Re: Question about usage of RCU in the input layer

[Arjan van de Ven <arjan@infradead.org>]

On Thu, 19 Mar 2009 19:07:50 -0700
"Paul E. McKenney" <paulmck@linux.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@in.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..

> 
> 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... 



-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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