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

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Date:	Sat, 21 Mar 2009 21:26:08 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	paulmck@...ux.vnet.ibm.com, 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

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 ?

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