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

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Message-ID: <20090321210745.GE7148@linux.vnet.ibm.com>
Date:	Sat, 21 Mar 2009 14:07:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Arjan van de Ven <arjan@...radead.org>, 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 Sat, Mar 21, 2009 at 09:26:08PM +0100, Eric Dumazet wrote:
> Arjan van de Ven a écrit :
> > 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)
> 
> Interesting stuff...
> 
> I thought you mentioned i2c drivers being source of the udelays(),
>  but I cant see them in this svg, unless its async_probe_hard ?

Arjan, another thought -- if the udelays() are not under rcu_read_lock(),
you should be able to finesse this by using CONFIG_PREEMPT_RCU, which
will happily ignore spinning CPUs as long as they are not in an RCU
read-side critical section.

							Thanx, Paul
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