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

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Message-ID: <20090323012737.GO7148@linux.vnet.ibm.com>
Date:	Sun, 22 Mar 2009 18:27:37 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Eric Dumazet <dada1@...mosbay.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

On Sun, Mar 22, 2009 at 04:16:37PM -0700, Arjan van de Ven wrote:
> On Sun, 22 Mar 2009 16:03:31 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Sun, Mar 22, 2009 at 03:44:33PM -0700, Arjan van de Ven wrote:
> > > On Sun, 22 Mar 2009 13:52:12 -0700
> > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > > On Sun, Mar 22, 2009 at 12:46:32PM -0700, Arjan van de Ven wrote:
> > > > > On Sun, 22 Mar 2009 09:53:24 -0700
> > > > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > > > 
> > > > > > > > How many synchronize_rcu() calls are you seeing on the
> > > > > > > > boot path?
> > > > > > > 
> > > > > > > I see 20 that hit the above code path (eg ones that wait)
> > > > > > > until userspace starts.
> > > > > > 
> > > > > > So with well-behaved readers, the full sequence would be worth
> > > > > > something like 50-60 milliseconds.
> > > > > 
> > > > > yeah it's about 10% if the total kernel boot time.. so it does
> > > > > start to matter
> > > > 
> > > > Half-second boot, eh?  That would indeed be impressive.
> > > 
> > > half a second until calling init.. that's what I have today (with
> > > all drivers built in).. nothing really special needed for it
> > > (well a few small patches that are pending for 2.6.30 ;-)
> > 
> > I thought the measurement was until the desktop was running with no
> > more disk activity.  ;-)
> 
> yeah that's like 2 seconds after that ;)

I must confess that I still dream of it booting faster than I perceive.
Of course, this is becoming ever easier to achieve as I continue to
grow older...  ;-)

> > But in any case, I will see what I can do about speeding up
> > synchronize_rcu().  I will likely start with TREE_RCU, and I may need
> > some sort of indication that boot is in progress.
> 
> system_state == SYSTEM_BOOTING is not a very elegant test but it works

Fair enough...  This would be until init, correct?

							Thanx, Paul
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