path to do the 
reformatting of the skb. :/

Bye,
Hannes