Re: [PATCH v3] net:Add sysctl_max_skb_frags

[Eric Dumazet]

On Wed, 2016-02-03 at 20:20 +0800, 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.

No penalization :

- default is the optimal value

- TCP stack tends to build skb with 32KB frags anyway. It is very rare
to actually get to 17 frags per skb (pathological sendpage() with tiny
parts, or tiny write() on many sockets from one thread). 

> 
> Besides, you have to implement the linearisation anyway because
> of virtualisation.

Sure.

We use a similar patch here at Google, since bnx2x has in some cases a
limit of 13 frags per skb. This driver calls linearize which can fail
under memory fragmentation. TCP usually retransmits, so only effect of
failures is extra latencies.

I am actually okay with this patch.

Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>