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

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Date:	Fri, 20 Mar 2009 15:21:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
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, Mar 20, 2009 at 07:31:04AM -0700, Paul E. McKenney wrote:
> On Fri, Mar 20, 2009 at 06:50:58AM -0700, Arjan van de Ven wrote:
> > On Thu, 19 Mar 2009 21:45:41 -0700
> > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > 
> > > > single CPU is soooo last decade ;-)
> > > > But seriously I no longer have systems that aren't dual core or SMT
> > > > in some form... 
> > > 
> > > OK, I will ask the stupid question...
> > > 
> > > Why not delay bringing up the non-boot CPUs until later in boot?
> > 
> > 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?
> 
> If so, here are some follow-on questions:
> 
> 1.	How many synchronize_rcu() calls are you seeing on the
> 	critical boot path and what value of HZ are you running?
> 
> 	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.  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.
> 
> 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?

And one other thing...  CONFIG_CLASSIC_RCU's synchronize_rcu() normally
runs faster than CONFIG_PREEMPT_RCU, if that helps.

							Thanx, Paul
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