linux-kernel - Re: Question about usage of RCU in the input layer

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Message-ID: <20090321125115.0a76ac27@infradead.org>
Date:	Sat, 21 Mar 2009 12:51:15 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	dipankar@...ibm.com, linux-input@...r.kernel.org,
	dmitry.torokhov@...il.com, linux-kernel@...r.kernel.org
Subject: Re: Question about usage of RCU in the input layer

On Fri, 20 Mar 2009 18:27:46 -0700
"Paul E. McKenney" <paulmck@...ux.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@...ux.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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