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

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Message-ID: <20090320044541.GE6807@linux.vnet.ibm.com>
Date:	Thu, 19 Mar 2009 21:45:41 -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 Thu, Mar 19, 2009 at 08:20:32PM -0700, Arjan van de Ven wrote:
> On Thu, 19 Mar 2009 19:07:50 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Thu, Mar 19, 2009 at 07:18:41AM -0700, Arjan van de Ven wrote:
> > > On Thu, 19 Mar 2009 14:26:28 +0530
> > > Dipankar Sarma <dipankar@...ibm.com> wrote:
> > > 
> > > > On Wed, Mar 18, 2009 at 09:58:12PM -0700, Arjan van de Ven wrote:
> > > > > Hi,
> > > > > 
> > > > > the input layer does a "synchronize_rcu()" after a
> > > > > list_add_tail_rcu(), which is costing me 1 second of boot
> > > > > time..... And based on my understanding of the RCU concept, you
> > > > > only need to synchronize on delete, not on addition... so I
> > > > > think the synchronize is entirely redundant here...
> > > > 
> > > > The more appropriate question is - why is synchronize_rcu() taking
> > > > 1 second ? Any idea what the other CPUs are doing at the time
> > > > of calling synchronize_rcu() ?
> > > 
> > > one cpu is doing a lot of i2c traffic which is a bunch of udelay()s
> > > in loops.. then it does quite a bit of uncached memory access, and
> > > the lot takes quite while.
> > > 
> > > > What driver is this ? How early
> > > > in the boot is this happening ? 
> > > 
> > > during kernel boot.
> > > 
> > > I suppose my question is also more generic.. why synchronize when
> > > it's not needed? At least based on my understanding of RCU (but
> > > you're the expert), you don't need to synchronize for an add, only
> > > between a delete and a (k)free.....
> > 
> > I don't claim to understand the code in question, so it is entirely
> > possible that the following is irrelevant.  But one other reason for
> > synchronize_rcu() is:
> > 
> > 1.	Make change.
> > 
> > 2.	synchronize_rcu()
> > 
> > 3.	Now you are guaranteed that all CPUs/tasks/whatever
> > currently running either are not messing with you on the one hand, or
> > 	have seen the change on the other.
> 
> ok so this is for the case where someone is already iterating the list.
> 
> I don't see anything in the code that assumes this..

I must let the networking guys sort this out.

> > It sounds like you are seeing these delays later in boot, however,
> 
> yeah it's during driver init/
> 
> > Alternatively, again assuming a single-CPU system
> 
> 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?
The first patch in my earlier email (which is in mainline) will shortcut
synchronize_rcu() whenever there is only one CPU is online, at least
for Classic RCU and Hierarchical RCU.

							Thanx, Paul
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