Subject: Re: lots of cpu time spent in skb_copy_bits() in net-next with small mtu

From: John Dykstra <[email-masked]>
Date: Tue, 16 Jun 2009 19:20:00 +0000

On Fri, 2009-06-12 at 22:57 +0300, Ilpo Järvinen wrote:
> On Fri, 12 Jun 2009, Benjamin LaHaise wrote:
> 
> > Just a heads up: something in net-next is resulting in lots of cpu time 
> > being spent in skb_copy_bits() (called from tcp_collapse()) when an ethernet 
> > interface mtu is lowered to 576 on the source and dest machines running 
> > netperf.  This behaviour does not appear in 2.6.29.  I'll try to bisect 
> > it this weekend.
> 
> I'd suggest somebody goes through DaveM's abstraction patch 915219441d56 
> (I'm totally out of time so somebody else has to do it if a timely 
> solution is desired)... It was quite messy on those parts and I already 
> found one issue from it (2df9001edc3) and wouldn't be too surprised if 
> there would be some more lurking around... 

I couldn't find anything wrong with tcp_prune_queue() and its helpers as
they appear in today's net-next.

Take that with a grain of salt if you wish--I'm going to go take a
couple grains of aspirin.

  --  John