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SubjectRe: Question about usage of RCU in the input layer
On Fri, 20 Mar 2009 18:27:46 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Fri, Mar 20, 2009 at 11:13:54AM -0700, Arjan van de Ven wrote:
> > On Fri, 20 Mar 2009 07:31:04 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > > >
> > > > that'd be throwing out the baby with the bathwater... I'm
> > > > trying to use the other cpus to do some of the boot work (so
> > > > that the total goes faster); not using the other cpus would be
> > > > counter productive to that. (As is just sitting in
> > > > synchronize_rcu() when the other cpu is working.. hence this
> > > > discussion ;-)
> > >
> > > OK, so you are definitely running multiple CPUs when the offending
> > > synchronize_rcu() executes, then?
> >
> > absolutely.
> > (and I'm using bootgraph.pl in scripts to track who's stalling etc)
> > >
> > > If so, here are some follow-on questions:
> > >
> > > 1. How many synchronize_rcu() calls are you seeing on the
> > > critical boot path
> >
> > I've seen only this (input) one to take a long time
>
> Ouch!!! A -single- synchronize_rcu() taking a full second??? That
> indicates breakage.
>
> > > and what value of HZ are you running?
> >
> > 1000
>
> K, in absence of readers for RCU_CLASSIC, we should see a handful
> of milliseconds for synchronize_rcu().

I've attached an instrumented bootgraph of what is going on;
the rcu delays are shown as red blocks inside the regular functions
as they initialize......

(svg can be viewed with inkscape, gimp, firefox and various other tools)


--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org
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