Subject: Re: [PATCH/RFC] synchronize_rcu(): high latency on idle system

From: Stephen Hemminger <[email-masked]>
Date: Sat, 12 Jan 2008 17:52:26 -0800

On Sat, 12 Jan 2008 19:35:58 +0100
Andi Kleen <[email-masked]> wrote:

> On Saturday 12 January 2008 18:51:35 Benjamin LaHaise wrote:
> > On Sat, Jan 12, 2008 at 03:37:59AM +0100, Andi Kleen wrote:
> > > > And yes, the 
> > > > network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
> > > > is a little more involved.
> > > 
> > > ... but the correct solution.
> > 
> > There has to be at least 1 synchronize_rcu() or equivalent in the 
> > unregister_netdev() path.  I suspect the easiest way to fix it might be to 
> > use call_rcu() to actually free the network device, as anything else will 
> > limit performance of single threaded teardown (ie, when an l2tp daemon 
> > gets terminated via kill -9).  This means an API change that exposes 
> > rcu for unregister_netdev().
> 
> The call_rcu() could be in free_netdev() couldn't it?

I think it should be in netdev_unregister_kobject().  But that would
only get rid of one of the two calls to synchronize_rcu() in the unregister_netdev.
The other synchronize_rcu() is for qdisc's and not sure if that one can
be removed?

-- 
Stephen Hemminger <[email-masked]>