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SubjectRe: 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@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?

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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