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

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Message-ID: <20090320111354.679ab53d@infradead.org>
Date:	Fri, 20 Mar 2009 11:13:54 -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 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
>  and what value of HZ are you running?

1000

> 
> 	If each synchronize_rcu() is taking (say) tens of jiffies,
> then, as Peter Zijlstra notes earlier in this thread, we need to focus
> 	on what is taking too long to get through its RCU read-side
> 	critical sections

I know that "the other guy" is not optimal and takes waaay too long.

>  Otherwise, if each synchronize_rcu() is
> 	in the 3-5 jiffy range, I may finally be forced to create an
> 	expedited version of the synchronize_rcu() API.

I think a simplified API for the "add to a list" case might make sense.
Because the request isn't for a full sync for sure...

(independent of that .. the open question is if this specific case is
even needed; I think the code confused "send to others" with "wait
until everyone sees"; afaik synchronize_rcu() has no pushing behavior
at all, nor should it)

> 
> 2.	If expediting is required, then the code calling
> synchronize_rcu() might or might not have any idea whether or not
> expediting is appropriate.  If it does not, then we would need some
> sort of way to tell synchronize_rcu() that it should act more
> aggressively, perhaps /proc flag or kernel global variable indicating
> that boot is in progress.
> 
> 	No, we do not want to make synchronize_rcu() aggressive all
> the time, as this would harm performance and energy efficiency in
> 	the normal runtime situation.
> 
> 	So, if it turns out that synchronize_rcu()'s caller does not
> 	know whether or not expediting is appropriate, can the boot
> path manipulate such a flag or variable?
> 
> 3.	Which RCU implementation are you using?  CONFIG_CLASSIC_RCU,
> 	CONFIG_TREE_RCU, or CONFIG_PREEMPT_RCU?

CLASSIC

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
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