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Message-ID: <568F87AC.60405@oracle.com>
Date: Fri, 8 Jan 2016 10:55:56 +0100
From: Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
To: 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>,
"\" 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
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.
Hans
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