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SubjectRe: Question about usage of RCU in the input layer
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@linux.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?

Thanx, Paul


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