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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
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